CCIE - BGP question (2)

Author

BGP question (2)

rootstock

2001-08-12, 3:57 am

a-b-c-d
a and c run ibgp, b run igp, a b c in as 1
c and d run ebgp, d in as 2

d has a loopback 192.1.1.1 could not be ping by a
a can learn 192.1.1.1 bye show ip route and sh ip b
the next hope is itself with can reach
d can ping serail of a

how can i thinking of the problem, I and in sea!

LANlord

2001-08-15, 12:44 pm

Sounds like you may want to try the command "ebgp multihop." Since you're using a loopback interface, BGP has to go an extra hop to get to the interface. Give that a shot. Good luck!

rootstock

2001-08-16, 8:07 am

A and C are neighbor in same AS
C and D are neighbor in different AS
I seems need not connected A and D as neighbor.

I wanted to survey a route travesing a AS, that is why I don't want to build multihop

I think A can ping D, but maybe I did somethings wrong, could you help me out?

LANlord

2001-08-16, 8:50 am

It sounds like BGP is operating correctly. If A and D are not directly connected, then they shouldn't be BGP neighbors. Neighbors must be directly connected. If you're able to ping from A to D then the route to D was learned from and IGP in AS2. Unless I'm misunderstanding this scenario, BGP seems to be working correctly. Make sense?

rootstock

2001-08-16, 10:15 am

Neighbors must be directly connected£¬ I think so as well, if not, multihop is key,

a-b-c

a and b in as 1
c in as 2
a and b runing ibgp
b and c runing ebgp

I can ping c from b without multihop£¬but if add a router between a and b(just running IGP), every thing changed!!!

thanks for answer

LANlord

2001-08-16, 2:53 pm

If you add a router between A and B, then your BGP neighbors are no longer directly connected. You will have to distribute the BGP route into the IGP, or else the router between A and B will not know how to get outside the AS.

dmaftei

2001-08-17, 11:17 am

quote:
Originally posted by rootstock
Neighbors must be directly connected
...
but if add a router between a and b(just running IGP), every thing changed!!!

If you add a router between a and b you get:

a--X--b--c

with a, X and b in AS1, and c is AS2, right? In this case a, X and b are all internal neighbors, and they have to be fully meshed:
a must be neighbor with X and b,
X must be neighbor with a and b,
b must be neighbor with a and X.

Note that iBGP neighbors need NOT be directly connected. Also note that the recommendation to use loopback addresses for neighbor IDs refers to iBGP; the recommended way to connect eBGP neighbors is directly, and use the connected interface address for the neighbor ID, like this:

b(ser0)----(ser0)c

routerb
...
interface serial 0
ip address 192.168.0.1 255.255.255.0
...
router bgp 1
neighbor 192.168.0.2 remote-as 2
...

routerc
...
interface serial 0
ip address 192.168.0.2 255.255.255.0
...
router bgp 2
neighbor 192.168.0.1 remote-as 1
...

With your setup (a, X, b and c all running BGP), you don't even need an IGP in AS1.

HTH

LANlord

2001-08-17, 12:17 pm

While all of that is true, the scenario stated that the router that was added (router x in your description) is only running an IGP and not BGP. So, your last statement doesn't apply in this scenario.

rootstock

2001-08-17, 7:57 pm

quote:
Originally posted by LANlord
If you add a router between A and B, then your BGP neighbors are no longer directly connected. You will have to distribute the BGP route into the IGP, or else the router between A and B will not know how to get outside the AS.

what you said I inconsiderated, I know this thoery but careless£¬I should map a and d as neighbor or did a reflect that should be work

one more thing, a and c as nighbor throughout b which running igp, is it not the not the directely connected?

rootstock

2001-08-17, 8:03 pm

dmaftei, thanks for answer, but I don't think X can be a neighbor with a and b , because x only running IGP, and about using loopback interface between EBGP, you are right, I should do this way, but I got a little lazy, it needs input extra extra command ..........

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