[c-nsp] Static routes in routing table

[Gert Doering]

Hi,

On Thu, Oct 06, 2005 at 05:34:36PM -0700, Chen, Qinxue wrote:
> We have two static routes to the same destination through two
> different next-hops. One static route has less prefered admin
> distance. We want a graceful failover happening when the primary
> static route fails. Now the question is how soon can Cisco routers
> take out the failed static route. If the next hop is on a
> direct-connected network (not necessaryly a point-to-point link,)
> that will depend on how soon the ARP table cleared on the Cisco
> router. 

Actually, no.  The static route will never be withdrawn in that case,
unless the ethernet interface goes to "down" (cable unplugged).

If there is no ARP entry, the router will try to create one ("that's
the way IP and ARP work").

> There are two cases:
> 
> 1) If cisco router -> next-hop is a point-to-point link, will the Cisco router flash the ARP entry for the next-hop right after the next-hop is down?

If the link is down, the static route will be deactivated (*if* the
static route points to the interface+next-hop - otherwise it might be
recursively resolved, and point elsewhere, if there is another route
to the next-hop).

> 2) If the next-hop is connected to a cisco switch but on a direct-connected network as the Cisco router, will the Cisco router has to wait for the ARP timeout for the next hop to take out the failed static route? Or any other way to make it instantly.

If the interface is up, the route will stay.

You could achieve something with IP SLA and route object tracking, but
a much easier way would be to run a routing protocol ("that's what they
are for").

gert
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Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de