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HSRP Behavior

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HSRP Behavior

Issue:

I have 2 Cisco 3560 Switch connected and a Trunk link is configured between them allowing all the VLAN. Also HSRP is configured with virtual IP address as 192.168.1.100. A server is connected with Teaming/Multipath configured in its NIC. SWITCH 1 is ACTIVE & SWITCH 2 is STANDBY.

By Chance if the trunk link between the switches goes down both the switches will become ACTIVE.

In this scenario, how the traffic flow path is going to be? Since the virtual IP MAC will be given by both the switch, which path the Server NIC will choose?

Resolution:

If only one NIC is active then any traffic coming into the switch with the standby NIC connection destined for the server would need to be sent across the switch interconnect.

If the interconnect fails between the switches this does not affect the active server NIC. The active server NIC will stay the same and therefore the server will continue to use the switch it was using before the failure. Obviously though return traffic may very well not reach the server as it may come in on the other switch.

Best practice is always to run more than one physical link between 2 switches doing HSRP, so you should definitely look to use Etherchannel for link redundancy.

Source:-

Comments

We run HSRP in R1 and R2, both are connected a L2 switch. Switch port 1 is connected with R1 and port 2 connected R2. R1 is currently active for HSRP. switch forward all packet to port no 1 (active gateway) because of switch have MAC table and the HSRP MAC address is stored against the port no 1. R1 goes down but LAN device is sending continuedly packet to gateway and switch also send the packet through port 1 as per MAC table. by default switch MAC table aging is 300 second. After 10 second R2 will be act as a active gateway for HSRP.

 

Question is:

1. How L2 switch understand HSRP mac now reachable through port 2?

2. Then how switch move the same mac address against port no 2 without having MAC flapping error?