Multipath routing on single router without BGP4 ?

randal

Golden Member
Jun 3, 2001
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Hey guys --
I have a DS3 uplinked to Fortis Communications (which ups to cogent & yipes, ugh!) which has been pretty shakey. To avoid possible downtime, we purchased another drop from XSpedius, who is in our building, as a failover link. This has been installed and tested, but there are no customers on it. We will be implementing BGP4 in ~ 2 weeks, but in the interim I have a routing question.

We have 6 /24s through Fortis, 4 /24s through XSpedius and 3 more /24s that customers own. Fortis announces their own blocks as well as our 2 customer class Cs. XSpedius announces their own blocks. Hopefully we'll have our own ARIN assignment shortly. All of this is handled by a Cisco 7206.

The problem I have is that our default route right now is out through Fortis and I am concerned that if I add another default route (via XSpedius) traffic from the wrong blocks of IPs will start transiting the wrong network, which will really make things not work - for example, Customer A on Fortis IP addresses might get sent out the XSpedius link, making the traffic never go anywhere / come back.

The only thing I can think of is some sort of source routing, where the router somehow determines that if traffic sources from w.x.y.z/24 it should take this route.

I know this is the wrong way to do it (hence why there are no customers live on the new link), as I should really have another router in there to handle the new network, but finding out whether or not this can be done is gnawing at my brain.

Thanks!
Randal
 

spidey07

No Lifer
Aug 4, 2000
65,469
5
76
I can't really picture what you're asking, a diagram would be nice.

You're asking about having too default routes? As far as announcing it sounds like you have that covered.

If you have two providers theres 3 way to do it, with associated pluses and minuses.

1) accept defaults only
2) accept partial routes from each provider (routes withing their own respective AS) and defaults (which is what I'd recommend)
3) accept full table

you'll have to watch out that you don't become a transit AS and wind up routing between them, although this should all be worked out in advanced with your providers and will be setup with your BGP policy. I'd recommend really laying out what you want to do and what you don't want to do - your policy will come from that.

Also for good routing help go to forum.cisco.com. plenty of more knowledgible BGP guys. I can get around with it but am not an expert.
 

randal

Golden Member
Jun 3, 2001
1,890
0
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routing pic

What I'm wondering is if there is some way to put customers on the XSpedius b.b.1-4.0 IPs and force them to go out through the XSpedius fa0/0 route, even though the default route is through Fortis.

It would be super easy to do with two routers, but I am wondering if it's possible to do on one router -- force some IP addresses to go out via X interface while other IPs get forced out of Y interface.

I've already got our BGP4 configs ready to go, just waiting on the upstreams to get themselves ready to go - this is purely a what-if exercise.

thanks!
randal

 

Garion

Platinum Member
Apr 23, 2001
2,329
6
81
A couple of comments..

First of all, you can add two default routes, as long as one of them is a higher cost than the other. It will use the low cost route first, then roll to the next cost route if the primary default router goes down (Spidey & crew, correct me if I'm wrong here, it's been a while since I've done this).

Unfortunately, the point is totally moot. Getting traffic OUT of a network is the easy part. The hard part is getting it BACK. Right now, all your subnets are advertised through Fortis. Remember that how traffic gets OUT and how it gets back IN are not related. When you send packet to a host, it doesn't always come back the same way it got there - The receiving host uses it's own routing tables and the Internet routing tables to get the traffic back to the sending server. So, if your Fortis link failed, you might be able to get traffic out of the XSpedius link, but all the IP's are still advertised through Fortis and the reply would fail.

In short, there's really nothing you can do without BGP, sorry.

- G