Resielient router interface: line protocol didn't went down

Unanswered Question
Sep 14th, 2010


This is regarding the routing when some link went down but line protocol didn't went down.

LAN is accessing the service

I have two routers which have two links to each other. on the top link, there are three layer 2 switches as per diagram bellow.

OSPF running across the link. if i loose the Router2 top link interface , the routing works as router2 removes the connected interface from routing as line protocol went down, but if the link between switch 2 and switch 3 lost, router doesn't remove the first link connected inferace from routing as line protocol up and hence access to is not working as traffic is forwared to switch3 on broken link and can't reach how could i achive in a dynamic way that if the switch 2 and switch 3 losses interface, router2 routes that traffic over the bacup down link to router1 and so on. subnet cann't be change due to design.

There are few ways i am thinking like router1 & 2  advertizes and into the ospf thus route preferred over cconnected. or second method using route tracking. is there any suitable way to accoplish that. if the link between switch 2 and 3 lost, the router should shut down the first interface



Router1  ------------------ Switch1-------Switch2------------Switch3--------------Router2----------- LAN                 (First TOP Link)                                                                                                  

  |                                                                                                                         |

  |---------------------------------------------------------------------------     ------------|                                                     (Secondary Backup down link)

I have this problem too.
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gatlin007 Sat, 09/18/2010 - 10:53

If is the only host that's necessary to access in this fault scenario and this addresses hosts in the Router2 LAN then install a /32 route on router1 for pointed out the L2 switched network Ethernet interface.

ip route fa0/0 ### L2 switched network

Redistribute this route into OSPF to be advertised over both media.  Router2 will learn this /32 via both OSPF adjacencies.  If needed tune the interface bandwidth attribute for optimum route selection. 

When the OSPF adjacency goes down over the L2 network Router2 will have a /32 route for via the surviving link.


mrbzumrbzu Mon, 09/20/2010 - 07:10

Hi Christopher,

If we have the route on Router2 pointing through fa 0/0 to switch 3 then if the switch 2 and switch 3 link fail then router 2 still think that the route is availble n forward the traffic to switch 3 and get dropped over there. This will not help. switch 2-3 failure will not tell router 2 that the link is not available any more even through OSPF. Any other suggestions please.


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