Routing failover between sites

[Mokhalil82]

Hi 

Please see the attached diagram. I have Site A (Existing) and Site B (New). Some users will be moving over to site B so we need to extend the network into site B. We have a 24Core fiber between the sites so the plan is to install a L3 switch at site B and link it up to site A using 20 or 40G. However we will be using new subnets at site B to avoid L2 broadcasts over the site to site link.

All traffic from site B needs to use the redundant ISP connections at site A, so the default route will point to site A. We have however purchased a small connection at site B, incase the link between the sites ever fails.

Site A uses BGP to the ISP. Looking at this topology I am trying to workout a automatic routing failover for site B incase the link to site A ever fails. I don't mind if this failover involves some manual change but I want it to be a small change.

The solution I had in mind is to have 2 default routes at site B, one to site A with a lower admin distance and the other to the ISP at site B. Using IPSLA i could drop the primary default route if the link between the sites ever drops resulting in all traffic then using the local link to the ISP to go out. As for return traffic the ISP will have to do some magic at their end to accommodate this failover (Im thinking they need to use BGP metrics?). 

Does this sound like a good solution or can anyone advise of anything else. Also it will be helpful to have advice on how the ISP could accommodate this failover.

Thanks

[Richard Bradfield]

your setup is very similar to one of mine, in my case site B is the DR site. As you are running BGP across an MPLS network I take it you have other remote sites.

So I  would have site B using BGP into the MPLS network.and the link active( there might be traffic from site Bto the other sites anyway)

Use something like EIGRP across the fiber links

Now in the BGP setup at both sites  put all the required subnets from both sites in the Network statement. then use a Neighbor Prepend and filter statements. to control the advertising of routes thru BGP.

I can give you details of what is required if you post the configs

[Mokhalil82]

Thanks Richard

So from what I understand, I can use BGP AS prepend to make Site B appear to be further away so other sites will still use the links at site A. We want everything to enter via site A primarily.

As with EIGRP over the fiber. I take it, it does not have to be redistributed into BGP as EIGRP will only be running between the two sites so traffic between the sites will primarily use the EIGRP routes because of the lower admin distance, and can then use the BGP routes for intersite traffic if the fiber fails.

Would you say that seems sensible enough?

[Jon Marshall · Accepted solution]

It's not clear from your diagram if site B has a router to connect to the MPLS network.

If you are only using the L3 switch and running BGP on it then you can't use EIGRP across the backup link because EBGP AD 20 is better than EIGRP AD 90 and so all traffic would go via MPLS.

You could obviously modify the AD of one of the routing protocols to make it route the way you want.

If however you had a router and you redistributed BGP into EIGRP then you could do that because the L3 switch would choose the EIGRP internal over external routes.

It's also not clear how you are propagating the default route from site A to B or whether it is statically configured ?

Jon

[Mokhalil82]

Site B does have a link into the MPLS. I was thinking whether to purchase a router or not, but I might go ahead and purchase one as it can do BGP with the ISP router.

Traffic between the site will be statically routed for now on the core switches.

[Richard Bradfield · Accepted solution]

Yes that would be a good idea, I would use eigrp between the sites, and BGP on the MPLS network,