[c-nsp] Anycast GW for L2 subnet

Tim Stevenson tstevens at cisco.com
Tue Aug 4 21:57:17 EDT 2015

Hi Adam, please see inline below:

At 03:58 PM 8/4/2015  Tuesday, Adam Vitkovsky quipped:
>Has anyone played with Anycast HSRP with fabric path please?
>Just would like to confirm I understand it correctly.
>So ISIS calculates best path to the anycast switch ID advertising the HSRP MAC

There are no MAC advertisements in FP, routing is based on switch IDs (SID).

>  and since I can manipulate metrics on links 
> between spine and leaf switches I should be 
> able to dictate which leaf switches should be using which GWs right?

You could do that but the idea with anycast HSRP 
is that all participating HSRP routers equally 
distribute the L3 switching load.

>Because only paths to anycast switch ID with 
>equal costs are considered for multipathing 
>right? (i.e. there’s no unequal cost load sharing correct?)

Correct, it is ECMP only.

The model is that all anycast HSRP routers have 
their own unique SID but also an emulated SID 
shared among them all. All advertise that ESID, 
and any FP switch with equal path cost to 2 or 
more of those will load balance traffic destined to the HSRP MAC among them.

Typical topology is spine/leaf but any topology 
will work. Note that only the control-plane 
Active router is the one that responds to ARP & 
sources HSRP hellos with the HSRP MAC (using the 
ESID as the source SID in FP frames).

See section 10 here for a bit more:

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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