Introduction
When you run the Hot Standby Router Protocol (HSRP) between two routers connected via a LAN switch, you may observe instability in HSRP. This often happens during a network disruption or an active router transition, such as an HSRP router with a higher priority and preempt configured being added to the LAN. This document explains why this instability occurs and how you can avoid it.
Prerequisites
Requirements
There are no specific requirements for this document.
Components Used
This document is not restricted to specific software and hardware versions.
Conventions
For more information on document conventions, refer to Cisco Technical Tips Conventions.
Configure
In this section, you are presented with the information to configure the features described in this document.
Note: To find additional information on the commands used in this document, use the Command Lookup Tool (registered customers only) .
Network Diagram
This document uses the network setup shown in the diagram below.
Configurations
This document uses these configurations:
|
Router A |
|---|
interface FastEthernet1/0 |
|
Router B |
|---|
interface FastEthernet3/0 |
Verify
There is currently no verification procedure available for this configuration.
Troubleshoot
This section provides information you can use to troubleshoot your configuration.
Troubleshooting Commands
Certain show commands are supported by the Output Interpreter Tool (registered customers only) , which allows you to view an analysis of show command output.
Note: Before you use debug commands, refer to Important Information on Debug Commands.
-
debug standby
Sample Debug Output
In the above diagram, when Router A is added to the network, you can observe the Router B HSRP state flapping from Active to Standby. Running debug standby on Router B yields the following output:
RouterB# debug standby
*Mar 1 02:55:56: SB0:FastEthernet3/0 Hello out 10.144.220.2 Active pri 110 hel 3
hol 10 ip 10.144.220.1
*Mar 1 02:56:08: SB0:FastEthernet3/0 Hello in 10.144.220.3 Active pri 120 hel 3
hol 10 ip 10.144.220.1
*Mar 1 02:56:08: SB0: FastEthernet3/0 state Active -> Speak
*Mar 1 02:56:08: SB0:FastEthernet3/0 Resign out 10.144.220.2 Speak pri 110 hel 3
hol 10 ip 10.144.220.1
*Mar 1 02:56:08: SB0:FastEthernet3/0 Hello out 10.144.220.2 Speak pri 110 hel 3
hol 10 ip 10.144.220.1
*Mar 1 02:56:09: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet3/0,
changed state to down
*Mar 1 02:56:11: SB0: FastEthernet3/0 state Speak -> Init
*Mar 1 02:56:13: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet3/0,
changed state to up
*Mar 1 02:56:13: SB0: FastEthernet3/0 state Init -> Listen
*Mar 1 02:56:14: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet3/0,
changed state to down
*Mar 1 02:56:14: SB0: FastEthernet3/0 state Listen -> Init
*Mar 1 02:56:20: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet3/0,
changed state to up
*Mar 1 02:56:20: SB0: FastEthernet3/0 state Init -> Listen
*Mar 1 02:56:30: SB0: FastEthernet3/0 state Listen -> Speak
*Mar 1 02:56:40: SB0: FastEthernet3/0 state Speak -> Standby
*Mar 1 02:56:41: SB0: FastEthernet3/0 state Standby -> Active
*Mar 1 02:56:41: SB: FastEthernet3/0 Adding 0000.0c07.ac00 to address filter
*Mar 1 02:56:41: SB0:FastEthernet3/0 Hello out 10.144.220.2 Active pri 110 hel 3
hol 10 ip 10.144.220.1
*Mar 1 02:56:44: SB0:FastEthernet3/0 Hello in 10.144.220.3 Active pri 120 hel 3
hol 10 ip 10.144.220.1
*Mar 1 02:56:44: SB0: FastEthernet3/0 state Active -> Speak
From the output above, it is clear that the HSRP state of Router B is continuously changing from Active to Speaking to Standby to Active, and so on.
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