BGP prepending

Unanswered Question
Sep 9th, 2009

All,

I've got an issue that I'm not understanding. I've attached a diagram. The topology represents something that I need to do in production.

RTRA advertises a route to the ISP and TransAS. TransAS then advertises this route to RTRB, which in turn will readvertise to the ISP. The ISP shows both routes in its BGP table.

RTRB prefers the route through the TransAS instead of the ISP, which isn't what I want. I created a route map on TransAS that would prepend its AS for any routes that come through it because I always want the ISP to be preferred.

Once I applied this route map to the neighbors on TransAS, the routes still get advertised to RTRB, but RTRB now no longer advertises to the ISP. I can shut down RTRA's interface to the ISP and RTRB will advertise the network out.

Why will RTRB not advertise the network until RTRA goes away? Is this the correct way to do this, or should I just consider running OSPF or EIGRP over this link with the "TransAS?"

Thanks,

John

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Edison Ortiz Wed, 09/09/2009 - 08:28

Once I applied this route map to the neighbors on TransAS, the routes still get advertised to RTRB, but RTRB now no longer advertises to the ISP. I can shut down RTRA's interface to the ISP and RTRB will advertise the network out.

Isn't that what you want? You want RTRA to advertised to ISP and RTRB receive this route from the ISP?

RTRB can't advertise to the ISP and receive the same route from the ISP at the same time.

You have to choose whether you want RTRB to advertise to ISP or receive the route from the ISP, not both.

You've explored both options per the post and it seems you aren't satisfied with either option.

I personally don't understand what you are after and actually, your BGP project seem quite confusing based on your post in the last 3 days :)

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Edison.

John Blakley Wed, 09/09/2009 - 09:07

"Isn't that what you want?"

Yes, but I thought that the route would still be in the bgp table as an alternate route, but it's not.

RTRB can't advertise to the ISP and receive the same route from the ISP at the same time.

Makes sense, but I still thought it would be seen as a backup route. My mistake.

"...it seems you aren't satisfied with either option. "

It's not that I'm not "satisfied", but I'm more trying to learn WHY the problem was happening. I've already got a "workaround" for it by redistributing eigrp into BGP and weighting all BGP learned routes. We only peer with our ISP on an MPLS network, so I won't be receiving any routes outside of our network on these routers anyway.

Thanks,

John

Edison Ortiz Wed, 09/09/2009 - 10:37

Can you share your BGP table under the 2 scenarios you mentioned in your original post?

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Edison.

John Blakley Wed, 09/09/2009 - 11:31

Edison,

I see what you're saying. If the router learns of a route that it's also advertising, it won't readvertise that route. It's working now, and I've weighted my incoming routes on one router because I always want it to go out to the PE before coming to us, and I've prepended the "middle" router in both directions. Now neither RTRA or RTRB will choose to send over the TransAS unless it needs to. I also changed the weighting for a couple of routes for RTRA to weight higher so it will always send through TransAS.

Oh, and it always had the route on RTRB, but it wasn't advertising. I'm assuming (haven't debugged it) that when the "ISP" router loses the route it updates RTRB, which in turn looks in its BGP table and says "I have that route" (like EIGRP), here it is. Am I correct?

Thanks,

John

Edison Ortiz Wed, 09/09/2009 - 12:03

If the router learns of a route that it's also advertising, it won't readvertise that route.

To the router it learned from? No.

I'm assuming (haven't debugged it) that when the "ISP" router loses the route it updates RTRB, which in turn looks in its BGP table and says "I have that route" (like EIGRP), here it is. Am I correct?

Correct.

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