Is is

The IS-IS Routing Protocol

RST-208
3010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved. 2

Agenda

• IS-IS Overview • TLVs
• CLNS Addressing • Configuration
• IS-IS Levels • Design
Considerations
• IS-IS PDUs
• New Features
• LSP Header
• Deployment Scenarios
• Flooding

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IS-IS Overview

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Terminology

• AFI: Authority and Format Identifier (the first octet of all OSI NSAP
addresses—identifies format of the rest of the address)
• CLNP: Connection-Less Network Protocol (ISO 8473—the OSI
connectionless network layer protocol—very similar to IP)
• ES: End System (the OSI term for a host)
• IS: Intermediate System (the OSI term for a router)
• ES-IS: End System to Intermediate System routing exchange protocol
(ISO 9542—OSI protocol between routers and end systems)
• IS-IS: Intermediate System to Intermediate System routing exchange
protocol (the ISO protocol for routing within a single routing domain)
• IS-IS Hello: A Hello packet (defined by the IS-IS protocol)
• LSP: Link State Packet (a type of packet used by the IS-IS protocol)
• TLV: Type Length Value

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IS-IS Overview

• IS-IS was originally designed for use as
a dynamic routing protocol for the ISO
Connectionless Network Protocol (CLNP);
(ISO10589 or RFC 1142)
• Adapted for routing IP in addition to CLNP
(RFC1195) as integrated or dual IS-IS
• IS-IS is a Link State Protocol similar to the
Open Shortest Path First (OSPF)

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IS-IS Overview (Cont.)

• IS-IS is an Interior Gateway Protocol (IGP)
used for routing within an Autonomous
System (AS) also referred to as a routing
domain
• BGP is normally used dynamic routing
between IP domains
• ISO-IGRP is a Cisco proprietary routing
protocol that can be used between
CLNP domains
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• 3 network protocols play together to
deliver the ISO defined Connectionless
Network Service
CLNP
IS-IS
ES-IS—End System to Intermediate System
Protocol
• All 3 protocols independently ride
over layer 2
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• CLNP is the ISO equivalent of IP for datagram
delivery services (ISO 8473, RFC 994)
• IS-IS carries routing information; integrated
IS-IS works within the ISO CNLS framework if
even used for routing IP (ISO 8473, RFC 1142)
• ES-IS is a dynamic protocol for discovering
layer 2 adjacencies (ISO9542, RFC 995); hosts
and routers discover each other
via ES-IS

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CLNS Addressing

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CLNS Addressing

Area ID SEL

• CLNS addressing consists of 3 parts:
Area—variable
ID
SEL(ector)

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NSAPs and Addressing

• ISO/IEC 10589 distinguishes only 3 fields in the NSAP
address format

IDP DSP
AFI IDI High Order DSP System ID NSEL
Variable Length Area Address 6 Bytes 1 Byte

• Area address: Variable length field composed of high order
octets of the NSAP excluding the SystemID and SEL fields
• SystemID: Defines an ES or IS in an area; Cisco implements
a fixed length of 6 octets for the SystemID
• NSEL: Selector, also designated as N-selector; it is the last
byte of the NSAP and identifies a network service user
(transport entity or the IS network entity itself)
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NSAPs and Addressing (Cont.)

• NSAP: Network Service Access Point
• An NSAP has an address that consists
of 3 parts
 Variable length area-address
 6 Byte system ID
 Byte n-selector (indicating transport layer)
 Total length between 8 and 20 bytes
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NETs versus NSAPs

• NET: Network Entity Title
• Is the address of the network entity itself
• A NET is an NSAP where n-selector is 0
(common practice)
• A NET implies the routing layer of the IS itself
(no transport layer)
• ISs (routers) do not have any transport layer
(selector=0)
• Multiple NETs are like secondary IP addresses;
only use them when merging or splitting areas
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CLNS Addressing: NSAP Examples

• Example 1:
47.0001.aaaa.bbbb.cccc.00
Area = 47.0001, SysID = aaaa.bbbb.cccc, NSel = 00
• Example 2:
39.0f01.0002.0000.0c00.1111.00
Area = 39.0f01.0002, SysID = 0000.0c00.1111, NSel = 00
• Example 3:
49.0002.0000.0000.0007.00
Area = 49.0002, SysID = 0000.0000.0007, Nsel = 00

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CLNS Addressing: NSAP Examples (Cont.)

39.0f01.0003.6666.6666.6666.00
39.0f01.0002.4444.4444.4444.00

39.0f01.0002.3333.3333.3333.0
0

39.0f01.0004.7777.7777.7777.00

39.0f01.0001.2222.2222.2222.00

39.0f01.0001.1111.1111.1111.0
0
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CLNS Addressing: How Did Most
ISP’s Define System IDs?

The LOOPBACK IP Address: 192.168.3.25
The AREA the Router Under Is: 49.0001
IP Address Conversion Process to System ID:
192.168.3.25

192.168.003.025

1921.6800.3025

49.0001.1921.6800.3025

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IS-IS Levels

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Areas and Backbone Routers

• IS-IS has a 2 layer hierarchy
 The backbone (Level 2)
 The areas (Level 1)
• An IS can be
 Level 1 router (intra-area routing)
 Level 2 router (inter-area routing)
 Level 1-2 router (intra and inter-area routing)
• For each level (1 and 2) a DIS will be elected
on LANs
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Areas and Backbone Routers (Cont.)

• Level 1 router
 Has neighbors only on the same area
 Has the Level 1 LSDB with all routing information

for the area
 Use the closest Level 2 router to exit the area
 This may result in sub-optimal routing
• Level 2 router
 May have neighbors in other areas
 Has a Level 2 LSDB with all information about
inter-area routing
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• Level 1–2 router
May have neighbors on any area
Has two LSDBs:
 Level 1 for the intra-area routing
 Level 2 for the inter-area routing

If the router has adjacencies to other areas,
it will inform the Level 1 routers (intra-area)
it is a potential exit point for the area
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Area 49.001
L1

L1L2

Area 49.003
Area 49.0002

L1 L1
L1L2 L1L2

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• Backbone must be L2 contiguous
Area 3
L1 Only
L1L2

L2 Only
Area 2 L1L2

L1L2 L1 Only
Area 4

L1L2
L1 Only
Area 1

L1L2
L1 Only

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“I’m in area 2 and ALL Area 3
Area 1 my neighbors are in the Router F
Router A same area. I must be a Area 2
L1-only router ?” Router D

Area 2 Area 2
Area 2 Router E
Router B
Router C
Area 4
Router G
!! NO !!
Router C must have a full L2 LSDB

to route between areas 1, 3, and 4.
Remember, the backbone must be
contiguous.
Remember, the Backbone Must Be Contiguous:
IS-IS Router Cannot Determine If They Need to Be L1 or L1L2,
So All Routers Try to Be a L1L2 IS by Default
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SPF (Dijkstra) and Partial
Route Calculation

• SPF (Dijkstra) is run when topology
has to be calculated (SPF tree)
• PRC (Partial Route Calculation) is
executed when IP routing information
has to be calculated
• If an IS receives an LSP where only IP
information has changed, it will run
PRC only (less CPU)

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IS-IS PDUs

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IS-IS PDUs

• IS-IS packets are encapsulated directly
in a data-link frame
• There is no CLNS or IP header
 Hello PDUs (IIH, ISH, ESH)
 LSP
Non-pseudonode LSP
Pseudonode LSPs
 CSNP
 PSNP

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Encapsulation

Datalink Header
IS-IS Fixed Header
IS-IS (OSI Family
(First Byte Is 0x83) IS-IS TLVs
0xFEFE)

Datalink Header ESIS Fixed Header
ESIS (OSI Family 0xFEFE) (First Byte is 0x81)
ESIS TLVs

Datalink Header CLNS Header (with NSAPs)
CLNS (OSI Family 0xFEFE) (First Byte Is 0x80) User Data

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Mac Layer Addresses

• On LANs IS-IS PDUs are forwarded to the
following well known MAC layer broadcast
addresses

AllL1ISs 01-80-C2-00-00-14
AllL2ISs 01-80-C2-00-00-15
AllIntermediateSystems 09-00-2B-00-00-05
AllEndSystems 09-00-2B-00-00-04

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Hello PDUs

• IIHs are between routers (IS-IS)
• Exchanged by ISs to form adjacencies
 Point-to-point IIH
 Level 1 LAN IIH
 Level 2 LAN IIH
• Multipoint and P2P IIHs are padded
to full MTU Size
Useful to detect MTU inconsistencies

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Hello PDUs (Cont.)
Point-to-Point IS-IS Hello
• Circuit-type:
1—Level 1 only
2—Level 2 only (no IS-ES hello)
3—Level 1–2
• Source ID: Transmitting router’s network layer address
• Holding time: Time at which neighbors can legally declare
this route dead if they haven’t gotten a hello from it
• Packet length: The length of the entire IS-IS hello
message
• Local circuit ID: Identifier to the interface and unique
relative to the transmitting router’s other interfaces

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Hello PDUs (Cont.)
LAN IS-IS Hello
• Priority: The transmitting routers’ priority
for becoming designated router on the
LAN, with higher #s having a higher
priority
• LAN ID: The name of the LAN as assigned
by the DIS; it consists of DIS-ID + extra
octet to differentiate this LAN from others
with the same DIS

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Hello PDUs (Cont.)

ES Sends ESH

IS Send ISH for ES

IS-IS Adjacency through IIH

• ISs send IIH to establish IS-IS adjacencies
• ISs listen to ESH to discover ESs
• ISs send ISH for ESs
• Es sends ESH and listen to ISH
• ESs select IS as default router by listening to ISH

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Node and Pseudonode LSP

• 2 kinds of Link State PDUs
 Non-Pseudonodes represent routers
 Pseudonodes represents LANs
(created by the DIS)

• A Level 1 router will create a Level 1 LSP
• A Level 2 router will create a Level 2 LSP
• A Level 1–2 router will create
A Level 1 LSP and a Level 2 LSP
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Non-Pseudonode LSP Generation

• Each IS will create and flood a new
Non-Pseudonode LSP
 When a new neighbor comes up or
goes away
 When new IP prefixes are inserted
or removed
 When the metric of a link did change
 When refresh interval timer expires

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Pseudonode LSP Generation

• The DIS will create and flood a new
Pseudonode LSP
 When a new neighbor comes up or goes away
 When refresh interval timer expires
• Pseudonode LSP is created by the DIS
 One for each level (Level 1 and/or Level 2)
 One for each LAN
• Reduces adjacencies and flooding over
LAN subnets
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Pseudonode LSP Generation (Cont.)
DIS DIS

PSN

• Broadcast link represented as virtual node, referred to as
Pseudonode (PSN)
• PSN role played by the Designated Router (DIS)
• DIS election is preemptive, based on interface priority with
highest MAC address being tie breaker
• IS-IS has only one DIS; DIS helps routers on broadcast link
to synchronize their IS-IS databases
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LSPDB without Pseudonode

LSP for Router B
LSP for Router A IS: 10 A
IS: 10 B 10 C
10 C 10 D
10 D ES: 10 E
ES: 10 E

LSP for Router D
LSP for Router C IS: 10 A
IS: 10 A 10 B
10 B 10 C
10 D ES: 10 E
ES: 10 E

EndSystem E
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Pseudonode in the LSPDB

LSP for Router A
LSP for Router A IS: 10 P
IS: 10 P LSP for the
Pseudonode P
IS: 0 A
0B
0C
0D
ES: 0 E

LSP for Router A
IS: 10 P
LSP for Router A
IS: 10 P

EndSystem E
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CSNP/PSNP

• For both Level 1 and Level 2 databases,
we have CSNPs and PSNPs
 Level 1 CSNP
 Level 2 CSNP
 Level 1 PSNP
 Level 2 PSNP

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Complete Sequence Number PDU

• Describes all LSPs in your LSDB (in range)
 Contains an address range
 LSPid, seqnr, checksum, remaining lifetime
• Used at 2 occasions
 Periodic multicast by DIS (every 10 seconds)
 On p2p links when link comes up
• Created and flooded by the DIS
 Every 10 seconds
 On each LAN the IS is the DIS
• If LSDB is large, multiple CSNPs are sent
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Partial Sequence Number PDU
• PSNPs have 2 functions
 Exchanged by ISs on p2p links (ACKs)
 Acknowledge receipt of an LSP
 Request transmission of latest LSP
• PSNPs describe LSPs by its header
 LSP identifier
 Sequence number
 Remaining lifetime
 LSP checksum

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LSP Header

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LSP Header

• The LSP header contains
 LSP-id
 Sequence number
 Remaining lifetime
 Checksum
 Type of LSP (Level 1, Level 2)
 Attached bit
 Overload bit
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LSP Header (Cont.)

• LSP identifier consists of 3 parts
Source ID
System-ID of router (non-PN) or DIS
(Pseudonode)
Pseudonode ID
Zero for router LSP, non-zero for
Pseudonode LSP
LSP number
Fragmentation number
00c0.0040.1234.01-00

System ID Frag-Nr
PN-ID

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LSP Header (Cont.)

• LSP sequence number
Used to determine the newest LSP version
• LSP remaining lifetime
Used to purge old LSPs
• LSP checksum
• LSP type
Level 1 or Level 2

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LSP Header (Cont.)
LSP Attached Bit

• Set in the Level 1 LSP by a L1-L2 router if
it has connectivity to another area
• Indicate to the area routers (Level 1) that it
is a potential exit point of the area
• Level 1 routers select the closest (best
metric) Level 2 router with the ATT-bit set

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LSP Header (Cont.)
LSP overload bit
• Set by the IS when it has an overload problem
on its LSDB
Indicates that the router has an incomplete LS database, and
hence cannot be trusted to compute
any correct routes
Is used in the LSDB, but topology behind it is not calculated
Therefore other routers do not compute routes which would
require the PDU to pass through the overloaded router
Exception—ES neighbors—since these paths are guaranteed
to be non-looping

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LSP flooding

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Why do we need flooding

• All routers generate an LSP
• All LSPs need to be flooded to all routers
in the network
if LSPDB is not synchronised, routing loops or
blackholes might occur
• IS-IS’ two components are the SPF
computation and reliable flooding

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What triggers a new LSP ?

• When something changes …
Adjacency came up or went down
Interface up/down (connected IP prefix !)
Redistributed IP routes change
Inter-area IP routes change
An interface is assigned a new metric
Most other configuration changes
Periodic refresh
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What to do with a new LSP ?

• Create new LSP, install in your own
LSPDB and mark it for flooding
• Send the new LSP to all neighbors
• Neighbors flood the LSP further

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Basic flooding rules

• When receiving an LSP, compare with old
version of LSP in LSPDB
• If newer:
install it in the LSPDB
Acknowledge the LSP with a PSNP
Flood to all other neighbors
Check if need to run SPF

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Basic flooding rules

• If same age:

• If older:
Send our version of the same LSP
Wait for PSNP

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Sequence number

• Each LSP (and LSP fragment) has its own
sequence number
• When router boots, it sets seqnr to one
• When there is a change, the seqnr is
incremented, a new version of the LSP is
generated with the new seqnr
• Higher seqnr means newer LSP

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Remaining lifetime

• Used to age out old LSPs
• Periodic refresh needed to keep stable
LSPs valid
• IS-IS counts down from 1200 sec to 0
we allows to start at 65535 sec (18.7h)
• When lifetime expires, the LSP is purged
from the network
Header with lifetime = 0 is flooded

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Flooding on a P2P Link
LSP
id=x seqnr=22

RouterA
Receives LSP
id=x seqNr=22 RouterB
It’s new. Put it in
the LSPDB
LSP
Now flood it: id=x seqnr=22
Send over p2p. Received it. Local
copy has seqNr = 21.
So the received one is
newer. Install it in LSDB.
Received ack PSNP Acknowledge it. Maybe
id=x seqnr=22 flood further.
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The Designated Router

• DIS is like the DR in OSPF
• DIS is only on LANs, not on p2p
• DIS has two tasks
create/update pseudonode LSP
conduct flooding over the LAN
• DIS sends periodic CSNPs
LSPid, SeqNr, Checksum, Lifetime of all LSPs present in
the LSPDB

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The Designated IS

• No Backup DIS in ISIS
not necessary, no LSPDB resync
• DIS is elected by priority and MAC
actually is “self-elected”
• LAN circuitID shows who is DIS
use show clns interface

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Flooding on a LAN
Rtr-A DIS
Received new LSP
id=x seqNr=22
LAN
Install in LSPDB.
LSP Flood the LSP.
!!! Problem !!! LSP
Dropped LSP id=x seqNr=22

Local copies of LSP-y and CSNP
LSP-z are up-to-date but id=y seqnr=... Periodic CSNP
local copy of LSP-x is older. id=x seqNr=22 every 10 secs
Request latest LSP-x via id=z ...
PSNP
PSNP
id=x seqNr=21 Neighbor has an
old LSP, better
resend him latest
Got it. Install and LSP
run SPF id=x seqNr=22
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TLVs

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Level 1 TLVs

TLV Name Type Origin

Area Address 1 ISO 10589
Intermediate System Neighbors 2 ISO 10589
End System Neighbors 3 ISO 10589

Authentication Information 10 ISO 10589
IP Internal Reachability Information 128 RFC 1195
Protocols Supported 129 RFC 1195
IP Interface Address 132 RFC 1195

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Level 2 TLVs

TLV Name Type Origin
Area Address 1 ISO 10589
Intermediate System Neighbors 2 ISO 10589
Partition Designated Level 2 IS 4 ISO 10589
Prefix Neighbors 5 ISO 10589
Authentication Information 10 ISO 10589
IP Internal Reachability Information 128 RFC 1195
Protocols Supported 129 RFC 1195
IP External Reachability Information 130 RFC 1195
Inter-Domain Routing Protocol Information 131 RFC 1195
IP Interface Address 132 RFC 1195

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New TLVs
TLV Name Type Comments

Used in Place of TLV 2 for
Extended IS Reachability Information 22
Traffic Engineering (TE)

Router-Id 134 TE Extension to IS-IS

TE Extension to IS-IS,
Extended IP Reachability Information
135 Used in Place of TLV 128
or 130

For Dynamic Distribution
Dynamic Hostname Information 137 of Hostname to NET Mapping
via LSP Flooding

Reliable Point-to-Point
Point-to-Point Adjacency State 240
Adjacency Formation

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Old IS-IS Metrics

• ISO 10589 specifies 4 types of metric
Default—supported by all routers
Delay—measures transit delay
Expense—measures the monetary cost of link
utilization
Error—measures error probability
• Default metric type must be supported by all
implementations
• Other types specified for QoS routing are not
available most implementation
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Old IS-IS Metrics (Cont.)

Byte(s)
0 I/E Default Metric (6 bits) 1

• Maximum LINK_METRIC per interface is 63
• Maximum PATH_METRIC is 1023
• There is no automatic interpretation
based on interface bandwidth
• Cisco uses default of 10 on all
interfaces regardless of bandwidth
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New IS-IS Metrics (Wide Metrics)

• With the draft-ietf-isis-traffic-02.txt

Max Link_METRIC is 16777215 (2^24 – 1)
Max PATH_METRIC is 4261412864 (2^32 – 2^25)

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Configuration

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How to Configure?
R1 Configuration

e0
R1
!
i nt erf ac e Loopbac k0
i p addres s 172. 16. 1. 1 255. 255. 255. 255
!
i nt erf ac e Et hernet 0 e0
i p addres s 172. 16. 12. 1 255. 255. 255. 0 R2
i p rout er i s i s s0
!
rout er i s i s
pas s i ve- i nt erf ac e Loopbac k0
net 49. 0001. 1720. 1600. 1001. 00
! s0
R3

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How to Configure? (Cont.)
R2 Configuration

e0
! R1
i nt erf ac e Loopbac k0
i p addres s 172. 16. 2. 2 255. 255. 255. 255
!
i nt erf ac e Et hernet 0
i p addres s 172. 16. 12. 2 255. 255. 255. 0
i p rout er i s i s e0
R2
!
i nt erf ac e Seri al 0 s0
i p addres s 172. 16. 23. 1 255. 255. 255. 252
i p rout er i s i s
!
rout er i s i s
pas s i ve- i nt erf ac e Loopbac k0 s0
net 49. 0001. 1720. 1600. 2002. 00 R3
!

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Looking at the Show Commands
R1#show cl ns nei ghbor
Syst em I d I nt er f ace SNPA St at e Hol dt i me Type Pr ot ocol
R2 Et 0 0000. 0c47. b947 Up 24 L1L2 I S- I S

R1#show cl ns i nt er f ace et her net 0
Et her net 0 i s up, l i ne pr ot ocol i s up
Checksum enabl ed, M
s TU 1497, Encapsul at i on SAP
Rout i ng Pr ot ocol : I S- I S
Ci r cui t Type: l evel - 1- 2
I nt er f ace number 0x0, l ocal ci r cui t I D 0x1
Level - 1 M r i c: 10, Pr i or i t y: 64, Ci r cui t I D: R2. 01
et
Num ber of act i ve l evel - 1 adj acenci es: 1
Level - 2 M r i c: 10, Pr i or i t y: 64, Ci r cui t I D: R2. 01
et
Num ber of act i ve l evel - 2 adj acenci es: 1
Next I S- I S LAN Level - 1 Hel l o i n 5 seconds
Next I S- I S LAN Level - 2 Hel l o i n 1 seconds
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Looking into the Database
R2#show cl ns nei ghbor
Syst em I d I nt er f ace SNPA St at e Hol dt i me Type Pr ot ocol
R1 Et 0 0000. 0c09. 9f ea Up 24 L1L2 I S- I S
R3 Se0 * HDLC* Up 28 L1L2 I S- I S

R2#show i si s dat abase
I S- I S Level - 1 Li nk St at e Dat abase:
LSPI D LSP Seq Num LSP Checksum LSP Hol dt i me ATT/ P/ OL
R1. 00- 00 0x0000008B 0x6843 55 0/ 0/ 0
R2. 00- 00 * 0x00000083 0x276E 77 0/ 0/ 0
R2. 01- 00 * 0x00000004 0x34E1 57 0/ 0/ 0
R3. 00- 00 0x00000086 0xF30E 84 0/ 0/ 0
I S- I S Level - 2 Li nk St at e Dat abase:
LSPI D LSP Seq Num LSP Checksum LSP Hol dt i me ATT/ P/ OL
R1. 00- 00 0x00000092 0x34B2 41 0/ 0/ 0
R2. 00- 00 * 0x0000008A 0x7A59 115 0/ 0/ 0
R2. 01- 00 * 0x00000004 0xC3DA 50 0/ 0/ 0
R3. 00- 00 0x0000008F 0x0766 112 0/ 0/ 0
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Looking into the Database Detail
R2#show i si s dat abase R2. 00- 00 det ai l
I S- I S Level - 1 LSP R2. 00- 00
LSPI D LSP Seq Num LSP Checksum LSP Hol dt i m e ATT/ P/ OL
R2. 00- 00 * 0x00000093 0x077E 71 0/ 0/ 0
Ar ea Addr ess: 49. 0001
NLPI D: 0xCC
Host nam e: R2
I P Addr ess: 172. 16. 2. 2
M r i c: 10
et I P 172. 16. 12. 0 255. 255. 255. 0
M r i c: 0
et I P 172. 16. 2. 2 255. 255. 255. 255
M r i c: 10
et I P 172. 16. 23. 0 255. 255. 255. 252
M r i c: 10
et I S R2. 01
M r i c: 10
et I S R3. 00
I S- I S Level - 2 LSP R2. 00- 00
LSPI D LSP Seq Num LSP Checksum LSP Hol dt i m e ATT/ P/ OL
R2. 00- 00 * 0x0000009A 0x5A69 103 0/ 0/ 0
Ar ea Addr ess: 49. 0001
NLPI D: 0xCC
Host nam e: R2
I P Addr ess: 172. 16. 2. 2
M r i c: 10
et I S R2. 01
M r i c: 10
et I S R3. 00
M r i c: 10
et I P 172. 16. 23. 0 255. 255. 255. 252
M r i c: 10
et I P 172. 16. 1. 1 255. 255. 255. 255
M r i c: 10
et I P 172. 16. 3. 3 255. 255. 255. 255
M r i c: 0
et I P 172. 16. 2. 2 255. 255. 255. 255
M r i c: 10
et I P 172. 16. 12. 0 255. 255. 255. 0

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Looking into the Routing-Table

R1#show i p r out e i si s
i L1 172. 16. 2. 2/ 32 [ 115/ 10] vi a 172. 16. 12. 2, Et her net 0
i L1 172. 16. 3. 3/ 32 [ 115/ 20] vi a 172. 16. 12. 2, Et her net 0

R2#show i p r out e i si s
i L1 172. 16. 1. 1/ 32 [ 115/ 10] vi a 172. 16. 12. 1, Et her net 0
i L1 172. 16. 3. 3/ 32 [ 115/ 10] vi a 172. 16. 23. 2, Ser i al 0

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Show IS-IS SPF-Log

R1#show i si s spf - l og
Level 1 SPF l og
When Dur at i on Nodes Count Fi r st t r i gger LSP Tr i gger s
04: 07: 42 12 5 1 PERI ODI C
03: 52: 41 12 5 1 PERI ODI C
03: 37: 40 12 5 1 PERI ODI C
00: 37: 31 12 5 1 PERI ODI C
00: 22: 31 21 5 1 PERI ODI C
00: 07: 30 19 5 1 PERI ODI C

RST-208

Show IS-IS LSP Log

R1#show i si s l sp- l og
Level 1 LSP l og
When Count I nt er f ace Tr i gger s
5d05h 1 Ser i al 1 DELADJ
5d05h 1 ATTACHFLAG
5d04h 2 Et her net 0 NEWADJ DI S
5d04h 3 Et her net 0 CONFI G DELADJ DELADJ
5d04h 1 Ser i al 1 NEWADJ
00: 23: 10 1 Loopback0 CONFI G

RST-208

Areas and levels

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Hierarchy

• IS-IS has 2 layers of hierarchy
the backbone is called level-2
areas are called level-1
• Same algorithms apply for L1 and L2
• A router can take part in L1 and L2
inter-area routing (or inter-level routing)

RST-208

Level-1 Routers

• Neighbors only in the same area
• L1 has information about own area
• L1-only routers look at the attached-bit in
L1 LSPs to find the closest L1L2 router
• L1-only routers install a default route to
the closest L1L2 router in the area

RST-208

Level-2 routers

• May have neighbors in other areas
• L2 has information about L2 topology
• L2 has info on what L1 destinations are
reachable and how to reach them via the L2
topology
• L2 routers often also do L1 routing
so called L1L2 routers

RST-208

Adjacency levels

L1-Adjacency L2-Adjacency

Router with adjacencies within
the same area.

However, needs to have a L2
database as well since it is a transit
node

Therefore L1L2 adjacency is required

L2-Adjacency
L2-Adjacency

L1L2 L1L2
Adjacency Adjacency

RST-208

Level-1, Level-2 & Level-1-2 Routers
• Backbone MUST BE L2 contiguous
L1-only

L2-only

L1-L2

L1-only

L1-only

L1-L2
L1-L2

L1-only
This router has to behave as level-2
as well in order to guarantee backbone
L1-L2 continuity

L1-only

RST-208

Level-1, Level-2 & Level-1-2 Routers
• Backbone MUST BE L2 contiguous
L1-only

L2-only

L1-L2

L1-only

L1-L2

L1-L2
L1-L2

L1-only
This router has to behave as level-2
as well in order to guarantee backbone
L1-L2 continuity

L1-only

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Design Considerations

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Design guidelines
Overload-bit

• 10589 defines for each LSP a special bit
called the LSPDB Overload Bit
• While having problems, a router could set
the OL bit, and other routers would route
around it
• Connected IP prefixes still reachable

RST-208

Design guidelines
Overload-bit

• With IS-IS you can manually set the
overload bit in the router’s LSP
• This router will therefore never be used for
transit during the path calculation, but it is
still reachable

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Design guidelines
Overload-bit
R1 R2

When R1 computes SPT, he will find R5
that R5 LSP has Overload-bit set. R5-LSP Overload-bit R3
Therefore R5 cannot be used as transit Neighbors: R1, R4
node and shortest path to R4 is:
R1->R2->R3->R4

• Why/When use Overload-Bit ? R4

When the router is not ready to forward
traffic for ALL destinations
Typically when ISIS is up but BGP not yet
When the router has other functions (Network Management)

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Design guidelines
Overload-bit

• BGP will typically converge much slower than
the IGP (a few minutes)
• During this time, other routers in the AS will use
this new router for transit
• But if the new router does not have all BGP
routes yet, it will drop traffic
• New router should first converge BGP before
carrying traffic

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Design guidelines
Overload-bit

• IS-IS can set the OL bit after each reboot,
and allow BGP to converge before it
advertises itself as transit by unsetting the
OL bit
• Network admin needs to specify how long
IS-IS should wait for BGP to converge
typically 2 to 5 minutes

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Design guidelines
Overload-bit

• BGP can tell IS-IS to unset the Overload-
bit immediately
• Default BGP update delay is 2 min
• When BGP never informs ISIS, the
Overload-bit will be cleared after 10
minutes

RST-208

Design guidelines
Overload-bit

• Overload-bit on-startup recommended in
MPLS networks
• During boot-up a router may have all IGP
routes but not all labels
• During this time it’s better not to use the
router as a transit point
router isis
set-overload-bit on-startup 120

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Set over Load Bit (Cont.)

router isis
set-overload-bit
set-overload-bit on-startup <sec>
set-overload-bit on-startup wait-
for-bgp

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Set over Load Bit (Cont.)

• Enhanced configuration:
Router IS-IS
set-overload-bit [ on-startup [ <timeout> | wait-for-bgp] ]

• keyword “wait-for-bgp”
• When BGP doesn’t inform IS-IS it is ready
and “wait-for-bgp” is configured, the over
Load Bit will be cleared after 10 minutes

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Database Timers

Timer Default Value Cisco IOS Command

Maxage 1200s IS-IS Max-lSP-Interval

LSP Refresh Interval 900s IS-IS Refresh-Interval

LSP Transmission Interval 33ms IS-IS lSP-Interval

LSP Retransmit Interval 5s IS-IS Retransmit-Interval
CSNP Interval 10s IS-IS CSNP-Interval

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IP routing specifics

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Dynamic Host Name

• All ISPs configure STATIC mappings of
system-IDs
• This process has dis-adv of maintaining
huge (identical) databases on all the routers
• Adding a router to the network, means
updating this static mappings on all
the routers

RST-208

Dynamic Host Name (Cont.)

• TLV 137
• RFC 2763
• Floods the host names dynamically
• Show isis topology shows the NSAPs
getting dynamically mapped to the
hostname

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L1 advertised into L2

• All L1L2 routers advertise all the IP
prefixes they learn via L1 into L2
• Only advertise routes you use
• Summarization possible
At L1->L2 or when redistributing

RST-208

Route Leaking

• ISIS feature/capability described in
draft-ietf-isis-domain-wide
• Allows L1L2 routers to insert in their L1 LSP
IP prefixes learned from L2 database if also
present in the routing table
• ISIS areas are not stubby anymore

RST-208

Route Leaking
1. Level-1 LSP with
IP prefix:
L1L2
L1L2 10.14.0.0/16

L1
L1L2

2. Level-2 LSP with
IP prefix:
10.14.0.0/16

L1L2 L1L2
3. Level-1 LSP with
IP prefix:
10.14.0.0/16
Up/Down-bit set

L1
L1

3. At this point prefix
4. At this point prefix 10.14.0.0/16 will be inserted
10.14.0.0/16 will NOT be in L1 LSP since route leaking
inserted in L2 LSP since is configured AND the prefix is
it has the Down-bit set present in the routing table as
a L2 route
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Route Leaking
3. Level-2 LSP with
IP prefix: 10.1.0.0/16
L1L2
L1L2
4. Level-2 LSP with L1
IP prefix: 10.1.0.0/16
L1L2
3. Level-1 LSP with
IP prefix: 10.1.0.0/16
2. Level-2 LSP with Up/Down-Bit set
IP prefix: 10.1.0.0/16 2. Level-2 LSP with
IP prefix: 10.1.0.0/16

L1L2 L1L2

5. At this point the prefix
10.1.0.0/16 will NOT be inserted
in the L1 LSP since a L1 route is
preferred in the routing table
L1
1. Level-1 LSP with L1
IP prefix: 10.1.0.0/16

RST-208

Route Leaking

• For IP only
• Prefixes MUST be present in the routing
table as ISIS level-2 routes
Otherwise no leaking occurs
Same criteria than L1 to L2
Inter-area routing is done through the routing
table

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Route Leaking

• Solution for several issues:
• optimal inter-area routing
• BGP shortest path to AS exit point
• MPLS-VPN

RST-208

Route Leaking

• When leaking routes from L2 backbone into
L1 areas a loop protection mechanism need
to be used in order to prevent leaked routes
to be re-injected into the backbone

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Route Leaking

• UP/Down bit
Extended IP Reachability TLV (135) contains Up/Down bit
Described in draft-ietf-isis-traffic

• UP/Down bit is set each time a prefix is
leaked into a lower level
• Prefixes with Up/Down bit set are NEVER
propagated to a upper level

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Route Leaking

• Recommendation:
use wide Metric TLV (TLV 135)
• Configured with:
Router isis
metric-style wide

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Route Leaking

• Route leaking is implemented in 12.1
Cisco IOS 12.1 command
redistribute isis ip level-2 into level-1 distribute-list <100-199>

RST-208

Summarization is possible …..

• From L1 areas into the L2 backbone,
• From L2 leaking down into L1 areas,
• When redistributing into L2 or L1
router isis
summary address 192.1.0.0 255.255.0.0

RST-208

RST-208

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