Redundant Link Failover From the same switch to the same router

[Magdy]

Hi,

I’m having difficulties finding the best solution for my problem.

I have a Cisco 3850 Switch, with one interface (G0/47) connected to a packet shaper, then to a Cisco 3900 Router (G0/1) (The packet shaper appears transparent to the switch and the router in the show CDP neighbours command)

AND

Another link from the same switch (G0/48) going directly to the same router (G0/2)

The reason of having this connection is that sometimes the packet shaper fails to process the packets, so it kills all traffic where nothing is passed to the router, at the same time the interface is not down.

My target is to point all traffic to pass through the packet shaper (G0/47) interface, but when the packet shaper fails, traffic to be redirected to the other interface (G0/49) to bypass the packet shaper.

Any suggestions?

Thanks 

[Jon Marshall]

The packet shaper device should really fail through ie. if is stops doing what it is meant to do it should still pass packets. 

From the Cisco perspective you have two options - 

1) run a dynamic routing protocol between the switch and router and then modify the metrics so the packet shaper link is preferred eg. EIGRP use delay, OSPF you can modify cost. 

This assumes that if the packet shaper stops processing traffic then it will also not pass IGP hellos etc. so the switch and router will see the only available path via the other link. 

2) use static routing at both ends and use IP SLA on the main link to monitor the state of the connection. 

Personally I use routing protocols wherever I can but you may find if the packet shaper link is intermittently up and down you would get more route flapping using an IGP. 

Either way would need testing. 

Jon

[Seb Rupik · Accepted solution]

Hi there,

what you need is an IP SLA job, combined with static route which monitors its status.

ip sla monitor 1 will send ICMP packets to the router sourced from Gi0/47. If the packet shaper fails, the track 10 process will notice this change in reachability. This in turn will remove the static route (with metric 1) and the secondary static rotue with metric 2 will take precedence. The config will look something like this:

!
ip sla monitor 1
  icmp-echo <router_gi0/1> source-interface Gi0/47
  timeout 50
  frequency 5
!
ip sla monitor schedule 1 life forever start-time now
track 10 ip sla 1 reachability
!
ip route 0.0.0.0 0.0.0.0 <router_gi0/1> track 10
ip route 0.0.0.0 0.0.0.0 <router_gi0/2> 2
!

cheers,

Seb.