BGP Re-advertise default route

[dano2112]

Hello,

I have a L3 switch that is doing BGP peering with two other routers, RouterA and RouterB.  I am learning a default route (via BGP) from RouterA and devices that use my L3 switch as their default gateway are able to reach the Internet without issue.

Now, I am trying to "re-advertise" the default route I am learning from RouterA and push that into the BGP routing table running on RouterB.  On my L3 switch, if I go into my BGP routing instance and configure "default-information originate", and then if I do "show ip bgp neighbor RouterB advertised-routes", I see a default route being pushed to RouterB.  But, the tech that manages RouterB says he doesn't see a default route come in from my L3 switch.  If I configure a static 0.0.0.0 route on my L3 switch and then do "redistribute static" in my BGP routing process, RouterB starts to pick up the default route from my L3 switch but at the same time, I create a routing loop between my L3 switch and RouterA and so I lose my Internet connection.

I'm hoping someone can help me sort this out.

Thank you!

Daniel

[Meheretab Mengistu]

Hi,

Please try "neighbor x.x.x.x default-originate".

HTH,

Meheretab

[dano2112]

Hi Meheretab,

Yes, I forgot to mention in my original post that I did try "neighbor x.x.x.x default-originate" but after configuring that and then doing "show ip bgp neighbor x.x.x.x advertised-routes", I did not see the default route 0.0.0.0 being sent over to RouterB.

[Meheretab Mengistu]

If you did not see the default route 0.0.0.0 being sent over to RouterB, could you check whether BGP community (no-advertise or no-export) is configured somewhere.

HTH,
Meheretab

[Jon Marshall]

If you see a default route being advertised then router B must be dropping it for some reason. 

You haven't said if these peerings are EBGP or IBGP so can you provide a few more details ? 

In terms of the static route what is the next hop you are using ?

Jon

[dano2112]

Hi Jon,

The guy that manages router B told me that he is allowing 0.0.0.0 in his prefix list but since I can't really see his router, I just have to take his word for it.

BGP is not really my forte so I'm doing my best to get up to speed on it. If I do "show ip bgp neighbor <RouterA_IP> received-routes", the majority of the routes I see listed have "i" under the Origin column so I'm going to assume that means this is iBGP. Some of the routes have "?" listed under Origin so I'm not sure about those. Is there an easier way to tell which one is being used? Please excuse my ignorance!

If I let BGP send the default route info from RouterA to my L3 switch, the source of the route is x.x.17.2, which is the RouterA IP. When I configured the static route on the L3 switch, I just did "ip route 0.0.0.0 0.0.0.0 x.x.17.2" and then configured "redistribute static" under my BGP process.

[Richard Burts]

Jon

When I read the original post I assumed that router A and router B were running EBGP with ISPs and were running IBGP with the switch. And the symptoms were make a lot of sense if that is the case. But after reading your post and re-reading the original post I realize that we do not know what the BGP relationships are. Perhaps the original poster can clarify?

HTH

Rick

[dano2112]

Richard,

My apologies; I needed Jon' assistance in determining the BGP relationships but after reading Jon's reply, I have determined that everything is eBGP (the AS numbers for my L3 switch, RouterA, and RouterB are all different).

[Jon Marshall]

The "i" does not  mean IBGP. 

On your router in the configuration will be a "router bgp <AS number>" 

Also you will have neighbor statements for the other routers eg.

"neighbor <router A IP address> remote-as <AS number>"

If the AS number in the "router bgp ..." statement is the same as the AS number in the neighbor statement it is IBGP, if it is different it is EBGP. 

Jon 

[dano2112]

Jon,

Got it! Again, sorry that I am not more up to speed on BGP.

So to answer your question, my AS number is different than the number being used for both RouterA and RouterB so based on your description above, this would all be eBGP.

[Jon Marshall]

Sorry, just to clarify, router A and router B are using different AS numbers as well ie. each router is using a different AS ? 

Jon

[dano2112]

Jon,

Yes, my L3 switch, RouterA, and RouterB are all using different AS numbers.

[Richard Burts]

If router A is in a different AS and router B is in a different AS then I wonder why you want to advertise the default route to B. That will make you a transit network between A and B. Which makes me realize that we do not know anything about the relationship you have with A or B. Can you clarify this?

HTH

Rick

[Jon Marshall]

Thanks and as Rick says no need to apologise BGP is not something you use on a daily basis unless you are an ISP really. 

If all this is EBGP and you can see the default being advertised to router B I would come back to the original point, router B must be dropping it from some reason. 

If they are filtering the routes received it sounds like they are blocking it. 

Jon

[Richard Burts]

In reading back through the thread I want to come back to this part of one of the posts

if I do "show ip bgp neighbor <RouterB_IP> advertised-routes", I see the following at the top of the output:

Network Next Hop Metric LocPref Path Origin
------------------- -------------------------------- ---------- ---------- ------------- ------
0.0.0.0/0 <RouterA_IP> 65400... ?

If router B does not have a valid route to <RouterA_IP> then B would (appropriately) drop the advertised route. So you do need to check with the administrator of B and check to be sure that they do have a route for that destination.

HTH

Rick