Cisco – Why ARP Cache clear when HSRP is already running

ciscohsrpmac address

I have a scenario to discuss. There are three nodes that I am talking about. lets give those sites the name A, B and C.

Site A and C has vendors routers installed. Vendor has no reachability for these sites A, B and C so it has asked me (service provider) to serve its customer. Now to provide services to vendor's customer we have installed our switches and Media converters at site A, B and C.
If the links gets down at site C. we started trouble shooting fall from site A to site C. 

A---------B (fiber) -------> A has vendor router and my switch.
B---------C1 (E1 circuit)--> B has only my switch to provide connection to C1.
C1--------C (Radio) -------> C1 is my office site where my equipment is placed
                               and C is my vendor's router.

Now I need to see the connectivity. The IP at A is .129 at B is .130 and at C is .56

  1. Checking A—-B I shut my port at B and bind IP .56 at B switch… Ping stats for .129 and .56 were shared (No change was made at site C – because port was shut at my end)
  2. Checking B—C because it was a VLAN traffic from Fiber, I shut the end at my switch and bind IP .159 at B. No change done at A again.

Also note that vendor is using HSRP protocol at its routers on site A and C so it keeps updating IP and MAC Addresses.

As soon as I am done with my troubleshooting after 4 hours, I reverted back those IPs to their respective locations. Means vendor should now receive correct information of MAC addresses that was originially assigned because of this HSRP Protocol..

Now my question is, why do i need to tell me vendor to clear its ARP Cache to get the sites working back other wise sites will remain down???? When HSRP keeps updating its neighbor routers information of ip against MAC.. why they need to clear it?

Best Answer

When HSRP triggers a failover, it also issues Gratuitous ARPs to update all the ARP caches and Switchport MAC Address tables that the default gateway is now being served by a new interface/router.

Its not terribly clear from your question, but as far as I understand, you are not actually running HSRP with your vendor, but instead are shutting and manually configuring what would have been the HSRP VIP IP address on different Routers. Given that you are doing this Manually and not within the HSRP protocol, the gratuitous ARP is not being sent to update everything, so a manual clearing of the ARP cache is necessary. This allows each device to "re-learn" the MAC address of its default-gateway, as well as the switched-path to get there.

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