The HSRP process uses multicast address 224.0.0.2 to communicate hello packets with the other HSRP routers. If connectivity is lost, or an HSRP router with higher priority is added to a network, the HSRP states can start flapping as shown above. When running HSRP on certain router platforms (see Note below) and a higher priority router is added to the network, the HSRP state of the lower priority router changes from Active to Speaking, and a link-state change occurs. The port of the switch detects this link-state change and a spanning tree protocol transition takes place. The port takes approximately 30 seconds to go through the listening, learning, and forwarding stages. This time period exceeds the default timeouts of the HSRP hello processes, so that the lower priority router, after reaching the Standby state, becomes Active because no hello packets were received from the Active router.
Since the routers do not see each other's HSRP hello packets, they both become active. When the switch ports transition to the Learning state it is possible that the switch sees the same virtual MAC address out of two different ports.
Note: Physical link-state changes caused by HSRP state changes occur specifically on the network module-Fast Ethernet (NM-FE) interfaces on Cisco 2600, Cisco 3600 and Cisco 7200 series routers. This behavior no longer occurs in Cisco IOS® Software release 12.1(3) and higher.
For more information, see Cisco Bug ID CSCdr02376 (registered customers only) .
Troubleshooting Procedure
Perform one of the following tasks in order to workaround the problem described above.
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Configure the switch with the set spantree portfast enable, which allows the switch to bypass the spantree states and go straight into the Forwarding state.
If the router is configured to bridge packets on this interface/port, then this workaround cannot be used, because the immediate forwarding on such a link could make the network prone to a forwarding loop outage.
Note: This restriction is also true for switch ports that are connected to other switches or bridges.
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Change the HSRP timers so that the spanning tree forward delay (default of 15 seconds) is less than half the HSRP holdtime (default of 10 seconds).
We suggest an HSRP holdtime of 40 seconds.
Note: Increasing the HSRP holdtime makes HSRP slower in detecting that the Active router is down and making the Standby router active.
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Ensure that there are no packet storms on the network (IPX is prone to packet storms).
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Configure the standby use-bia command, which forces the HSRP active router to use the burned-in address.
This accomplishes two things. Since HSRP no longer needs to change (or add) a unicast MAC address to the MAC address filter list, the Ethernet interface does not get reset. It also keeps the switch from learning the same address on two different ports. Refer to What is the standby use-bia Command and How Does It Work? for more information.
Note: Using the standby use-bia command has the following disadvantages:
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When a router becomes Active the virtual IP address is moved to a different MAC address. The newly Active router sends a gratuitous Address Resolution Protocol (ARP) response, but not all host implementations handle the gratuitous ARP correctly.
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Proxy ARP breaks when standby use-bia is configured. A standby router can not cover for the lost proxy ARP database of the failed router.
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Due to internal limitations, the standby use-bia command is not supported on the Multilayer Switch Feature Card 2 (MSFC2). For more information, refer to the Configuration Guidelines and Restrictions section of Configuring IP Unicast Layer 3 Switching on Supervisor Engine 2.
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