[c-nsp] Anycast GW for L2 subnet

Adam Vitkovsky Adam.Vitkovsky at gamma.co.uk
Fri Aug 7 04:25:41 EDT 2015

Hi Tim,
Thank you very much for your explanation, I really appreciate it.

> Tim Stevenson
> Sent: 05 August 2015 02:57
> 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
> There are no MAC advertisements in FP, routing is based on switch IDs (SID).
Aah yes, sure, must have thinking evpn :)
Right the MAC learning is done in data-plane.

> >  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.
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.

> >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).
I see now, that's how everyone associates the HSRP MAC with the ESID and can start loadsharing.
Thank you very much.

> See section 10 here for a bit more:
> http://www.cisco.com/c/en/us/products/collateral/switches/nexus-7000-
> series-switches/white_paper_c11-687554.html
> Hope that helps,
> Tim
> Tim Stevenson, tstevens at cisco.com
> Routing & Switching CCIE #5561
> Distinguished Engineer, Technical Marketing
> Data Center Switching
> Cisco - http://www.cisco.com
> +1(408)526-6759

        Adam Vitkovsky
        IP Engineer

T:      0333 006 5936
E:      Adam.Vitkovsky at gamma.co.uk
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