HSRP

[Suprit Chinchodikar]

Hello,
As per the below topology. I have 3 switches connected to each other.
Port has been blocked between S1 and S2 due to STP.
All three switches have IP's configured as the gateways.
Host H1 is connected to S3 and has it's default gateway configured as .3
So, when H1 send the packet it will come to its default gateway i.e. towards .3
But how does S3 knows that it has to forward traffic to S1 and now S2?

[Gopinath_Pigili]

Hello  Suprit Chinchodikar,

Using HSRP concept,  both S1 and S2 will assume one virtual router that is assigned with Unique ip address and ficticious mac address. All Hosts use this ip address as their default gateway and the mac address for the Layer 2 header. Also S1 and S2 go with election process and one router will become active (S1) and other will become stand-by (S2). Only Active router forwards the traffice. Standby is backup and just it keeps monitors the Active router by sending hellos every 3 seconds by default.

When S3 receives the traffic destined for  virtual ip and mac address, based on the virtual mac-address it sends the traffice to S1. Because  S1 is active for the mac-address. In the event of S1 is down, then S2 will becomes active for that same Virtual ip and mac-address, so S3 sends the traffic to S2.

Best regards
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[MHM Cisco World · Accepted solution]

No need l2 link between SW1 and SW2 and hence there is no STP blk port 

And SW3 will use MAC of VIP of HSRP to send traffic toward SW1 or SW2 (which one is active)'

And how SW3 add MAC to table 

The active HSRP send GARP this help SW in domain to add MAC to table.

Note:-only active send GARP 

MHM

[Giuseppe Larosa · Accepted solution]

Hello @Suprit Chinchodikar ,

HSRP provides an emulation of an always on virtual router at OSI Layers 3 and OSI Layer 2 by using a special MAC address that is related to the HSRP group number.

As explained by @MHM Cisco World , whatever switch is HSRP active it sends out Gratuitos ARP to tell that the HSRP VIP MAC address is on his port.

So also S3 is aware of HSRP MAC address and it is able to bridge the frame with destination MAC = HSRP MAC address to the active HSRP switch S1 in your network diagram.

if S1 fails , S2 will take ove becoming the new HSRP active router and it will update all devices in the same VLAN broadcast domain by sending out GARP.

Hope to help

Giuseppe

[Gopinath_Pigili]

Yes...,  S1  send GARP (Gratuitous ARP) to S3. This helps to S3 to learn the mac-address of active...!!

Best regards
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[Georg Pauwen]

Hello,

can you post the running configs of all three switches ?