[c-nsp] Quick question on HSRP...

[Jean-Francois.Dube at videotron.com]

Hi Jeff,

My understanding is that you are basically going to replace the default
gateway for in a couple of vlans. (Same IP but different MAC.)

Active HSRP router will issue gratuitous ARP (gARP) when it becomes Active
so there should be little disruption for the hosts inside the vlan trying
to reach the default gateway.

If hosts ignore gARP then they will try to forward frames to the wrong
destination MAC for as long as their ARP entry is valid.

Servers usually have ARP timeout of a few seconds to a few minutes where
routers can have timeout of 4 hours (like Cisco) or more.

HSRP is one thing. You still need to make sure the new routers know how to
route packets the same way the old ones did.

Good luck,

JF

Jean-François Dubé
Technicien, Opérations Réseau IP
Ingénierie Exploitation des Réseaux
Vidéotron

"cisco-nsp" <cisco-nsp-bounces at puck.nether.net> a écrit sur 2013-12-30
18:27:12 :

> De : Jeff Kell <jeff-kell at utc.edu>
> A : "cisco-nsp at puck.nether.net" <cisco-nsp at puck.nether.net>,
> Date : 2013-12-30 18:30
> Objet : [c-nsp] Quick question on HSRP...
> Envoyé par : "cisco-nsp" <cisco-nsp-bounces at puck.nether.net>
>
> Quick question for someone that's been there, done that, as I'm a bit
> rushed to try to lab test this...
>
> We're adding some new routers (4500Xs) for an upgraded server farm
> arrangement with a number of server-side vlans / VRFs.  The plan was to
> trunk it with the existing L3 router, and fire up HSRP (v2) across them
> to transition the L3 routing to the new router without being too
> terribly disruptive.  Not sure if we want to leave the HSRP in place
> (thinking yes) or remove it (and the old router) after the migration,
> but will cross that bridge when we get there.
>
> HSRP would place the current "default gateway" as the virtual IP, and I
> presume it will pick up a new MAC address.  I'm concerned this will
> affect the active hosts with the ARP cached for their gateway.  The MAC
> address would still be valid (should match the original gateway) but the
> traffic would be directed to the original (now virtual) IP, as opposed
> to the new "physical" gateway on the router.
>
> So just how "disruptive" will introducing HSRP really be?
>
> Jeff
>
> _______________________________________________
> cisco-nsp mailing list  cisco-nsp at puck.nether.net
> https://puck.nether.net/mailman/listinfo/cisco-nsp
> archive at http://puck.nether.net/pipermail/cisco-nsp/