[c-nsp] Anycast GW for L2 subnet

Tim Stevenson tstevens at cisco.com
Fri Aug 7 11:27:56 EDT 2015

At 01:25 AM 8/7/2015  Friday, Adam Vitkovsky quipped:
> > You could do that but the idea with anycast HSRP
> > is that all participating HSRP routers equally
> > distribute the L3 switching load.
> >
>Yes I agree but in my case the DC would span across two geographic 
>locations and so would the VLANs.
>So I was thinking I could enforce VMs in either location to 
>primarily use local GWs as the closest exit from the VLAN.
>So kind of L2 routing or anycasting of the GWs.
>When I do the same in L3 i.e. anycast the VLAN prefix I should get a 
>nice split and no tromboning of traffic between the locations.

Yes, this will work from an Anycast HSRP standpoint, ie, if you have 
two sites, two AC routers at each, leaf switches in each site would 
only vector gwy traffic to the 2 local (closest) AC routers.

A couple caveats:
- in theory you could have more sites, each with an AC pair, but we 
only support 4 AC gwys today
- FP still builds the MD trees as it always has, ie you avoid 
tromboning of unicast, but multidestination (bc/mc/uuc) still has the 
potential to 'take the scenic route' through the other site, 
depending on the topology of each tree

We are actually working on solutions to both those limitations but I 
am speaking only of what we have today.

Hope that helps,

Tim Stevenson, tstevens at cisco.com
Routing & Switching CCIE #5561
Distinguished Engineer, Technical Marketing
Data Center Switching
Cisco - http://www.cisco.com

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