AD ON STATIC ROUTES

[carl_townshend]

I have a small question about floating static routes, how does the route know when the next hop has gone down and to use the next route with higher AD ? does it use some simple icmp checks ?

[Giuseppe Larosa · Accepted solution]

Hello Carl,

without object tracking the router will consider  the IP next-hop reachable until the interface where the IP subnet of the next-hop is up.

In case of LAN switched environment, the L3 interface can be an SVI and the next-hop can die without L3 interface to fail too.

In this case the next-hop is considered valid until the associated ARP entry is valid up to 4 hours on a cisco device with default settings.

In a LAN environment : or move to dynamic routing or you need to use reliable static routing

see

http://www.cisco.com/en/US/docs/ios/12_3/12_3x/12_3xe/feature/guide/dbackupx.html

the primary static route next-hop has to be tracked using an tracking object or an IP SLA (IOS version dependent)

when the tracked object / IP SLA fails the primary route is removed from IP routing table and secondary route (with higher AD) is installed.

Hope to help

Giuseppe