Setup routing failover on Cisco Router

Posted on 2013-01-15
Medium Priority
Last Modified: 2013-01-16
I have a Cisco 3945 router (running 15.0(1r)M13).

I have 2 separate connections coming into this router.  Both links connect to my 2nd office location.

I have a preferred path for traffic to use (link A) and a secondary link (link B) that I can use if the Link A ever were to go down.

All I need to do it go into my router and change the static route for the network in question, from using Link A to Link B, and that's all I need to do to failover.

My question is - is there an easy way to set this up to do this failover automatically?  Can I put in multiple static routes and assign a cost?  Is there anyway to *easily* do this without complicating my other hardware pieces?

Essentially all I need is for the router to somehow know primary path is down, and to use the secondary path.

Of course that means there has to be some logic for switching back too, once the link is back up, I haven't really thought about that yet.

Any suggestions would be helpful, but I'd like a simple solution, not something that complicates my environment much more.
Question by:Vjz1
  • 2

Accepted Solution

Leeeee earned 2000 total points
ID: 38779388
Yes, IP SLA and floating statics come to mind.

Floating statics can be configured to assign a secondary default route with a higher AD. For example:

ip route (Primary)
ip route 10 (secondary)

The 10 you see at the end changes the default AD from 1, which is used for static routes, to 10 so the primary default route is preferred. When that primary route goes away, the secondary will become active. When the primary comes up, it will be re-instated into the routing table.

What I'd recommend configuring is IP SLA (cant have IPBASE). IP SLA assigns a track to a default route to check it's availability at specific intervals. So, we could be pinging a public DNS server out on the internet or your ISP's gateway using your primary link. If a specified threshold isn't met or trouble is detected over that link, the IP SLA will remove the primary static route and the secondary will become active automatically. This is beneficial over using floating statics as sometimes links remain up/up, but no traffic is flowing over the link.

Here's a good guide IP SLA:
LVL 10

Expert Comment

ID: 38782348
Why not just use a routing protocol such as OSPF or EIGRP and turn cost/delay up on the backup link? routing protocols are made for finding alternate paths. The example above relies on the fact that the interface with the primary route goes down. Only then the route disapears. With routing protocols you have processes on both routers that need to communicate with eachother. Once the communication over one path is blocked all traffic is rerouted over the other one.

Author Comment

ID: 38782590
Sounds like an interesting option.  I just don't know anything about OSPF or EIGRP.  I guess I need to start reading up on them.
LVL 10

Expert Comment

ID: 38782863
Sure, and your example is a very easy one.

Another option in your situation would be to use some multilink technology such as EtherChannel/LAG or PPP Multilink depending on the media and the router capabilities.  With this you could use both links while they are available and have still one active in case of the failure of the other one.

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