05-24-2008 07:05 AM - edited 03-03-2019 10:04 PM
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
05-24-2008 07:15 AM
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.
05-24-2008 06:03 PM
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
05-24-2008 08:12 PM
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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05-24-2008 09:24 PM
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
05-25-2008 01:42 AM
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.
05-25-2008 07:04 AM
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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