Getting Started with HP ProCurve Switching and Routing, Rev. 9.41

1. ProCurve Networking by HP
Student guide
Technical training
IP Routing Foundations
Version 5.21

3. Contents
Overview
Introduction ............................................................................................ Overview–1
Course objectives.................................................................................... Overview–1
Prerequisites ........................................................................................... Overview–1
Course module overviews ...................................................................... Overview–2
Course agenda ........................................................................................ Overview–3
Additional information ........................................................................... Overview–4
Module 1: IP Routing Basics
Objectives ............................................................................................................. 1–1
General network connectivity goals ..................................................................... 1–2
Scenario: ProCurve University............................................................................. 1–3
Router interfaces and port state ............................................................................ 1–4
Route tables and local address ranges .................................................................. 1–6
The route table...................................................................................................... 1–6
Multinetted interface ............................................................................................ 1–8
When multinetting is appropriate ......................................................................... 1–8
Loopback interface ............................................................................................. 1–10
Learning about remote networks ........................................................................ 1–11
Routing protocol categories................................................................................ 1–12
RIP and OSPF..................................................................................................... 1–13
Standard IGPs for IP networks ........................................................................... 1–14
The disadvantage of RIP .................................................................................... 1–14
Link-state protocols ............................................................................................ 1–15
Router1 RIP update to Router2 .......................................................................... 1–16
Cost..................................................................................................................... 1–16
RIP v2 use of multicast....................................................................................... 1–17
Router2 updates its route table ........................................................................... 1–18
Router2 RIP update to Router1 .......................................................................... 1–19
Router2 RIP update to Router3 .......................................................................... 1–20
Router3 updates its route table ........................................................................... 1–21
Assessing this topology ...................................................................................... 1–22
Providing a routed mesh..................................................................................... 1–23
Split horizon in a routed mesh............................................................................ 1–24
Processing inbound RIP updates ........................................................................ 1–25
Link failure recovery in mesh (1) ....................................................................... 1–27
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4. IP Routing Foundations
Link failure recovery in mesh (2) ....................................................................... 1–28
Link failure recovery in mesh (3) ....................................................................... 1–29
Poisoned Reverse................................................................................................ 1–30
Connecting to a core router ................................................................................ 1–31
Connecting to a core routing switch................................................................... 1–32
Connecting to redundant core............................................................................. 1–33
Routing among locations at ProCurve University.............................................. 1–34
Dynamic route exchange .................................................................................... 1–35
Network summarization ..................................................................................... 1–36
Summarization of address space using static routes........................................... 1–37
Route table lookup.............................................................................................. 1–39
Advertising static routes ..................................................................................... 1–40
Equal cost multipath ........................................................................................... 1–41
Module 1 summary............................................................................................. 1–42
Module 2: OSPF Routing
Objectives ............................................................................................................. 2–1
OSPF at ProCurve University ...................................................................... 2–2
Basic OSPF interactions ....................................................................................... 2–3
OSPF routing protocol ................................................................................. 2–4
OSPF hierarchy: Routers and networks ....................................................... 2–5
OSPF Router ID .......................................................................................... 2–5
OSPF adjacencies ........................................................................................ 2–5
OSPF network types .................................................................................... 2–6
OSPF area .................................................................................................... 2–7
OSPF hierarchy: Autonomous System ........................................................ 2–9
OSPF router boots up................................................................................. 2–10
Hello messages .......................................................................................... 2–10
Exchanging Hello packets.......................................................................... 2–11
Two-way neighbor recognition .................................................................. 2–13
Designated Router election ........................................................................ 2–14
Exchanging Database descriptions............................................................. 2–15
Link State Request packet.......................................................................... 2–17
Link State Update packet ........................................................................... 2–18
Updating the Link State Database.............................................................. 2–19
Originating new LSAs ............................................................................... 2–20
Flooding LSAs in Link State Update packet ............................................. 2–21
R1A’s LSA ................................................................................................ 2–22
SPF tree and IP route table......................................................................... 2–23
Summary of OSPF packet types ................................................................ 2–25
Summary of OSPF LSA types confined to a single area ........................... 2–27
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5. Contents
Distribution of link state changes ....................................................................... 2–28
Impact of link state changes....................................................................... 2–29
Connecting to existing multi-access network ............................................ 2–30
Recognizing a new router on a multi-access network................................ 2–31
Database synchronization .......................................................................... 2–32
Adjacencies established, database synchronized ....................................... 2–33
Flood new LSAs......................................................................................... 2–34
Acknowledging flooded LSAs................................................................... 2–35
Designated Router adjacency responsibilities............................................ 2–36
Designated Router LSA flooding responsibilities ..................................... 2–37
Non-DR LSA flooding responsibilities...................................................... 2–38
OSPF network types................................................................................... 2–39
Finding the shortest path ............................................................................ 2–41
OSPF’s performance in large intranet........................................................ 2–42
OSPF scalability......................................................................................... 2–44
Area Border Router (ABR) ....................................................................... 2–44
Multiple areas and adjacency ..................................................................... 2–45
ABR link state database synchronization................................................... 2–46
LSA flow between areas ............................................................................ 2–47
Flooding Summary LSAs........................................................................... 2–48
Hierarchical addressing enables summarization ........................................ 2–49
Summary of OSPF LSA types ................................................................... 2–50
External route information ................................................................................. 2–51
Redistributing non-OSPF network information ......................................... 2–52
ASBR ......................................................................................................... 2–53
Stub-area type: Injecting the default route ................................................. 2–54
Locating the ASBR .................................................................................... 2–55
Stub and “totally stubby” area ................................................................... 2–56
Not-so-stubby area (NSSA) ....................................................................... 2–57
Module 2 summary .................................................................................... 2–58
Module 3: Default Gateway Redundancy Protocols
Objectives ............................................................................................................. 3–1
Redundant router interfaces.................................................................................. 3–2
Redundant links: Physical view............................................................................ 3–3
Redundant links: Logical view............................................................................. 3–4
Impact of device failure........................................................................................ 3–5
Edge switch failure ............................................................................................... 3–5
Router failure........................................................................................................ 3–5
Providing a second router..................................................................................... 3–7
Why failover is not automatic (1)......................................................................... 3–8
Why failover is not automatic (2)......................................................................... 3–9
Why failover is not automatic (3)....................................................................... 3–10
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6. IP Routing Foundations
Automatic failover for default gateway.............................................................. 3–11
Common characteristics and operations ............................................................. 3–12
Virtual Router Redundancy Protocol ................................................................. 3–14
Virtual routers in VRRP ..................................................................................... 3–15
VRRP: Actual and virtual IP addresses.............................................................. 3–16
VRRP: Master and Backup states....................................................................... 3–17
VRRP: Virtual MAC address ............................................................................. 3–18
VRRP Master broadcasts “gratuitous ARP” ...................................................... 3–19
Master accepts traffic sent to virtual MAC address ........................................... 3–20
Virtual MAC address enables automatic failover .............................................. 3–21
VRRP advertisements......................................................................................... 3–22
VRRP advertisement packet format ................................................................... 3–23
VRRP support for load sharing .......................................................................... 3–24
Considering link failure vs. device failure ......................................................... 3–25
Mixed virtual router states (1) ............................................................................ 3–26
Mixed virtual router states (2) ............................................................................ 3–27
Proprietary variations and enhancements ........................................................... 3–28
VRRPE: Virtual and actual IP addresses............................................................ 3–29
XRRP.................................................................................................................. 3–30
Module 3 summary............................................................................................. 3–31
Module 4: ACL Theory
Objectives ............................................................................................................. 4–1
Device security and access control....................................................................... 4–2
Identity-based security.......................................................................................... 4–2
Role-based security .............................................................................................. 4–2
Rule-based security .............................................................................................. 4–3
Basic security principles: Physical security example........................................... 4–4
Security threats ..................................................................................................... 4–5
Basic security principles: Additional layer of physical security .......................... 4–6
Comparing physical and virtual security.............................................................. 4–7
Planning for rule-based access control ................................................................. 4–8
Rule-based access control example .................................................................... 4–10
Selection criteria in IP header............................................................................. 4–11
Determine which port(s) will filter traffic .......................................................... 4–12
A rule that may be applied to ingress or egress ports......................................... 4–13
The implied “deny any” rule .............................................................................. 4–14
Impact of applying Rule 1 at ingress port .......................................................... 4–15
Impact of applying Rule 1 at egress port............................................................ 4–16
Associating users with resource requirements ................................................... 4–17
Inbound ACL recommendations ........................................................................ 4–17
Outbound ACL recommendations...................................................................... 4–18
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7. Contents
Define characteristics of resources ..................................................................... 4–19
Strategies for defining inbound ACLs................................................................ 4–20
Access control for faculty users ......................................................................... 4–21
Access control criteria in TCP and UDP headers............................................... 4–22
Permit faculty user access to curriculum server network ................................... 4–24
Permit faculty user access to SMTP services ..................................................... 4–25
Deny faculty user access to administrative servers ............................................ 4–26
Permit faculty user Internet access ..................................................................... 4–27
Access control for student users ......................................................................... 4–28
Permit student access to web registration server................................................ 4–29
Deny student traffic destined for administrative servers.................................... 4–30
Student Internet access ....................................................................................... 4–31
Access control of admin users............................................................................ 4–32
Permit admin user access to web registration server.......................................... 4–33
Permit admin access to HR and admin servers .................................................. 4–34
Access control for guests.................................................................................... 4–35
Deny guest access to intranet destinations ......................................................... 4–36
Permit guest access to Internet destinations ....................................................... 4–37
Module 4 summary............................................................................................. 4–38
Learning Check Answers
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8. IP Routing Foundations
6 Rev. 5.21

9. Overview
Introduction
IP Routing Foundations provides the basic knowledge of routing technologies
necessary to prepare for Routing Switch Essentials. Designed to be delivered as a
self-paced prestudy or in the classroom, IP Routing Foundations focuses on
standards, theories, and technologies and is not dependent on ProCurve products or
features.
Before taking IP Routing Foundations, students should complete Adaptive EDGE
Fundamentals or have attained equivalent background. The topics in Adaptive
EDGE Fundamentals include:
Basic Ethernet technology
IP addressing
VLANs
Spanning Tree
Link Aggregation
Fundamentals of switch technology
Traffic prioritization
Course objectives
During this course, you will:
Learn basic routing and traffic filtering technologies, including redundant
default gateway protocols, Router Information Protocol (RIP), Open Shortest
Path First (OSPF), and Access Control Lists (ACLs)
Prepare for the Routing Switch Essentials instructor-led course
Prerequisites
Adaptive EDGE Fundamentals
Rev. 5.21 Overview – 1

10. IP Routing Foundations
Course module overviews
Module 1, “IP Routing Basics,” describes RIP, static routes, and other information
necessary to develop routed networks in the contemporary enterprise.
Module 2, “OSPF Routing,” introduces the basic features and processes of the
OSPF routing protocol.
Module 3, “Default Gateway Redundancy and Protocols,” describes the Virtual
Router Redundancy Protocol and other technologies designed to ensure the
availability of default gateways.
Module 4, “ACL Theory,” describes the theory and planning for ACLs.
Overview – 2 Rev. 5.21

11. Overview
Course agenda
IP Routing Foundations is designed to be a self-paced prestudy for Routing Switch
Essentials. Students should complete each section and its related Learning Check
before moving to the next topic.
Rev. 5.21 Overview – 3

12. IP Routing Foundations
Additional information
Additional information
• The HP Certified Professional (HPCP) program is a world-class
certification program benchmarked around the world to ensure
validation of the technical and sales competencies and expertise
needed to plan, deploy, support and service HP technology and
solutions
• ProCurve participates in the Sales and Integration Tracks within HPCP
• This course, along with Routing Switch Essentials, prepares you for
the required exam for ASE – Routing Switch Essentials
• The exam number for this course is HPO-790
• For more information on HPCP, go to www.hp.com/certification
• For more information on HP ProCurve Training and Certification, go to
http://www.hp.com/rnd/training/certifications.htm
Rev 5.21 Student Guide: Overview–4 5
IP Routing Foundations is part of a series of courses on ProCurve products. For
more information, visit the ProCurve Web site.
Overview – 4 Rev. 5.21

13. IP Routing Basics
Module 1
Objectives:
After completing this module, you will be able to:
Categorize sources of routing information
• Static and dynamic
• Interior and exterior
• Distance vector and link state
Describe how a router builds its route table and how it chooses the best match
from the tables entries
Describe reasons for defining multinetted interfaces
Explain the value of a loopback interface
Describe the process a router uses to choose a path when its route table
includes multiple equal cost paths to the same destination
Rev. 5.21 1–1

14. IP Routing Foundations
General network connectivity goals
General network connectivity
goals
Establish connectivity among clients and resources
• Routers must obtain enough information to find the best path to each
address range and collect the information in a route table
Routing efficiency, economy, scalability
• Each route table entry specifies an address range that may represent:
– A single network (broadcast domain)
– A range of networks whose address space can be expressed as a
starting address and mask
• Summarize address space whenever possible to minimize the number
of route table entries
Enable selective forwarding based on resource needs
• Arrange clients and addressing scheme to selectively enable access to
resources
• Goals of limiting resource access may be based on traffic shaping or
security requirements
• Alternate paths for link failover
– Unlike STP, all links active (no blocked links)
Rev 5.21 Student Guide: 1–2 3
In general, routers exist to connect clients and resources. Routers learn the most
efficient way to reach each address range, collect the information, and organize it
in a route table. To enable routers to function efficiently, a medium-to-large
enterprise will use a hierarchical addressing scheme. Hierarchical addressing
enables an administrator to summarize the address range at remote locations using
the smallest number of route table entries. This is only possible when hosts within
an IP address range are at the same physical location. A sound IP addressing
scheme enables an intranet to scale to a very large size without exceeding the
capabilities of its routers.
Routers enable any-to-any communication. However, not all users are necessarily
able to reach all resources. This is true for two reasons:
1. Users simply don’t need all intranet resources.
2. Some user/resource pairs must be disallowed to conform to security policies.
The actual mechanisms used for traffic filtering are beyond the scope of this
module and will be discussed later in the course. However, to enable the
development of efficient traffic filters, administrators must take great care when
planning their IP addressing schemes. Basically, the IP addresses of clients with
common resource requirements should be within a range that can easily be
expressed by a starting address and mask. This module will provide more detail on
this topic.
1–2 Rev. 5.21

15. IP Routing Basics
Scenario: ProCurve University
Scenario: ProCurve University
The university comprises three campuses
Each campus supports a variety of users
• Students and guests
• Faculty and administration
Each campus supports a variety of applications, including web, e-mail,
and multimedia conferencing
10 GbE 10 GbE Northeast
Northwest High-speed
campus core campus
10 GbE
Southwest
campus
Rev 5.21 Student Guide: 1–3 4
This module and the rest of IP Routing Foundations will refer to ProCurve
University whenever it is useful to illustrate a basic technology principle. The
fictional university consists of three campuses connected by a high-speed core.
The university supports four types of users—students, guests, faculty, and
administrators—and a typical array of enterprise applications.
The university will appear more regularly in Routing Switch Essentials, which
focuses heavily upon the deployment and configuration of ProCurve routing
switches.
Rev. 5.21 1–3

16. IP Routing Foundations
Router interfaces and port state
Router interfaces and port
state
Every vendor’s router supports one or more of the following
interface types:
• Physical
– Created by assigning an IP address and mask to a physical port
– Interface state may be “up” only if the physical port state is “up”
• Virtual
– Associates IP address and mask with a VLAN
– Interface state may be “up” if at least one of the ports in the VLAN
is “up”
• Loopback
– Assigns IP address and mask to an interface whose state is not
bound to a physical port state
– Interface state is always “up”
• Multinetted
– Assigns two or more IP address/mask combinations to a physical,
virtual, or loopback interface
Rev 5.21 Student Guide: 1– 4 5
Every router in an enterprise, regardless of the vendor who provides it, must
enable communication among multiple networks. All routers accomplish this by
enabling administrators to define one or more of the following types of router
interfaces:
1. Physical
As its name suggests, the physical interface is created by assigning an IP
address and mask to a physical port. The rest of this module will focus
heavily on this type of interface, which is the “traditional” router interface.
2. Virtual
Common in contemporary enterprises, the virtual interface associates an IP
address and mask with a VLAN. This enables packets for multiple broadcast
domains to be forwarded through a single port.
3. Loopback
The loopback interface defines an IP address and mask that is not bound to
any port or VLAN. It is often used as the interface for management
communication.
4. Multinetted
In a multinetted configuration, two or more IP addresses and masks are
assigned to a single port, VLAN, or loopback interface.
1–4 Rev. 5.21

17. IP Routing Basics
Whether they are virtual or physical, router interfaces function in the same way in
terms of Layer 3 forwarding. Differences among the types of interfaces are
confined solely to Layer 2 forwarding issues. The physical interface associates
each router port with a different broadcast domain and thus a different address
range, while the virtual interface enables you to associate an arbitrary set of ports
with a broadcast domain/address range.
Rev. 5.21 1–5

18. IP Routing Foundations
Route tables and local address ranges
Route table and local address
ranges
• For each interface whose state is “up,” the router derives the local address
range by applying the mask to the assigned IP address
• Route table entries for local address ranges usually have a cost of “0”
• Router forwards traffic destined for local networks using port indicated in route
table
– Drops traffic destined for address ranges not represented in the table
IP Route Table
Network address Mask Gateway Port Cost Type
10.1.10.0 255.255.255.0 0.0.0.0 If 1 0 Local
10.1.30.0 255.255.255.0 0.0.0.0 If 2 0 Local
If 1 Router1
Port 1: 10.1.10.1/24
Port 2: 10.1.30.1/24
If 2
Switch1: 10.1.10.3/24 Switch2: 10.1.30.3/24
Router forwards traffic
Hosts in range 10.1.10.0/24 Hosts in range 10.1.30.0/24 among its local address
DG: 10.1.10.1 DG: 10.1.30.1
ranges
Rev 5.21 Student Guide: 1–6 6
In this example, a router has two interfaces defined. Because the physical port “If
1” is connected to Switch1, the interface state is up. Because the interface is
defined in the router’s configuration as 10.1.10.1/24, the router applies the mask to
the address and derives a range of addresses that it expects to find through that
port.
In this case, the range of local addresses the router puts in the route table is
10.1.10.0 with a mask of 255.255.255.0. When this dotted decimal mask is
converted to binary, the mask includes 24 “1” bits and eight “0” bits. In the
application of the mask to the address, each of the “1” bits indicates the number of
high order—that is, “most significant”—bits in the address that are common to all
of the hosts connected to this interface. The “0” bits of the mask represent the low
order—that is, “least significant”—bits in each host’s address that may have any
value. All of the combinations of these eight bits—from 0000 0000 to 1111
1111—are considered part of the address range. However, lowest value (0) and the
highest value (255) are not permissible as addresses for individual hosts. The
lowest value is the network address, also known as the “starting address.” The
highest value is the broadcast address. The same principles apply to If 2.
The route table
A router bases forwarding decisions on the content of its route table. While a
Layer 2 forwarding device, such as a switch, floods traffic destined for unknown
MAC addresses, a router drops traffic whose destination IP address does not match
any of the entries in the route table.
1–6 Rev. 5.21

19. IP Routing Basics
The graphic on the previous page shows route table entries for two networks—
10.1.10.0 and 10.1.30.0. Although routers from different vendors may display
routing information differently, all route tables contain the same basic information.
Common fields include:
The “Gateway” field for each address range is sometimes labeled as the
“Next Hop” field, but its function is to tell the router how to reach the address
range. In this case, because all three address ranges are local, this router uses
all zeros in dotted decimal format. Once again, different vendors represent
this in different ways.
The “Port” field indicates which of the router’s interfaces leads toward the
best path to the destination.
The “Cost” field provides information about the distance to the network.
Because the address ranges in the example are local, Router1 records the
“Cost” for each route as “0.” Although the end stations in networks
10.1.10.0/24 and 10.1.30.0/24 are connected to a downstream switch,
Router1 considers the addresses to be “local” because Router1’s interfaces
are in the same broadcast domain as other hosts in the same address range.
The switch is transparent from an IP routing perspective because it forwards
traffic based on Layer 2 information rather than Layer 3. The switch’s own IP
address, which is assigned for management purposes, does not affect this
transparency.
The “Type” field indicates the source of the routing information. Because all
of these address ranges are local, their type is “D” which represents “directly
connected.” We will cover other sources of routing information later in this
module.
Because Router1 provides the default gateway for its local hosts, it can forward
traffic on their behalf and also deliver traffic that is destined for those hosts.
Because all hosts are local, the router uses ARP to obtain each destination host’s
MAC address and encapsulates each forwarded packet with a Layer 2 header that
contains its own MAC address in the source address field and the target host’s
MAC address in the destination address field.
The router does not change the source or destination IP address in the Layer 3
header. The source address field in the IP datagram header contains the address of
the sending host and the destination address field contains the address of the target
host. The router does not insert its own address into the IP datagram header as it
does with the Layer 2 header.
In most environments, a router is also required to forward traffic toward remote
networks.
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20. IP Routing Foundations
Multinetted interface
Multinetted interface
• Defined to provide default gateway addresses for hosts that are in same
broadcast domain but have different address ranges
• Each address range appears as route table entry
IP Route Table
Network address Mask Gateway Port Cost Type
10.1.10.0 255.255.255.0 0.0.0.0 If 1 0 Local
10.1.30.0 255.255.255.0 0.0.0.0 If 2 0 Local
172.16.150.0 255.255.255.0 0.0.0.0 If 2 0 Local
If 1 Router1
Port 1: 10.1.10.1/24
Port 2: 10.1.30.1/24
If 2
Switch1: 10.1.10.3/24 Switch2: 10.1.30.3/24
Hosts in range 10.1.10.0/24 Hosts in range 10.1.30.0/24
DG: 10.1.10.1 DG: 10.1.30.1
Hosts in range 172.16.150.0/24
DG: 172.16.150.1
Rev 5.21 Student Guide: 1–8 7
Multinetting enables an administrator to associate multiple IP addresses with a
single broadcast domain that might be physically bounded, using a physical
interface associated with a single router port, or virtually bounded, using a virtual
interface associated with a VLAN. Multinetting creates routing inefficiencies and
should be used only when necessary.
In contemporary networks, multinetting is usually not recommended, although it
was quite common in earlier periods, when physical router interfaces presented the
only router interface option. Furthermore, multinetting can create problems in
environments where hosts use DHCP to receive IP configuration information.
Hosts in a DHCP network usually will receive addresses in the same range;
consequently, hosts in a multinetted network may not receive an address in the
intended range.
When multinetting is appropriate
Multinetting can be necessary when the network includes a collection of hosts,
links, and legacy connectivity devices, such as hubs, that do not support VLANs.
The graphic above illustrates this point. Suppose that hosts in the 10.1.30.0/24
address range are used by clients who need access to the Internet. Their addresses
would be included in a range to be translated by a router, proxy server, or firewall
using NAT. However, the hosts in the range 172.16.150.0/24 are special-purpose
devices with statically defined addresses. Their access should be restricted. They
will never need to browse the Internet. An administrator might specifically omit
their address range from the range of addresses to be translated by the proxy,
firewall, or other NAT device.
1–8 Rev. 5.21

21. IP Routing Basics
Administrators might also implement multinetting as an interim step while
changing the IP addressing scheme. Suppose, for example, that an intranet
originally was configured to use statically defined public addresses and must now
be converted to a private addressing scheme where hosts dynamically obtain their
addresses. Enabling multinetting would enable the administrator to continue
providing connectivity for hosts whose addresses have not been converted, as well
as for those whose addresses have been converted to the new scheme.
Rev. 5.21 1–9

22. IP Routing Foundations
Loopback interface
Loopback interface
• Address range associated with loopback interface appears as a route table
entry
• May be used as source and/or destination for router’s host processes such as
SNMP, Telnet, and HTTP
IP Route Table
Network address Mask Gateway Port Cost Type
10.1.0.0 255.255.255.0 0.0.0.0 lb 1 0 Local
10.1.10.0 255.255.255.0 0.0.0.0 If 1 0 Local
10.1.30.0 255.255.255.0 0.0.0.0 If 2 0 Local
172.16.150.0 255.255.255.0 0.0.0.0 If 2 0 Local
If 1 Router1
Port 1: 10.1.10.1/24
Port 2: 10.1.30.1/24
If 2 Loopback 1: 10.1.0.1/24
Switch1: 10.1.10.3/24 Switch2: 10.1.30.3/24
Hosts in range 10.1.10.0/24 Hosts in range 10.1.30.0/24
DG: 10.1.10.1 DG: 10.1.30.1
Hosts in range 172.16.150.0/24
DG: 172.16.150.1
Rev 5.21 Student Guide: 1–10 8
A loopback interface is very useful for routers in an intranet that supports
redundant links. Because the state of a loopback interface is not dependent on the
state of any physical port, its IP address will be reachable if at least one other
router interface is up. Consequently, the loopback address often is used for in-band
device management.
Routers often are configured to use the loopback address for outbound
communication with network management stations or other routers. With no
loopback defined for this purpose, a router will send the packet through the
interface that is “closest” to the destination network; that is, the one that
corresponds with the route table’s next hop toward the destination network.
In the case of a network management station, administrators often set up filters
that allow the station to accept messages only from a set of source address ranges.
In a redundant network, one or more routers might choose different paths to the
network management station’s address range based on the physical state of some
of the intervening links. Consequently, it can be difficult to predict the address
from which a router will send a management message.
Furthermore, by using the loopback interface for all host-based communication
with the router, you can set up traffic filters that prohibit traffic produced by
typical management protocols—including HTTP, FTP, TFTP, Telnet and SSH—
from reaching any of the physical or virtual interfaces. The traffic can be permitted
to reach the loopback interface. All valid administrators would need to configure
and monitor the router using the loopback interface as a target address. (Traffic
filters will be discussed later in this course.)
1 – 10 Rev. 5.21

23. IP Routing Basics
Learning about remote networks
Learning about remote
networks
A router can learn of the existence of remote networks through
any combination of the following:
• Dynamic interaction with other routers that follow a common set of
rules for exchanging routing information
– These rules might include:
• Procedures for establishing relationships with neighboring
routers
• The frequency and format of messages exchanged with other
routers
• Static route configuration, which requires an administrator to:
– Specify an address range, expressed as starting address and mask
– Provide “next hop” information that will allow the router to send
traffic toward the address range
– Supply a cost to be associated with the path to the address range,
enabling router to choose the lowest-cost statically defined path
Network topology, including Internet and intranet connectivity,
determine appropriate methods for each situation
Rev 5.21 Student Guide: 1–11 9
A router can only forward traffic toward address ranges that appear in its route
table. If a router receives a routable packet with a destination address that does not
match with any route table entries, it drops the packet.
Routers may learn the information in their route tables dynamically through
interaction with other routers with which they share a common set of route
exchange rules known as a “routing protocol.” Routing protocols specify the
format of the information the routers exchange and the conditions that require a
router to send information to a neighboring router.
Administrators often choose to augment the dynamically learned information by
statically defining information that the router can use to reach specific address
ranges. In most contemporary networks, routers must be aware of remote networks
because most enterprise users require access to Internet and intranet resources.
Usually, route tables are populated with a combination of static and dynamically
learned routes.
In any case, routers cannot directly deliver traffic to remote hosts. Instead, they
deliver traffic destined for remote hosts to neighboring routers that provide the
best route to the remote address range.
Rev. 5.21 1 – 11

24. IP Routing Foundations
Routing protocol categories
Routing protocol categories
Interior Gateway Protocols (IGP)
• Facilitate exchange of information among routers under the same
organizational control; that is, within the same “autonomous system”
• Examples of standard IGPs:
– Routing Information Protocol (RIP)
– Open Shortest Path First (OSPF)
Exterior Gateway Protocols (EGP)
• Facilitate exchange of route information among routers in different
autonomous systems
• Border Gateway Protocol version 4 (BGP4) is current standard EGP for
Internet connectivity
Rev 5.21 Student Guide: 1–12 10
There are two types of dynamic interaction between routers:
1. Interior Gateway Protocols (IGP) involve communication among routers
that are under common administrative control and use the same protocol for
exchanging information; that is, in the same autonomous system.
2. Exterior Gateway Protocols (EGP) involve communication among routers
that are under different administrative control; that is, in different
autonomous systems.
An Internet Service Provider is likely to use a combination of interior and exterior
gateway protocols to facilitate exchange of routing information among the routers
that make up its own internal network as well as with the routers at subscriber
locations.
Not all Internet subscribers use an exterior gateway protocol; however, a very
large subscriber that load balances among multiple ISPs is the most likely
candidate for using a formalized exterior gateway protocol. Small-to-medium
sized subscribers are likely to use a combination of interior gateway protocols and
static routes to facilitate Internet connectivity.
1 – 12 Rev. 5.21

25. IP Routing Basics
RIP and OSPF
Several routing protocols have been formalized and are described in various
standards documents. In some cases, vendors implement these standards exactly as
written; other vendors enhance the protocols to optimize particular aspects or
functions. Other protocols are entirely proprietary, with their own reserved port
and/or protocol numbers. These protocols operate only with other routers from the
same vendor.
Two common routing protocols, RIP and OSPF, are both IGPs with the same high-
level goal: to enable connectivity within an autonomous system. In general,
because RIP and OSPF perform this task in completely different ways, each is best
suited for particular topologies. However, there is a large overlapping area of
applicability. Many intranets can deploy either protocol effectively.
Routing protocols specify the format of messages to be exchanged. As a fairly
simple routing protocol, RIP specifies only one type of message. On the other
hand, OSPF is a far more complex IGP that specifies several different types and
even sub-types of messages, specifying formal procedures for setting up
relationships with neighboring routers and types of messages that should be sent in
particular circumstances.
Routing protocols also specify the conditions that require a router to send an
advertisement. While a RIP router periodically sends routing information to its
neighbors, an OSPF router sends a particular type of message when it experiences
a change in the state of one of its links.
RIP will be described in more detail later in this module. A later module will
discuss OSPF.
Rev. 5.21 1 – 13

26. IP Routing Foundations
Standard IGPs for IP networks
Standard IGPs for IP networks
Distance vector: RIP
• Each router sends periodic updates containing a subset of its route
table entries to directly connected neighbor routers
• Information about remote networks is passed from router to router
based on each router’s perspective
• Time required for each router to find alternate path to an address
range after link failure depends on number of routers that separate it
from the address range
Link state: OSPF
• Each router reports to its neighbors the characteristics of its active
connections to local networks
• Updates are flooded to all routers within administratively defined
area, resulting in consistent picture of area’s routers and networks
• Each router builds a logical tree that calculates its shortest path to
each network address range
• Enables faster convergence – detection of alternate paths after link
failure – due to possession of first-hand information
Rev 5.21 Student Guide: 1–14 11
There are two types of standard IGPs available in IP networks:
1. Distance-vector protocols, such as RIP, require routers to integrate
information into their own tables and send the resulting entries, as modified,
from their own perspectives.
2. Link-state protocols, such as OSPF, require routers to establish neighbor
relationships with adjacent routers. Routers generate updates based on local
information and send the updates to neighbors, who then flood updates to all
their neighbors. Ideally, within a few milliseconds, every router in an
administratively defined area has identical information. Each router builds a
logical tree that traces out the shortest path to each advertised destination,
using itself as the root. As a result, every router has a consistent picture of the
network from its own perspective.
The disadvantage of RIP
While RIP and other distance-vector protocols are easier to configure than link-
state protocols, the distance-vector protocols have one serious disadvantage.
Changes in routing topology often propagate slowly because information in a
router’s table is acquired from other routers that may be as many as 15 hops away.
1 – 14 Rev. 5.21

27. IP Routing Basics
Suppose, for instance, that Router1 is directly connected to Network 1. When
Router1 loses its connection to Network 1, it immediately sends its neighbors an
update that reports the cost of Network 1 to be 16. In RIP, the cost of 16 represents
infinity and indicates the network is unreachable because the maximum number of
router hops in RIP is 15.
After Network 1 has been marked as unavailable, each router is free to accept
advertisements from other neighbors that offer a lower-cost path to Network 1.
Because there is a 30-second interval between RIP updates, and because RIP
updates move one hop at a time, several minutes may elapse before each router has
determined the lowest-cost path between itself and Network 1.
Link-state protocols
Link-state protocols avoid this issue because they do not rely on second-hand
information. A router sends an “advertisement” when it recognizes a link state
change. The update does not contain just the change, but the attributes of all of the
router’s currently active links. The router sends the advertisement to its immediate
neighbors. The neighbors are required by the protocol to immediately flood the
advertisement to all of their neighbors.
Unlike RIP routers, OSPF routers do not increment the costs as they flood updates.
In fact, an OSPF router is not permitted to make any changes to advertisements it
receives on one network before sending it out onto another network.
As a result, all of the routers in the area have a consistent picture of the
connections between all routers and networks in the area. Each router builds a tree
based on first-hand information that traces the shortest path between itself and
every router and network in the area. When a link state changes, the router
recalculates the tree based on the new information. Ideally, less than a second
passes between the time the router advertises its new state and the time when all of
the routers have found an alternate path, if one exists
Rev. 5.21 1 – 15

28. IP Routing Foundations
Router1 RIP update to Router2
Router1 RIP update to Router2
Ethernet header: Router1
Dest: 01005e-000009 Source: <R1 MAC>
IP datagram header: • Advertises entries in its
Source: 10.0.64.1 Dest: 224.0.0.9 route table through
UDP header: interface 3
Source: 520 Dest: 520
Routing Information Protocol: • Does not include the
Command: Response (2) Version: RIPv2 (2) address range associated
Network: 10.1.0.0 Mask: 255.255.255.0 Metric: 1 with interface 3
Network: 10.1.10.0 Mask: 255.255.255.0 Metric: 1
Network: 10.1.30.0 Mask: 255.255.255.0 Metric: 1 (10.0.64.0/24)
Network: 172.16.150.0 Mask: 255.255.255.0 Metric: 1
Network 10.0.64.0/24
If 3 If 3
10.0.64.1/24 10.0.64.2/24
RIP enabled
Loop 1: 10.1.0.1/24 Loop 1: 10.2.0.1/24
R1 R2
If 1 If 2 If 1 If 2
10.1.10.1/24 10.1.30.1/24 10.2.20.1/24 10.2.40.1/24
172.16.150.1/24
S1 S2 S3 S4
10.1.10.3/24 10.1.30.3/24 10.2.20.3/24 10.2.40.3/24
Hosts in Hosts in Hosts in Hosts in
10.1.10.0/24 10.1.30.0/24 10.2.20.0/24 10.2.40.0/24
172.16.150.0/24
Rev 5.21 Student Guide: 1–16 12
When RIP is enabled on an interface, the router prepares an update that advertises
the address ranges in its route table. In many cases, including the one above, each
address range in the table represents a network, a single broadcast domain.
However, this is not always the case. Sometimes the entries represent an address
range that includes many networks.
In the example above, Router1 advertises all of its connected networks with one
notable exception. A RIP advertisement doesn’t include the address range
associated with the interface through which the router sends the update. In this
case, the advertisement is being prepared for transmission over interface 3 (if 3),
which is associated with the address range 10.0.64.0/24. Accordingly, that network
is specifically omitted from the advertisement.
It is important to note that the update actually includes two distinct steps: the
preparation and the sending of the update. By default, this process occurs every 30
seconds; when this interval expires, the router must send advertisements through
all of its RIP-enabled interfaces.
Cost
Note that the cost associated with each of the advertised networks is 1. While
Router1 associates a cost of 0 with its locally connected address ranges, it
advertises these networks with a cost of 1. In some vendor implementations, the
cost used internally will be 1; however, the external cost is reported in the same
way by all router vendors.
1 – 16 Rev. 5.21

29. IP Routing Basics
RIP v2 use of multicast
The source address in the IP datagram that encapsulates the RIP advertisement is
the address of Router1’s interface on the network it shares with Router2. The
destination address is a multicast address, which is the requirement in RIP v2.
The use of multicast ensures that all routers connected to a network will receive
and process the update simultaneously. Routers or other devices on this network
that do not support RIP v2 will not process this update because they are not
members of the RIP Routers multicast group (224.0.0.9).
In the example, Router1 is the only RIP router on network 10.0.64.0. Note that
Router2 does not have RIP enabled. This does not affect Router1’s outbound RIP
updates. Because RIP is enabled on this interface, Router1 will continue sending
updates indefinitely.
Rev. 5.21 1 – 17

30. IP Routing Foundations
Router2 updates its route table
Router2 updates its route table
Network Gateway Port Cost Type • Router2 integrates
10.0.64.0/24 0.0.0.0 3 0 D networks from Router1’s
10.1.0.0/24 10.0.64.1 3 2 R
RIP update into its route
10.1.10.0/24 10.0.64.1 3 2 R
10.1.30.0/24 10.0.64.1 3 2 R table
10.2.0.0/24 0.0.0.0 Lo 1 0 D • “Gateway” associated with
10.2.20.0/24 0.0.0.0 1 0 D RIP-learned networks is
10.2.40.0/24 0.0.0.0 2 0 D source address from IP
172.16.150.0/24 10.0.64.1 3 2 R
datagram header of
Router1’s RIP update
Network 10.0.64.0/24
If 3 If 3
10.0.64.1/24 10.0.64.2/24
RIP enabled RIP enabled
Loop 1: 10.1.0.1/24 Loop 1: 10.2.0.1/24
R1 R2
If 1 If 2 If 1 If 2
10.1.10.1/24 10.1.30.1/24 10.2.20.1/24 10.2.40.1/24
172.16.150.1/24
S1 S2 S3 S4
10.1.10.3/24 10.1.30.3/24 10.2.20.3/24 10.2.40.3/24
Hosts in Hosts in Hosts in Hosts in
10.1.10.0/24 10.1.30.0/24 10.2.20.0/24 10.2.40.0/24
172.16.150.0/24
Rev 5.21 Student Guide: 1–18 13
In this example, RIP has been enabled on Router2’s interface on the 10.0.64.0/24
network. Router2 receives Router1’s RIP update and begins processing it. It
doesn’t matter if Router1’s RIP update arrived before Router2 sent any
advertisements over the network it shares with Router1 because each router’s
sending and receiving actions are independent.
When Router2 receives the advertisement, it compares each entry with the entries
already in its route table and immediately adds any advertised address range that
does not already appear there. In the example above, all of the address ranges are
new, so all are added. The cost of the RIP-learned address ranges is one number
higher than the cost advertised by Router1. This is only true if Router2’s
configured interface cost for interface 3 is at the default setting of “1.” While it is
possible to manipulate interface costs for the purpose of favoring one path over
another, it is usually not recommended for reasons discussed later in this module.
Every address range a router learns from a RIP update is set to type “R” (for RIP)
in the route table. The “Port” value is the interface through which Router2
received the update that advertised the address range.
In this example, every RIP-learned network in Router2’s route table has the same
next hop. This is because Router2 has only one neighbor.
1 – 18 Rev. 5.21

31. IP Routing Basics
Router2 RIP update to Router1
Router2 updates its route table
Network Gateway Port Cost Type • Router2 integrates
10.0.64.0/24 0.0.0.0 3 0 D networks from Router1’s
10.1.0.0/24 10.0.64.1 3 2 R
RIP update into its route
10.1.10.0/24 10.0.64.1 3 2 R
10.1.30.0/24 10.0.64.1 3 2 R table
10.2.0.0/24 0.0.0.0 Lo 1 0 D • “Gateway” associated with
10.2.20.0/24 0.0.0.0 1 0 D RIP-learned networks is
10.2.40.0/24 0.0.0.0 2 0 D source address from IP
172.16.150.0/24 10.0.64.1 3 2 R
datagram header of
Router1’s RIP update
Network 10.0.64.0/24
If 3 If 3
10.0.64.1/24 10.0.64.2/24
RIP enabled RIP enabled
Loop 1: 10.1.0.1/24 Loop 1: 10.2.0.1/24
R1 R2
If 1 If 2 If 1 If 2
10.1.10.1/24 10.1.30.1/24 10.2.20.1/24 10.2.40.1/24
172.16.150.1/24
S1 S2 S3 S4
10.1.10.3/24 10.1.30.3/24 10.2.20.3/24 10.2.40.3/24
Hosts in Hosts in Hosts in Hosts in
10.1.10.0/24 10.1.30.0/24 10.2.20.0/24 10.2.40.0/24
172.16.150.0/24
Rev 5.21 Student Guide: 1–19 13
When Router2 sends a RIP advertisement through its only RIP-enabled interface,
it does not include the address range 10.1.64.0/24 because that address range is
associated with interface 3.
Because Router2 has already received advertisements from Router1, it follows an
additional rule requiring that advertisements a router sends onto a network do not
include the address ranges for which the next hop is on that network.
In the example, none of the networks that Router2 learned from Router1 are
included in the RIP update Router2 sends onto network 10.0.64.0/24. Because
10.1.64.1 is the “next hop” for the address ranges 10.1.0.0/24, 10.1.10.0/24, and
10.1.30.0/24, and because the address range associated with interface 3 contains
the next hop address, these are omitted from the update.
The set of rules that govern which networks may be advertised is known as “Split
horizon.” The primary reason that RIP routers follow Split horizon rules is because
a neighbor simply doesn’t need to learn about networks for which it provides the
next hop. Other reasons for the Split horizon rules will be discussed later.
Rev. 5.21 1 – 19

32. IP Routing Foundations
Router2 RIP update to Router3
Router2 RIP update to Router3
IP datagram header:
• Router2’s RIP updates Source: 10.0.65.1 Dest: 224.0.0.9
through interface 4 UDP header:
include: Source: 520 Dest: 520
Routing Information Protocol:
– Locally defined Network: 10.0.64.0 Mask: 255.255.255.0 Metric: 1
networks Network: 10.1.0.0 Mask: 255.255.255.0 Metric: 2
– Routes to address Network: 10.1.10.0 Mask: 255.255.255.0 Metric: 2
Network: 10.1.30.0 Mask: 255.255.255.0 Metric: 2
ranges learned Network: 10.2.0.0 Mask: 255.255.255.0 Metric: 1
from a neighbor on Network: 10.2.20.0 Mask: 255.255.255.0 Metric: 1
interface 3 Network: 10.2.40.0 Mask: 255.255.255.0 Metric 1
Network: 172.16.150.0 Mask: 255.255.255.0 Metric: 2
Network 10.0.65.0/24
If 3 If 4 If 3
10.0.64.2/24 10.0.65.1/24 10.0.65.2/24
RIP enabled RIP enabled
Loop 1: 10.2.0.1/24 Loop 1: 10.3.0.1/24
R2 R3
If 1 If 2 If 1 If 2
10.2.20.1/24 10.2.40.1/24 10.3.10.1/24 10.3.30.1/24
Hosts in Hosts in Hosts in Hosts in
10.2.20.0/24 10.2.40.0/24 10.3.10.0/24 10.3.30.0/24
Rev 5.21 Student Guide: 1–20 15
In this example, Router2 has another neighbor that it reaches through a network
(10.0.65.0/24) associated with interface 4. Because Router3 does not have RIP
enabled, Router2 has not yet received any advertisements from Router3. Still,
because RIP is enabled on interface 4, Router2 sends periodic RIP updates
regardless of whether it has received any information from Router3.
The RIP update that Router2 sends to Router3 contains a completely different set
of address ranges than the update it sends to Router1. Following Split horizon
rules, the RIP advertisement Router2 sends through interface 4 does not include
the address range associated with interface 4, 10.0.65.0/24. However, it does
include all address ranges in its route table that are either local or learned from a
neighbor connected to an interface other than interface 4. Router2 advertises the
cost of these address ranges from its own perspective. In all cases except for local
networks, a RIP router advertises the cost that each address range has in its own
route table.
The “Gateway” or next hop value in the route table is the most important factor in
determining which address ranges Router2 will advertise through network
10.0.65.0/24. A RIP advertisement includes all local address ranges except the
network address associated with the interface over which the advertisement will be
transmitted. A remote address range will be included in the RIP advertisement
only if its associated “Gateway” or “next hop” IP address is outside the range of
the network associated with the interface over which the advertisement will be
transmitted.
1 – 20 Rev. 5.21

33. IP Routing Basics
Router3 updates its route table
Router3 updates its route table
• All routes known to Network Gateway Port Cost Type
Router3 are either local or 10.0.64.0/24 10.1.65.1 3 3 RIP
learned from 10.0.65.1 10.0.65.0/24 0.0.0.0 3 0 Direct
10.1.0.0/24 10.1.65.1 3 3 RIP
• Router3’s updates through 10.1.10.0/24 10.1.65.1 3 3 RIP
interface 3 include 10.1.30.0/24 10.1.65.1 3 3 RIP
networks not learned from 10.2.0.0/24 10.1.65.1 3 2 RIP
neighbors on the network 10.2.20.0/24 10.1.65.1 3 2 RIP
associated with that 10.2.40.0/24 10.1.65.1 3 2 RIP
10.3.0.0/24 0.0.0.0 Lo 1 0 Direct
interface
10.3.10.0/24 0.0.0.0 1 0 Direct
10.3.30.0/24 0.0.0.0 2 0 Direct
172.16.150.0/24 10.1.65.1 3 3 RIP
Network 10.0.65.0/24
If 3 If 4 If 3
10.0.64.2/24 10.0.65.1/24 10.0.65.2/24
RIP enabled RIP enabled RIP enabled
Loop 1: 10.2.0.1/24 Loop 1: 10.3.0.1/24
R2 R3
If 1 If 2 If 1 If 2
10.2.20.1/24 10.2.40.1/24 10.3.10.1/24 10.3.30.1/24
Hosts in Hosts in Hosts in Hosts in
10.2.20.0/24 10.2.40.0/24 10.3.10.0/24 10.3.30.0/24
Rev 5.21 Student Guide: 1–21 16
In the manner described earlier, Router3 increments the cost of all advertised
networks by the cost assigned to the interface through which the update arrives.
Everything that was advertised by Router2 with a cost of 1 appears in Router3’s
route table with a cost of 2. The address ranges reported with a cost of 2 have a
cost of 3 in Router3’s route table.
In this example, Router2 is Router3’s only neighbor, so the “Gateway” or next hop
router interface for every remote address range in Router3’s route table is
10.0.65.1, which is the IP address of Router2’s interface on the network that
connects the two routers. None of Router1’s interfaces appear in Router3’s route
table as a next hop because Router3 and Router1 do not share a network. The
“Type” column contains “RIP” for all address ranges that Router3 learns from
Router2’s advertisements.
When Router3 sends an advertisement to Router2, it will follow the Split horizon
rules described earlier. In this case, only three address ranges qualify for inclusion
in the RIP advertisement sent to Router2: 10.3.10.0/24, 10.3.30.0/24, and
10.3.0.1/24.
Rev. 5.21 1 – 21

34. IP Routing Foundations
Assessing this topology
Assessing this topology
Some of the problems with this topology include:
• Inefficient forwarding paths and potential bottleneck
– Traffic between Router1 and Router3 has to go through Router2
• Does not provide backup paths in the event of link failure
• Does not scale well
If 3 If 4
10.0.64.2/24 10.0.65.1/24
RIP enabled RIP enabled
Loop 1
R2
10.2.0.1/24
10.2.20.0/24 10.2.40.0/24 If 3
If 3
10.0.65.2/24
10.0.64.1/24
RIP enabled
RIP enabled
Loop 1: 10.1.0.1/24 R1 Loop 1 10.3.0.1/24
R3
10.1.10.0/24 10.1.30.0/24 10.3.10.0/24 10.3.30.0/24
172.16.150.0/24
Rev 5.21 Student Guide: 1–22 17
Although this topology is useful for describing RIP operations, it is clearly not an
efficient topology. If the links between routers have equal bandwidth, Router2 may
become a bottleneck because it must handle traffic between hosts connected to
Routers 1 and 3, as well as traffic coming from or destined for its locally
connected networks.
Furthermore, this topology also does not provide any redundancy. If either of the
links between Router2 and its neighbors should fail, many hosts would be isolated.
The above deficiencies would be magnified if this intranet needed to support more
than three routers. If we continued daisy-chaining routers in this manner, the
potential for bottlenecks and traffic delay would increase dramatically. The
vulnerability of the connections would also escalate.
1 – 22 Rev. 5.21

35. IP Routing Basics
Providing a routed mesh
Providing a routed mesh
A routed mesh
• Provides a dedicated link between each pair of routers
• Provides a backup path in the event of link failure
• Does not scale well beyond 3 or 4 nodes
10.0.64.0/24 10.0.65.0/24
Loop 1
R2
10.2.0.1/24
10.2.20.0/24 10.2.40.0/24
Loop 1: 10.1.0.1/24 10.0.66.0/24 Loop 1 10.3.0.1/24
R1 R3
10.1.10.0/24 10.1.30.0/24 10.0.10.0/24 10.3.30.0/24
172.16.150.0/24
Rev 5.21 Student Guide: 1–23 18
Creating a mesh of the routers would solve the problems relating to potential
bottlenecks and lack of redundancy. In a mesh, each device is connected to all
other devices. Rather than creating a bottleneck at Router2, the topology shown in
the example provides Router3 with a direct connection to Router1. If any of the
three links should fail, the remaining links would continue to provide connectivity
among all three routers. Of course, the potential for a bottleneck would then
increase until the mesh was restored.
However, the full mesh solution is not scalable. For every node added to the mesh,
the number of point-to-point connections increases dramatically. While it only
takes three links to create a full mesh among three nodes, six links are required to
fully connect four nodes. A full mesh for five nodes requires 10 point-to-point
links.
A full mesh for 10 nodes requires 45 point-to-point links. The number of links can
be calculated using the following formula: L = N(N-1)/2’where “L” represents the
number of point-to-point links and “N” represents the number of nodes to be
interconnected. The values for 10 nodes are 10*9/2=45.
Rev. 5.21 1 – 23

36. IP Routing Foundations
Split horizon in a routed mesh
Split Horizon in a routed mesh
Each router in a full mesh:
• Advertises to neighbors all networks learned from other neighbors
• Receives advertisements for each remote network from every neighbor
• Chooses the lowest cost path to each destination network
Next hop for 10.1.x.x traffic Next hop for 10.3.x.x traffic
(Do not advertise 10.1.x.x (Do not advertise 10.3.x.x
networks) networks)
Loop 1
Next hop for 10.2.x.x R2 Next hop for 10.2.x.x
10.2.0.1/24
traffic traffic
(Do not advertise (Do not advertise
10.2.x.x networks) 10.2.20.0/24 10.2.40.0/24 10.2.x.x networks)
Loop 1: 10.1.0.1/24 R1 Loop 1: 10.3.0.1/24
R3
Next hop for Next hop for
10.3.x.x traffic 10.1.x.x traffic
(Do not advertise (Do not advertise
10.3.x.x networks) 10.1.x.x networks)
10.1.10.0/24 10.1.30.0/24 10.3.10.0/24 10.3.30.0/24
172.16.150.0/24
Rev 5.21 Student Guide: 1–24 19
In the non-redundant topology described earlier, each router receives information
about a specific address range from only one neighbor. However, in a meshed
topology, such as the one shown, each router receives updates from both
neighbors. Consequently, there is some overlap in the advertised networks.
In the example above, Router3 will receive advertisements from Router1 and
Router2. Following Split horizon rules, Router2 advertises networks 10.2.x.x with
a cost of 1 because those networks are local to Router2. It also advertises networks
10.1.x.x and 172.16.150.0/24 with a cost of 2. If the update from Router2 is the
first one Router3 hears, it will add all seven of the advertised networks to its route
table. However, when the first RIP update from the neighbor Router1 arrives,
Router3 follows a very specific procedure for evaluating the shortest or lowest-
cost path.
It is important for RIP routers to follow Split horizon rules regardless of whether
routing loops exist. Even in the non-redundant topology illustrated earlier, failure
to follow Split horizon rules can result in significant confusion for the router.
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