ccna

1. © 2008 Cisco Systems, Inc. All rights reserved. Cisco Public
BRK-135T
CCNA Switching 1
CCNA Switching
Snezhy Neshkova, CCIE # 11931
Technical Manager
US Academy Conference 2008

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Agenda
ƒ
High Availability Campus Design Overview
ƒ
STP Operations
ƒ
STP Variances
ƒ
Rapid PVST+ Configuration
ƒ
Troubleshooting Spanning Tree Protocol

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Spanning Tree Protocol

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High Availability Campus Design
ƒ
Modularity, structure, and hierarchy
ƒ
Network redundancy and network protocols
Link redundancy (trunks, channels and fiber)
Redundancy in the distribution block
Redundancy and routing design
ƒ
Resilient campusnetwork designSiSiSiSi
Intranet
Internet

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Data Center
WAN
Internet
Si Si Si Si Si Si
Si Si
Si Si
Si Si
Si Si
Access
Core
Distribution
Distribution
Access
High Availability Campus Design Structure, Modularity, and Hierarchy

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Hierarchical Campus Network
Server Farm
WAN
Internet
PSTN
Si
Si
Si Si
Si Si Si
Si
Si Si Si
Si
Not This !!
Structure, Modularity and Hierarchy

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SiSiSiSiSiSiSiSiSiSiSiSi
Hierarchical Network Design
Building Block
Access
Distribution
Core
Distribution
Access•
Offers hierarchy—each layer has specific role
•
Modular topology—building blocks
•
Easy to grow, understand, and troubleshoot
•
Creates small fault domains— Clear demarcations and isolation
•
Promotes load balancing and redundancy
•
Promotes deterministic traffic patterns
•
Incorporates balance of both Layer 2 and Layer 3 technology, leveraging the strength of both•
Utilizes Layer 3 Routing for load balancing, fast convergence, scalability, and control
Without a Rock Solid Foundation the Rest Doesn’t Matter

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Access Layer Feature Rich Environment
ƒ
It’s not just about connectivity
ƒ
Layer 2/Layer 3 feature rich environment; convergence, HA, security, QoS, IP multicast, etc.
ƒ
Intelligent network services: QoS, trust boundary, broadcast suppression, IGMP snooping
ƒ
Intelligent network services: PVST+, Rapid PVST+, EIGRP, OSPF, DTP, PAgP/LACP, UDLD, FlexLink, etc
ƒ
Cisco Catalyst integrated security features IBNS (802.1x), (CISF): port security, DHCP snooping, DAI, IPSG, etc.
ƒ
Automatic phone discovery, conditional trust boundary, power over Ethernet, auxiliary VLAN, etc.
ƒ
Spanning tree toolkit: Portfast, UplinkFast, BackboneFast, LoopGuard, BPDUGuard, BPDUFilter, RootGuard, etc.
Access
Distribution
CoreSiSiSiSiSiSiSiSi

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SiSiSiSi
Distribution Layer Policy, Convergence, QoS, and High Availability
ƒ
Availability, load balancing, QoS and provisioning are the important considerations at this layer
ƒ
Aggregates wiring closets (access layer) and uplinks to core
ƒ
Protects core from high density peering and problems in access layer
ƒ
Route summarization, fast convergence, redundant pathload sharing
ƒ
HSRP or GLBP to provide first hop redundancySiSiSiSi
Access
Distribution
Core

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SiSiSiSi
Core Layer Scalability, High Availability, and Fast Convergence
ƒ
Backbone for thenetwork -connects network building blocks
ƒ
Performance and stability vs. complexity -less is more in the core
ƒ
Aggregation point for distribution layer
ƒ
Separate core layer helpsin scalability duringfuture growth
ƒ
Keep the design technology- independentSiSiSiSi
Access
Distribution
Core

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Foundation Services
ƒ
Layer 3 routing protocols
ƒ
Layer 2 redundancy—spanning tree
PVST+ - STP (802.1D-1998)
Rapid PVST+ - RSTP (802.D-2004)
ƒ
Trunking protocols—(isl/.1q)
ƒ
Unidirectional link detection
ƒ
Load balancing
Etherchannel link aggregation
CEF equal cost load balancing
ƒ
First hop redundancy protocols
VRRP, HSRP, and GLBP
Spanning
Tree
Routing
HSRP
GLBP
Trunking
LoadBalancing

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Best Practices— Spanning Configuration
ƒ
Onlywhen you have to!
ƒ
More common in thedata center
ƒ
Required when a VLAN spans access layer switches
ƒ
Required to protect against ‘user side’loops
ƒ
Use Rapid PVST+ for best convergence
ƒ
Take advantage of the Spanning Tree Toolkit
Data Center
WAN
Internet
Layer 3 Equal Cost Links
Layer 3 Equal Cost Links
Layer2 Loops
Same VLAN
Same VLAN
Same VLAN
Si Si Si Si Si Si
Si Si
Si Si
Si Si Si Si

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Multilayer Network Design
ƒ
Each access switch hasunique VLANs
ƒ
No layer 2 loops
ƒ
Layer 3 link between distribution
ƒ
No blocked links
ƒ
At least some VLANs span multiple access switches
ƒ
Layer 2 loops
ƒ
Layer 2 and 3 running over link between distribution
ƒ
Blocked linksSiSiSiSiSiSiSiSi
Vlan 10
Vlan 20
Vlan 30
Vlan 30
Vlan 30
Vlan 30
Layer 2 Access with Layer 3 Distribution

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Routing to the Edge Layer 3 Distribution with Layer 3 Access
ƒ
Move the Layer 2/3 demarcation to the network edge
ƒ
Upstream convergence times triggered by hardware detection of light lost from upstream neighbor
ƒ
Beneficial for the right environment
10.1.20.0
10.1.120.0
VLAN 20 Data
VLAN 120 Voice
VLAN 40 Data
VLAN 140 Voice
10.1.40.0
10.1.140.0
EIGRP/OSPF
EIGRP/OSPF
GLBP ModelSiSiSiSi
Layer 3
Layer 2
Layer 3
Layer 2
EIGRP/OSPF
EIGRP/OSPF

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STP Operations
STP Convergence Steps
Step 1: Elect a Root Bridge
The switch with the lowest Bridge ID wins; the standard Bridge ID is 2-byte priority followed by a MAC address unique for the switch
Step 2: Elect the Root Ports
The one port on the each switch with the least cost path back to the root
Step 3: Elect the Designated and Non-Designated Ports
When multiple switches connect to the same segment; this is the switch that forwards the least cost Hello onto the segment
Three step process STP uses to create a loop-free topology

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STP Operations STP Port States and BPDU Timers in the Operation of STP
Port States
Processes
Blocking
Listening
Learning
Forwarding
Disable
Receives and process BPDUs
9
9
9
9
8
Forward data frames received on interface
8
8
8
9
8
Forward data frames switched from another interface
8
8
8
9
8
Learn MAC address
8
8
9
9
8
1Return to blocking if not lowest cost path to root bridge
BPDU Timers
Hello Time
ƒ
The hello time is the time between each BPDU frame that is sent on a port
ƒ
This is equal to 2 seconds by default, but can be tuned to be between 1 and 10 seconds
Forward Delay
ƒ
The forward delay is the time spent in the listening and learning state
ƒ
This is by default equal to 15 seconds for each state, but can be tuned to be between 4 and 30 seconds
Maximum Age
ƒ
The max age timer controls the maximum length of time a switch port saves configuration BPDU information
ƒ
This is 20 seconds by default, but can be tuned to be between 6 and 40 seconds

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A Root
C Peer
D Peer
1
2
2
1
2
2
1
RP
DP
DP
RP
DP
DP
RP
NDP
1
Core
Distribution
B Peer
32768:000000000002
8192:000000000001
32768:000000000004
32768:000000000003
Root Ports: Port with Least Cost Path to the Root Bridge
Nondesignated Ports:
Ports in Blocking
Designated Ports: Ports Selected for Forwarding
Direction of BPDU Flow
Layer 2 Spanning Tree

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Layer 2 Hardening
ƒ
Place the Root where you want it
Root Primary/Secondary Macro
HSRP/GLBP
ƒ
The root bridge should stay where you put it
Rootguard
Loopguard
UplinkFast
UDLD
ƒ
Only end station traffic should be
seen on an edge port
BPDU Guard
Root Guard
PortFast
Port-securitySiSiSiSi
BPDU Guard or Rootguard
PortFast
Rootguard
UplinkFast
STP Root
Spanning Tree Should Behave the Way You Expect
Loopguard
Loopguard
Port-Security

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*Also Supported with MST and Rapid PVST+
Spanning Tree Features
ƒ
PortFast*:Bypass listening-learning phase for access port
ƒ
UplinkFast:Three to five seconds convergence after link failure
ƒ
BackboneFast:Cuts convergence time by Max_Age for indirect failure
ƒ
LoopGuard*:Prevents alternate or root port from becoming designated in absence of BPDUs
ƒ
RootGuard*:Prevents external switches from becoming root
ƒ
BPDUGuard*:Disable PortFast enabled port if a BPDU is received
ƒ
BPDUFilter*:Do not send or receive BPDUs on PortFast-enabled ports
Wiring Closet Switch
Distribution Switches
Root
F
F
F
F
F
X
BSiSiSiSi

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What Is Root Guard? SiSiSiSiSiSiSiSi
Root
GLBP Active
RP
DP
BP
ƒ
Root guard forces a Layer2 LAN interface to be a designated port
ƒ
If spanning-tree calculations cause a port to be selected as the root port, the port transitions to the root- inconsistent(blocked) state to prevent the customer's switch from becoming the root switch or being in the path to the root.
SW(config)# interface fa0/3
SWconfig-if)# spanning-tree rootguard

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UplinkFast
ƒ
Spanning Tree enhancement to reduce failover convergence time
ƒ
Used when recovery path is known and predictable
ƒ
Enabled on access switch
ƒ
Bypasses ‘listening’and ‘learning’stages of STP
ƒ
Reduces failover time to 2–3 seconds from 30 seconds
ƒ
Auto-populates upstream address tables (dummy mcast)
2
F
F
F
B
1
Root
F
F
1SiSiSiSi
Switch(config)# spanning-tree uplinkfast

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BackboneFast
ƒ
Spanning Tree enhancement to reduce failover convergence time
ƒ
Targeted at indirectfailures
ƒ
Enabled on all switches
ƒ
Bypasses ‘max-age’
ƒ
Reduces failover time to 30 seconds from 50 seconds
3
F
F
F
F
SiSiSiSi
F
B
Root
Switch(config)# spanning-tree backbonefast

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PortFastSiSiSiSiSiSiSiSi
Root
RP
DP
ƒUsed on Access Ports (Edge Port) that are NOT connected to other switches or hubs
ƒOptimizes the convergence by simply ignoring the Listening and Learning states of the port
ƒImmediately puts the port into Forwarding state when the port is physically working
BP
Switch(config-if)# spanning-tree portfast

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What Is BPDU Guard? SiSiSiSiSiSiSiSi
Root
ƒ
PortFast BPDU guard can prevent loops by moving PortFast-configured interfaces that receive BPDUs to errdisable,rather than running Spanning Tree across that port
ƒ
This keeps ports configured with PortFast from being incorrectly connected to another switch
Switch(config)# spanning-tree portfast bpduguard
or
Switch(config-if)# spanning-tree bpduguard enable
1w2d: %SPANTREE-2-BLOCK_BPDUGUARD: Received BPDU on port FastEthernet3/1 with BPDU Guard
enabled. Disabling port.
1w2d: %PM-4-ERR_DISABLE: bpduguard error detected on Fa3/1, putting Fa3/1 in err-disable state

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Link Redundancy—UDLD Aggressive Mode UDLD
ƒ
Default UDLD timers (15 second hellos) are intended to take down link prior to 802.1d spanning tree listening/learning transition
ƒ
Aggressive mode—after aging on a previously bi-directional link—tries eight times (once per second) to reestablish connection then err- disables port
ƒ
Aggressive mode protects against
One side of a link has a port stuck (both Tx and Rx)
One side of a link remains up while the other side of the link has gone down
! Global configuration
udld aggressive
udld message time 7
! Interface
interface GigabitEthernet8/1
udld port aggressiveSiSiSiSi
Tx
Tx
Rx
Rx

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Spanning Tree Verification
3550# show spanning-tree summary
Switch is in pvst mode
Root bridge for: VLAN0001
EtherChannel misconfig guard is enabled
Extended system ID is enabled
Portfast is disabled by default
PortFast BPDU Guard is disabled by default
Portfast BPDU Filter is disabled by default
Loopguard is disabled by default
UplinkFast is disabled
BackboneFast is disabled
Pathcost method used is short
Name Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
VLAN0001 0 0 0 2 2
VLAN0005 0 0 0 2 2
---------------------- -------- --------- -------- ---------- ----------
2 vlans 0 0 0 4 4
3550# show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 0005.ddc5.8300
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24577 (priority 24576 sys-id-ext 1)
Address 0005.ddc5.8300
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- ------------------------
Gi0/12 Desg FWD 19 128.12 P2p
Po1 Desg FWD 3 128.65 P2p
(etc, for additional vlans)

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Spanning Tree Best Practice “How Can I Have a Spanning Tree Loop? I Don’t Have Spanning Tree Enabled?”
Cisco Recommends Leaving STP-Enabled for the
Following Reasons:
ƒ
If there is a loop (induced by mispatching, bad cable, and so on), STP will prevent detrimental effects to the network caused by multicast and broadcast data
ƒ
Protection against an EtherChannelbreaking down
ƒ
Most networks are configured with STP, giving it maximum field exposure; more exposure generally equates to stable code
ƒ
Protection against dual attached NICs misbehaving (or bridging enabled on servers)
ƒ
The software for many protocols (such as PAgP, IGMP snooping, and trunking) is closely related to STP; running without STP may lead to undesirable results

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Spanning Tree Standards and Features Spanning Tree Toolkit, 802.1D, 802.1s, 802.1w
ƒ
802.1D/1998:Legacy standard for bridging and Spanning Tree (STP)
ƒ
802.1D/2004:Updated bridging and STP standard; includes 802.1s, 802.1t, and 802.1w
ƒ
802.1s:Multiple Spanning Tree Protocol (MSTP)—maps multiple VLANs into the same Spanning Tree instance
ƒ
802.1t:MAC address reduction/extended system ID—moves some BPDU bits to high-numbered VLANs from the priority field, which constrains the possible values for bridge priority; unique “MAC”per chassis not port
ƒ
802.1w:Rapid Spanning Tree Protocol (RSTP)—improved convergence over 1998 STP by adding roles to ports and enhancing BPDU exchanges
ƒ
Cisco Features:Per VLAN Spanning Tree (PVST), PVST+, UpLinkFast, BackboneFast, BPDU Guard, RootGuard, LoopGuard, UDLD
A
B
STOP

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STP Variance
DEC STP pre-IEEE
802.1t—802.1d maintenance
IEEE
802.1w—Rapid STP (RSTP)
802.1D—Classic STP
802.1s—Multiple STP (MST)

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Cisco Spanning-Tree toolkit
PortFast Lets the access port bypass the listening and learning phases
Root Guard Prevents external switches from becoming the root
STP enhancements to 802.1(d,s,w)
BackboneFast Cuts convergence time by MaxAge for indirect failure
UplinkFast Provides 3-to-5 second convergence after link failure
Loop Guard Prevents the alternate or root port from being elected unless Bridge Protocol Data Units (BPDUs) are present
BPDU Guard Disables a PortFast-enabled port if a BPDU is received
BPDU Filter Prevents sending/receiving BPDUs on PortFast-enabled ports

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STP Variance
Per-VLAN Spanning Tree Plus (PVST+) Provides a separate 802.1D spanning tree instance for each VLAN configured in the network. This includes PortFast, UplinkFast, BackboneFast, BPDU Guard, BPDU Filter, Root Guard, and Loop Guard.
MST Provides up to 16 instances of RSTP (802.1w) and combines many VLANS with the same physical and logical topology into a common RSTP instance. This includes, PortFast, BPDU Guard, BPDU Filter, Root Guard, and Loop Guard.
Cisco Proprietary
Rapid PVST+ Provides an instance of RSTP (802.1w) per VLAN. This includes PortFast, BPDU Guard, BPDU Filter, Root Guard, and Loop Guard.

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05101520253035PVST+Rapid PVST+ UpstreamDownstream
Optimizing L2 Convergence
ƒ
Rapid-PVST+ greatly improves the restoration times for any VLAN that requires a topology convergence due to link UP
ƒ
Rapid-PVST+ also greatly improves convergence time over Backbone fast for any indirect link failures
ƒ
PVST+ (802.1d)
Traditional Spanning Tree Implementation
ƒ
Rapid PVST+ (802.1w)
Scales to large size (~10,000 logical ports)
Easy to implement, proven, scales
ƒ
MST (802.1s)
Permits very large scale STP implementations (~30,000 logical ports)
Not as flexible as Rapid PVST+
Time to Restore Data Flows (sec)
PVST+, Rapid PVST+ or MST

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RAPID SPANNING TREE
802.1w

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RSTP: New Concepts
ƒ
New Port Roles and States
ƒ
Modified BPDU
ƒ
Proposal/Agreement messages between bridges
ƒ
BPDU handling
ƒ
New topology change mechanism
ƒ
PVST+/802.1D Compatibility

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Rapid PVST+
ƒ
Rapid PVST+ is the IEEE 802.1w (RSTP) standard implemented per VLAN.
ƒ
Each Rapid PVST+ instance on a VLAN has a single root switch. You can enable and disable STP on a per- VLAN basis when you are running Rapid PVST+.
ƒ
Rapid PVST+ uses point-to-point wiring to provide rapid convergence of the spanning tree.
ƒ
The spanning tree reconfiguration can occur in less than 1 second with Rapid PVST+ (in contrast to 50 seconds with the default settings in the 802.1D STP).

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Rapid PVST+ defines 4 port roles:
ƒ
Root port -If Rapid PVST+ selects a new root port, it blocks the old root port and immediately transitions the new root port to the forwarding state.
ƒ
Designated port
ƒ
Alternate port
ƒ
Backup port
Port state (blocking, forwarding, learning) is independent from the role
Rapid PVST+ roles (1)
Both blocking

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Rapid PVST+ roles (2)
Root
A
B
R
R
Root Port
R
Root Port:
Port receiving the best BPDU for the bridge – shortest path to the Root in terms of path cost

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Rapid PVST+ roles (3)
Root
A
B
R
D
D
D
R
Root Port
R
Designated Port
D
Designated Port:
Port sending the best BPDU on a segment

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Rapid PVST+ roles (4)
Root
A
B
R
D
D
D
R
Root Port
R
Designated Port
D
Alternate Port
Alternate Port:
Port blocked by BPDUs from a different bridge – redundant path to the Root

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Rapid PVST+ roles (5)
Root
A
B
R
D
D
D
R
Root Port
R
Designated Port
D
Backup Port
Alternate Port
Backup Port:
Port blocked by BPDUs sent from the same bridge – redundant path to a segment

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Modified BPDU
ƒ
Protocol version now 2 (was 0)
ƒ
No more distinct TCN BPDU
ƒ
Flag field changes
•
802.1D bridges drop RSTP BPDUs

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Modified BPDU (2)
ƒ
BPDUs now act as keepalives:
– All bridges send BPDUs every hello time (PVST+ used to relay BPDUs from the root)
– Port information invalidated in 3 x hello time max Ù3 BPDUs lost
– No more Max Age or Message Age fields – a hop count is used instead inside a region
ÖFaster failure detection

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Agreement/Proposal
ƒ
Explicit handshake mechanism between bridges
ƒ
Upon link up event, bridge sends a proposal to become designated for that segment (designated bridge is the one leading to the root)
ƒ
Response is an agreementif remote bridge selects the port on which it received the proposal as its root port
ƒ
As soon as agreement is received, port moves to forwarding

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Agreement/Proposal
A
B
C
B
Root
ƒ
Ahas a better priority than B
ƒ
Asends a proposal to Bto become designated
ƒ
Bcompares the received priority and replies with an agreement –B’s port becomes Root Port
•
The same process is repeated when a new bridge is inserted
Prop
Agree

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Rapid Transition
ƒFor a rapid transition to occur
• Make sure you properly identify:
» Edge ports (enabled via Portfast)
» Point-to-point links (derived from duplex mode by default)
ƒIt is very important to check duplex settings between two bridges –else, no Proposal/Agreement mechanism !

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Protocol Migration (1)
Mix of 802.1D and 802.1w Bridges:
ƒ
1D bridges drop 1w BPDUs Ö1D Bridges always end up sending BPDUs
•
After Migration Delay (3s), RSTP bridge starts sending 802.1D BPDU if it detects a legacy bridge (on a per-port basis).
A
(1w)
B
(1w)
C
(1D)
RSTP BPDU
STP BPDU

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Protocol Migration(2)
•
Cannot detect if 802.1D bridge is removed if it is not designated (otherwise it would send BPDUs).
•
If C goes away, force protocol migration manually (set spantree mst x/y redetect- neighbor)
•
1D-1w interoperability = slow convergence: Ötry to avoid it.
A
(1w)
B
(1w)
C
(1D)
STP BPDU

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Configuring Rapid-PVST+
ƒ
RSTP fast convergence on a per-VLAN basis
ƒ
Each VLAN runs its own RSTP instance and sends its own RSTP BPDU, just like PVST+
CiscoIOS# configure terminal
CiscoIOS(config)# spanning-tree mode rapid-pvst+

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Configuring Rapid-PVST+ in Distribution
Dist-2
Dist-1
FWD: VLANs 6-10 BLK: VLANs 1-5
Trunk
FWD: VLANs 1-5 BLK: VLANs 6-10
FWD: VLANs 1-5 BLK: VLANs 6-10
Root: VLANs 1-5 Sec. Root: VLANs 6-10
Root: VLANs 6-10 Sec. Root: VLANs 1-5
FWD: VLANs 6-10 BLK: VLANs 1-5
Cisco IOS
spanning-tree extend system-id
spanning-tree mode rapid-pvst+
! Set spanning tree mode to rapid-pvst+
spanning-tree vlan 1-5 root primary
spanning-tree vlan 6-10 root secondary

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Configuring Rapid-PVST+ in Access
Trunk
FWD: VLANs 1-5 BLK: VLANs 6-10
Root: VLANs 1-5 Sec. Root: VLANs 6-10
FWD: VLANs 6-10 BLK: VLANs 1-5
FWD: VLANs 1-5 BLK: VLANs 6-10
Root: VLANs 6-10 Sec. Root: VLANs 1-5
FWD: VLANs 6-10 BLK: VLANs 1-5
Acc-1
Acc-1
CiscoIOS
spanning-tree extend system-id
! Enable MAC address reduction. This is not a requirement
spanning-tree mode rapid-pvst+
! Set spanning tree mode to rapid-pvst+

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Spanning Tree Protocol Troubleshooting Methodology
ƒ
Start now—be proactive
ƒ
Divide and conquer
ƒ
Document Spanning Tree topology
ƒ
Implement Spanning Tree enhancement features
ƒ
Develop recovery plan to include data collection for root cause analysis
Data Center
WAN
Internet
Layer 3 Equal Cost Links
Layer 3 Equal Cost Links
Layer2 Loops
Si Si Si Si Si Si
Si Si
Si Si
Si Si
Si Si

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Spanning Tree Protocol Documenting Spanning Tree Topology
Corporate Network
3/1–4
3/1–4
1/1
1/1
Gig 0/1
155.3.77.254
Gig 0/1
155.3.76.254
VLAN186
VLAN187
Root: 1,3,5,7,9,187
0001.c912.7800
Root: 2,4,6,8,186
0003.6b73.9700
F: 1,2,3,4,5,6,7,8,9
B: 4
F: 1,2,3,4,5,6,7,8,9
4/1
4/1
4/2
4/2
4/3
4/3
4/4
4/4
1/1
1/2
1/1
1/2
1/1
1/2
1/1
1/2
0001.c967.7800
DIST-01
192.168.1.1
DIST-02
192.168.1.2
Closet1
192.168.1.11
Closet2
192.168.1.12
Closet3
192.168.1.13
Closet4
192.168.1.14
0001.c9dd.7800
0001.c932.9700
0001.c949.9700
F: 1,2,3
F: 1,4,5
F: 1,6,7
F: 1,8,9
B: 1,3
B: 2
F: 1,2,3
F: 1,4,5
F: 1,6,7
F: 1,8,9
B: 1,5
B: 1,7
B: 6
B: 1,9
B: 8

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Spanning Tree Protocol STP Loop Recovery
ƒ
Do not power off switches—pull/shut redundant links
ƒ
If possible, initially disable ports that should be blocking
ƒ
Check and physically remove the connections to the ports that should be blocking
ƒ
Set up remote access to your network and call the TAC

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IOS#show spanning-tree vlan 1 brief
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 1
Address 0060.8355.7b00
Cost 23
Port 1 (GigabitEthernet1/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0007.0e8f.0880
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Designated
Name Port ID Prio Cost Sts Cost Bridge ID Port ID
-------------------------- ----------- ------ ------- -------
GigabitEthernet1/1 128.1 128 4 FWD 67 32768 0005.5f33.dc01 128.1
FastEthernet3/48 128.176 128 19 FWD 48 32768 0030.7bdd.5080 128.16
Spanning Tree Protocol Troubleshooting Commands

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IOS#show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 32768
Address 0030.7b4e.4801
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768
Address 0030.7b4e.4801
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- ------------------------
Fa2/1 Desg FWD 19 128.129 P2p Peer(STP)
Spanning Tree Protocol Troubleshooting Commands

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IOS#show spanning-tree summary
Root bridge for: VLAN0010.
Extended system ID is enabled
PortFast BPDU Guard is enabled
EtherChannel misconfiguration guard is disabled
UplinkFast is disabled
BackboneFast is enabled
Default pathcost method used is short
Name Blocking Listening Learning Forwarding STP Active
------------------------------------- ------------- ------------- ---------------- --------
VLAN0001 0 0 0 2 2
VLAN0010 0 0 0 1 1
VLAN1002 0 0 0 1 1
VLAN1003 0 0 0 1 1
VLAN1004 0 0 0 1 1
VLAN1005 0 0 0 1 1
------------------------ ------------- ------------- ------------- ----------------
6 VLANs 0 00 7 7
Spanning Tree Protocol Troubleshooting Commands
Root for Listed VLANs

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IOS# show proc cpu
CPU utilization for five seconds: 1%/0%; one minute: 2%; five minutes: 2%
PID Runtime(ms) Invoked uSecs 5Sec 1Min 5Min TTY Process
1 0 1 0 0.00% 0.00% 0.00% 0 Chunk Manager
<…some output removed…>
79 0 256 0 0.00% 0.00% 0.00% 0 mls-msc Process
80 30508 461976 66 0.40% 0.43% 0.44% 0 Spanning Tree
81 108 27024 3 0.00% 0.00% 0.00% 0 Ethchnl
<…some output removed…>
162 12 41 292 0.00% 0.01% 0.00% 1 Virtual Exec
IOS# show spanning-tree summary
<…some output removed…>
Name Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
VLAN0001 1 0 0 1 2
<…some output removed…>
VLAN1005 0 0 0 1 1
---------------------- -------- --------- -------- ---------- ----------
282 vlans 1 0 0 282 283
Spanning Tree Protocol Troubleshooting Commands
Number of Spanning Tree Instances

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IOS#show spanning-tree vlan 1 detail
VLAN0001 is executing the ieee compatible Spanning Tree protocol
Bridge Identifier has priority 32768, address 0005.7495.9101
Configured hello time 2, max age 20, forward delay 15
Current root has priority 32768, address 0001.c912.7800
Root port is 70 (GigabitEthernet2/6), cost of root path is 4
Topology change flag not set, detected flag not set
Number of topology changes 4 last change occurred 02:17:20 ago
from Port-channel1
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 0, topology change 0, notification 0, aging 300
Port 70 (GigabitEthernet2/6) of VLAN0001 is forwarding
Port path cost 4, Port priority 128, Port Identifier 128.70.
Designated root has priority 32768, address 0001.c912.7800
Designated bridge has priority 32768, address 0001.c912.7800
Designated port id is 128.70, designated path cost 0
Timers: message age 2, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is point-to-point by default
BPDU: sent 7, received 4162
Port 833 (Port-channel1) of VLAN0001 is blocking
Port path cost 4, Port priority 128, Port Identifier 128.833.
Designated root has priority 32768, address 0001.c912.7800
Designated bridge has priority 32768, address 0001.c912.7800
Designated port id is 128.769, designated path cost 0
Timers: message age 1, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is point-to-point by default
BPDU: sent 4, received 134836
Spanning Tree Protocol Troubleshooting Topology Change
Don’t Forget PortFast

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