Multilink is a way of bundling more than one PPP WAN links and bundle them together into one logical interface. This is one of the method you can implement when you have a primary and secondary links in place and create a logical conduit where both links are present.
This method can be used to load balance data across the links and at the same time it will support redundancy when one one link fails.
I have tested this configuration on 2 of my 2500 Routers and I haven’t tested these on more recent IOS so some commands may vary. I will be checking them on a pair of 3845s/2691s on a later date and will add my findings to this post.
As you can see the Diagram, I am using R4 and R5 which are connected via Serial 0 and Serial 1 respectivly.
As you can see, since this is a lab scenario, I have set the clock rate on the link. If you are setting this up on a WAN link, you don’t need to do this as the circuit provider would be setting this up for you.
Also another thing to watch out for is to create the Multilink interface first then move onto configuring the Serial interfaces. If you do the other way around, it will not allow you to set the multilink group as it is not present on the router (See the error message below).
That is all to watch out for and you can see my R5 configuration below…
Here is how it should look…
Now I will bring Serial 0 interface down and test the Multilink…
As you can see, it works even when one interface is down… 🙂