BGP and Prefix-list

Unanswered Question
May 24th, 2008

guys just a quick one.........suppose there are three sites A,B,c all are connected via bgp and have seprate AS's.

now if site A is advertising site B network suppose 192.168.1.0/24. so site b is connted via BGP with c as well the route will be in site B table.....so that specific route would be advertise to site C by B by default as they are connected as well via BGP(keeping in mind that site c doesnt need to get this route)? if yes why if not why??? is this why we use prefix-list?? ic ant get my head around it. Thanks guys in advance

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Edison Ortiz Sat, 05/24/2008 - 07:15

now if site A is advertising site B network suppose 192.168.1.0/24.

Site A owns the route in BGP

so site b is connted via BGP with c as well the route will be in site B table.....so that specific route would be advertise to site C by B by default as they are connected as well via BGP

Site B will have the route as "connected" on its routing table but it isn't advertising this route into BGP per your description.

Just because this route is "connected" and it's a preferred route for Site B, does not mean it will be advertised in BGP.

For Site C to reach 192.168.1.0/24, it will go to Site A, then Site B.

Now, the return traffic may go from Site B directly to Site C (asymmetrical routing). It all depends what networks are advertised from Site C into BGP.

HTH,

__

Edison.

The_guroo_2 Sat, 05/24/2008 - 18:03

Thanks for that well how come site C will go to site A directly as they are not conected.....secondly first you said that the route will be advertise by default then u have a statement

"Just because this route is "connected" and it's a preferred route for Site B, does not mean it will be advertised in BGP"

Sorry i didnt get that. Now i have tried this scenario in lab at my home but it didnt advertise which is bit starnage......kindle see my tapologu and configs.

Router A

int serial 0/0 ip address 195.1.1.1 255.255.255.0

router Bgp 123

network 192.168.1.0 mask 255.255.255.0

neibour 195.1.1.2 remote-as 345

Router B

int serial serial 0/0 ip address 195.1.1.2 255.255.255.0

router bgp 345

neigbour 195.1.1.1 remote-as 123

now by doing that the router B got 192.168.1.0 route in his table now router B configs for zrouter c are:

router B

router bgp 345

neibour 196.1.1.2 remote as 555

Router C

router bgp 555

neibour 196.1.1.1 remote-as 345

so after that all bgo sessions were up but i couldntt see the route (192.168.1.0) route in C routing table or bgp table???? why is that as u said that by default b will be sending it to c....thanks

Edison Ortiz Sat, 05/24/2008 - 20:12

Thanks for that well how come site C will go to site A directly as they are not connected.

Based on your original post:

a) They are neighbors

b) Site A is advertising 192.186.1.0/24, not Site B.

secondly first you said that the route will be advertise by default then u have a statement

"Just because this route is "connected" and it's a preferred route for Site B, does not mean it will be advertised in BGP"

Sorry i didnt get that.

You said default, I didn't. The Bold portion of my post are quotes from your post.

I reiterate, in order for a route to be on BGP, you need to advertise such route under the BGP process. A connected route won't be advertised in BGP by default. You need to use the network command or redistribute static. Per your post, only Site A is advertising the route so Site C will go to Site A as that location is the only one advertising the route via BGP.

so after that all bgo sessions were up but i couldntt see the route (192.168.1.0) route in C routing table or bgp table????

Verify 192.168.1.0 is in the routing table on Router A. It needs to learn that route from Router B in order to install it in BGP. This process is often done either with a static route in Router A or an IGP (Interior Gateway Protocol) running between Router B and Router A. On your original post, I assume that task was completed.

HTH,

__

Edison

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sdoremus33 Sat, 05/24/2008 - 21:24

You can also determine which routes are bein g adversied by BGP by using the show ip bgp neigh x.x.x.x received routes. This tells you what routes are being learned on B via BGP. HTH

The_guroo_2 Sun, 05/25/2008 - 01:42

Thanks for the posts guys.....now i will re phrase my question......site A is connected to site B and site C is connected with B. site C is not connected with site A. Now my question is if router A advertise a network to router B. Now router B has that route in its bgp table........router B is not advertising that route to c under network command. so will router C get that route automatically as router b and c are connected via BGP??? guys i wanna know the behavior of BGP??? i hope this is much better way to ask.

Edison Ortiz Sun, 05/25/2008 - 07:04

Now my question is if router A advertise a network to router B. Now router B has that route in its bgp table........router B is not advertising that route to c under network command. so will router C get that route automatically as router b and c are connected via BGP?

Yes, router C will receive that route by default.

__

Edison.

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