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Host connectivity or ping issues due to ARP or CEF table changes in Catalyst 3750 switch stack

Core issue

On a Catalyst 3750 stack, network connectivity can be impacted if the MAC address or VLAN ID associated with an IPv4 or IPv6 addresses were to change,, which causes a change in the ARP and CEF adjacency tables.

When the change occurs, the update is not correctly propagated to the stack members.

This issue is documented in Cisco bug ID CSCsh11040.

In some scenarios, if the host changes from DHCP to a static IP, then back to dynamic, the problem goes away.This can be better understood in this example:

There are two hosts, X and Y, and there are two DHCP servers. When X asks for a DHCP address, it seems to get one DHCP address assignment offers from each of the servers. These are IP addresses A and B.

The host accepts the address A, and not B, and informs the first DHCP server with an accept message.

The ARP table of the switch learns of both the addresses, standard of Cisco IOS  learning from gratuitous ARP messages. The switch thinks A and B are two IPs that share the MAC and VLAN. There is no problem here.

The host Y comes on line and requests a DHCP address. It can get two assignments, IP address C and B, and the order does not matter. The host accepts one of them and rejects the other DHCP address.

Again through ARP/Cisco IOS, the switch learns address C and B mapped to the same MAC, but different from the MAC address of A.

This triggers an adjacency MAC string change update in the code, with respect to address B. Address C is a true adjacency add message. The message for B clobbers the rewrite info for C, so C becomes a victim.


For a workaround, use the clear ip arp command, which resolves it until another such change occurs.

This issue is resolved in Cisco IOS 12.2(35)SE2, which can be downloaded from Cisco Downloads.

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Last update:
‎06-22-2009 05:12 PM
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