Convergence of CAM Table of a Switch during HSRP Failover

Answered Question
May 10th, 2007

Hello,

I have one switch and two routers connected to this switch. HSRP is configured on both the lan interfaces of the routers.

Tracking is configured on the lan interfaces to track each serial link connected to each router to remote networks.My PC

is configured with the virtual HSRP IP as its default gateway.

When the active router or it's connected serial fails, the standby router takes up the active role and continue

to send all my lan traffic through it. My doubt is how the virtual HSRP mac address in the switch's mac address table is

mapped to the correct exit port(port connected to the now active router) which earlier mapped to a different port(port

connected to the now failed router) as HSRP hellos are exchanged only between the routers and not to the switch. I

understand that mac address table enteries are learnt from source mac addresses of incoming frames.

Could someone explain me the process of mac address table population of the switch during HSRP failover.

Best Regards,

Pavan

I have this problem too.
0 votes
Correct Answer by royalblues about 9 years 8 months ago

Pawan,

The active HSRP router is responsible for all the quries sent to the virtual Mac-address.

When the standby takes over as the active HSRP router, it starts responding to the virtual mac-address queries.

This mac-address is now learnt by the switch on the port which was conencted to the standby HSRP router and hence updates its CAM tabe accordingly.

eg. R1 & R2 are 2 routers running HSRP (gr#2). R1 connected to port1 & R2 to port2.

So the HSRP mac-address 0000.0c07.ac02 will be learnt at port1 by the switch. during failover this same address is learnt on port2 as R2 takes over the HSRP active role

HTH, rate if it does

Narayan

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Correct Answer
royalblues Fri, 05/11/2007 - 00:47

Pawan,

The active HSRP router is responsible for all the quries sent to the virtual Mac-address.

When the standby takes over as the active HSRP router, it starts responding to the virtual mac-address queries.

This mac-address is now learnt by the switch on the port which was conencted to the standby HSRP router and hence updates its CAM tabe accordingly.

eg. R1 & R2 are 2 routers running HSRP (gr#2). R1 connected to port1 & R2 to port2.

So the HSRP mac-address 0000.0c07.ac02 will be learnt at port1 by the switch. during failover this same address is learnt on port2 as R2 takes over the HSRP active role

HTH, rate if it does

Narayan

pavansuraj Fri, 05/11/2007 - 01:42

Narayan,

Initially the switch's mac table is empty and in order for switch to fill it's mac table with virtual mac, a frame with this mac as source has to enter the switch.

Is it the active router which sends this frame or any device in the lan by sending an ARP broadcast for virtual HSRP IP?

Thanks for bearing with me

Pavan

mohammedmahmoud Fri, 05/11/2007 - 01:48

Hi Pavan,

Narayan has explained it really nice, when the active routes fails and the standby router takes over, the active router stops responding on quires and the standby which is going to active starts replying instead of it, and thus the MAC entry on the switch changes, its that simple.

HTH,

Mohammed Mahmoud.

pavansuraj Fri, 05/11/2007 - 20:58

Thankyou freinds,

Now I am able to visualize the process.

Best Regards

Pavan

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