[c-nsp] OSPF and BGP relationship

[Adam Vitkovsky]

Ok so to break this down:
You have two AS exists. 
Each AS exit receives a default route from upstream. 
Each AS exit generates an OSPF default route (setting different metric to influence as-exit policy). 
So you are using OSPF default route as an attractor to get the traffic to AS exits.
But since the AS exits only have a default route there's not much they can offer.  
When you say "as specific as I like" what do you have on your mind please?

If you would have the full BGP table coming from upstream providers and being exchanged between the two AS exits i.e. BGP speakers A and B. 
You could advertise OSPF default route with the same metric from both of them -just to attract traffic to the AS exits (this would distribute the AS egress traffic  nicely based on the closest proximity (best metric) to a particular AS exit from a given node in the network). 
Once the packet enters one of the BGP speakers A or B it will be routed based on the longest match route (so not following the default) for the packets destination -this could mean it could either use the AS exit local to the router or can be forwarded towards the other AS exit based on a better local reference for that given subnet for instance. 
But this "playing" with AS exits where some destinations are preferred over one AS exit and other destination preferred via other AS exit will necessarily introduce asymmetric routing that is packets leaving via exit A the return traffic might come back via B.


adam

> -----Original Message-----
> From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of
> CWO Network Operations
> Sent: 02 March 2015 21:40
> To: cisco-nsp at puck.nether.net
> Subject: [c-nsp] OSPF and BGP relationship
> 
> I have a question about the common practice of using OSPF and (i)bgp.
> 
> Here is my setup:
> 
> 
> I have 4 Cisco routers (A, B, C & D). All routers are connected to each other
> through metro ethernet connections. The 4 routers have other “stuff”
> behind them speaking only OSPF and require a injected default route.
> Router A and B are connected to different internet backbone providers using
> BGP.
> Internally I use iBGP and OSPF. I do not redistribute OSPF routes into BGP,
> nor do I do the opposite.
> Router A injects a default route into the network using OSPF’s default-
> information originate metric 100.
> Router B also injects a default route into the network using OSPF’s default-
> information originate metric 110.
> 
> So, right now all my outbound traffic goes out through router A (because of
> the metric 100). Inbound traffic comes through both internet connections,
> based on the preferred BGP route.
> Since the IGP (ospf) has the lower IGP metric (in comparison to ibgp) the
> ospf default routes (0.0.0.0/0) routes determine the flow of outbound
> traffic. Because of that, I can’t seem to “direct” outbound traffic using a local
> route map (local-preference). Ideally I would like to be able to direct
> outbound traffic as specific as I like.
> 
> What is the common setup, in terms of BGP and OSPF, on networks that
> resemble ours?
> 
> Thanks
> JB
> 
> 
> 
> 
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