On Wed, 19 Dec 2001 09:34:50 GMT, "Harri Suomalainen"
>> On re-reading your question that link might not address how the switch
>> handles the change and I apologize for that. When the router interface
>> down the switch port also goes down and the arp entry is removed.
>HSRP hello packects are sent from the active router as follows:
>-Source MAC = HSRP virtual MAC
>-Sourtce IP = real router IP
>-Destination = Multicast 22.214.171.124
>Therefore, when a router becomes active it will start sourcing packects
>from HSRP MAC and the switch will update it's CAM table to include the
>HSRP address in the new port. (Just like the mac had moved to another port).
This doesn't seem to be the case as per this document:
"Routers running HSRP communicate HSRP information between each other,
via HSRP hello packets. These packets are sent to the destination IP
multicast address 126.96.36.199 (reserved multicast address used to
communicate to all routers) on User Datagram Protocol (UDP) port 1985.
These hello packets are sourced with the configured IP address on the
interface and the burned-in MAC address of the interface, as opposed
to the HSRP or virtual IP and MAC address. This use of source
addressing is necessary so that HSRP routers can correctly identify
The only exception to the above behavior is for Cisco 2500, 4000, and
4500 routers. These routers have Ethernet hardware that only
recognizes a single MAC address. Therefore, these routers will use the
HSRP MAC address when they are the active router, and their burned-in
address for HSRP hello packets."
Strangely enough, however, the RFC says this:
"If and only if the router is in the Active state, the router MUST use
the group's virtual MAC address as the source MAC address for its
Hello messages. This is necessary in order to allow learning bridges
to be able to determine which LAN segment the virtual MAC address
currently belongs to."
My guess is that what Cisco's site says is accurate, primarily because
the RFC also states that a router must send out a gratuitous ARP
broadcast upon transitioning to the active state. Seems to me this
would negate the need for the Hello messages to use the virtual MAC
address as the source.