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HSRP in VPC

Rishav Garg
Beginner
Beginner

Hi All,

If i am using HSRP on SVI's in VPC environment. Both would be active - active in VPC that i know.

But what would if i manually shut down the one peer device SVI(Suppose Primary Switch) HSRP SVI. Because from downhost prespective both link is UP which is a VPC member ports. So if by load balancing if it choose path towards Primary Switch on which L3 SVI is down.

 

What will happen? will my traffic be lost only for that link or VPC will do something else?

3 Replies 3

Leo Laohoo
Hall of Fame
Hall of Fame

Duplicate posts.  

Go HERE.

InayathUlla Sharieff
Cisco Employee
Cisco Employee

HI,

vPC in essence makes both HSRP routers the 'Active' router. Both are populated with the G flag and can forward traffic equally. This should in essence make it unnecessary to have staggered VLAN/HSRP deployments. The only thing that the Active router will do more is to respond to ARP requests, but then the L2 switch forwarding the traffic to the vPC HSRP pair will use it's load balancing algorithm to make sure both vPC/HSRP switches get a healthy equal dosage of the traffic. 

Check out this document beginning with page 24:

http://www.cisco.com/en/US/prod/collateral/switches/ps9441/ps9670/C07-572835-00_NX-OS_vPC_DG.pdf

 

So to put it in simple words:-

When you shut down the SVI interface the switch will send gratious arp saying that to forward all traffic to sw2.

which would get populated downstream. I dont think for the traffic destined to the vip will use agian the sw1 which was primary. In case if it selects that link the traffic will get dropped but as said switches are intelligent enough to route the packet to the secondary switch now as the G flag is set and that will take care.

 

Hope this answer the query.

 

I think I have to disagree with the last part. I have no idea what kind of gratious arp the switch is supposed to send when its VLAN interface has been shut.

The downstream switches have no intelligence whatsoever about the northbound topology. All they have is a single port-channel to a single (logical) switch. They do switch packets to the port-channel where they expect the VIP to be. If etherchannel loadbalancing results in the frame arriving at the switch with the shut SVI, then I believe we see the packet being dropped.

 

Regards

 

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