bgp router with 4 different routers to 2 isps

[jwilde]

I have a network where i have 4 border routers connected to 2 isps (i'm migrating to 2).  I am participating with ibgp between the routers.  The border routers are not physically connected but are connected through a layer 3 switch.  The border routers participate with the layer3 switch through OSPF and the border routers advertise default routes to their peers. The problem i have is that when one of the routers receives a packet and notices through ibgp a better route (through another router) the border router passes it back to the layer3 switch.  Well, the layer3 switch passes it back to the router which creates a loop.  I can get by this by doing tunneling, but was hoping there is some configuration command to have that packet delivered to the best route border router.  

[Jon Marshall]

I'm not sure which devices you are advertising the default route to ?

In terms of the L3 switch are you redistributing all BGP routes into OSPF ?

Jon

[jwilde]

OK... the border routers advertise default router to the layer 3 switches.  There is no redistribution of bgp into ospf. bgp only sits on my border routers.  The border router gets ibg information from the layer3 switches and in return gives the layer3 switches a default route so they know how to get to the Internet.

[Jon Marshall]

If the L3 switch only gets a default route from each border router it will obviously have no way of knowing which router to send the traffic to.

The usual options would be -

1) the L3 switch participates in IBGP

or

2) redistribute into OSPF. As long as you don't use "bgp redistribute-internal" on your border routers then each border router should only redistribute it's EBGP routes into OSPF which is what you want ie. if the router prefers an IBGP route then that must mean it received it from one of it's peers so you wouldn't want that router to advertise it to the L3 switch.

Other than those options I can't see an easy way of doing it without an awful lot of manual route preference configuration.

Jon

[jwilde]

yea i figured i would have to connect them one way or another.  I do have it working on other routers, but i created tunnels to them.

[Jon Marshall]

If you did redistribute BGP into OSPF then your border routers that had the IBGP routes would actually have OSPF routes via the L3 switch.

I didn't ask you but are all the border routers connected to the L3 switch on a common subnet or with separate L3 routed links ?

Edit - how do you do with tunnels ie. how do you know which routes to prefer over which tunnels ?

Just interested.

Jon

[jwilde]

no they are /30s but i suppose the same subnet would work too.

[Jon Marshall]

Should work both ways.

If the L3 switch and other L3 devices that would also receive the OSPF routes can handle the number of routes that is probably what I would look to do

Jon

[jwilde]

ummmm no they are 1st generation 3750s which i'm switching to asr's or something else.  have not decided yet.