Hsrp

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Right, came accross an issue today where two back to back routers connected via HSRP were dropping traffic. Upon looking in the syslog I noticed on the standby a duplicate address warning for the physical ip of the fa0/0
that connects to the active router. I thought it was a routing or STP loop where the standby was seeing its own hello packets. Apparently not. My collegue changes the local vmac on the standby router to the bmac of the fa0/0 int and it worked, no more dup addresses. I asked him how this worked as he is ccie, but he failed (couldnt be arsed) to explain it to me. Does anyone know how this fix workes, in eliminating dup addresses?
Its beyond me

 

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Im not to sure, im yet to cover the switching element of the CCNP track (which im looking forward to)

a quick google revealed http://supportwiki.cisco.com/ViewWiki/index.php/HSRP_standby_IP_address_is_reported_as_a_duplicate_IP_address_and_the_user_receives_the_%22STANDBY-3-DUPADDR%22_error_message

look at the links near the bottom for further recommended documentation

 

I'm writing our CCIE R&S product, and in my research, all I found was that this error occurs when an HSRP router receives its own packet or when two HSRP routers become active at the same time. These issues occur because of an STP loop, an EtherChannel configuration problem, or some other problem that causes duplicate frames to be transmitted. You can configure an access list to prevent an HSRP router from receiving its own packets, stopping the error message, but it doesn't fix the problem - it only fixes the symptom. Despite the error, HSRP will still work.

I didn't know you could change the VMAC... I thought it was set to 0000.0c07.acxx... then again, I haven't worked hands-on with HSRP.