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HSRP confusion

! !

+r1-------+r2

! !

!---sw-----!

in above fig. r1 nad r2 is my router and sw is switch. in hsrp r1 is active router and r2 is standby router...as we know when link of r1 to internet goes down hsrp trak that link and fine the link is down so r2 now become as a active router...till that ok

now confusion is data from switch follow which path...sw to r1 to r2 to internet or

sw to r2 to internet.

2 REPLIES

Re: HSRP confusion

Hi Devang,

Because it is a layer 2 switch so question is not that will it go to switch or not because it will just bypass the switch and hit the lan interface of r2 router directly and from there it will go to internet.

I hope your switch is connetced to both the routers on lan interfaces.

HTH

Ankur

Hall of Fame Super Silver

Re: HSRP confusion

Devang

I believe that Ankur has pointed in the right direction. To understand the answer to your question we need to think of layer 2 operations.

In HSRP the 2 routers share an IP address which should be configured as the default gateway for the end stations on the subnet/VLAN. And the routers also share a special MAC address which is related to the HSRP group. When an end stations sends an ARP request for its default gateway address, the response has the HSRP MAC and the switch layer 2 forwarding table associates that MAC with the port for r1. So the end stations sends to its default gateway using the shared MAC. IF r1 is the active router then its advertises that MAC. And if r1 fails and r2 takes over as the active HSRP router then it sends out an ARP advertisement and the switch updates its layer 2 forwarding table (MAC table or CAM table depending on the type of switch) so that it knows that the special MAC address is now associated with the port for r2. And the packets from the end station are sent to the port of r2.

HTH

Rick

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