BGP Route Exclusion

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We need to exclude a route (One ISP) when traversing a path, because of high latency. Is there any way user can do it?
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Are you trying to stop your router from learning that route from your BGP neighbor?  If so then do the following.

First you have to see the exact network that you want to block.   To do that perform the following.

show ip bgp neigh <ip of neigh> received-routes

That will show you all the networks that you are recieving from that neighbor relationship.  If there is one you want to block from coming in you will have to build a prefix-list denying that route.

To create the prefix list do the following.

conf t
ip prefix-list <name> seq <#> deny <network/mask>

please note that the network will have to be inputted in CIDR notation (ie  Ensure that you enter the exact network that you want to block as it is being recieved by your router.

Next you have to put that prefix list into your bgp asn.

conf t
router bgp <asn>
neighbor <ip of neigh> prefix-list <name> in

The prefix list name is case sensative.  When entering it into BGP you have to type it exactly as you created it.

After applied re-sync your bgp relationship by doing the following.

Clear ip bgp <ip of neigh> soft

After about a minute  you can do a show ip route and that network should be removed from your route table.  If you do the show command I listed earlier you should no longer see that route as being recieve on your router.  The other end however will be able to see that network being advertised and there will be a RIB failure on their end since you applied that list.  they might notifiy you about that if you do not let them know that you are filtering the route.

There is not a lot of detail in your question so that is the best that I can answer you right now.  If you are not trying to filter a incoming route then please let me know exactly what you are trying to accomplish and I will see if I can help you out.

SSgt Dain
Network Engineer USMC

One more thing.  In order to see that information that you are receiving from your neigh router you have to have soft-reconifguration enabled on that ASN.  To do that do the following.

Conf t
router bgp <asn>
neigh <ip of neigh> soft reconfiguration inbound

Doing this will let you see the specific routes that you are recieving from your neighbors.  If you do not have this set that first show command will not work.


Thanks for your input Romandain, Actaully we route voice traffic in/out of Mexico. One of their ISP is giving us hard time. GlobalCrossing gives them the traffic and they put a big latency/jitter that the voice quality gets completely distorted. We checked it using connection server test tool. The ping stats are over 200ms.

Coming to my questions, yes on the other end mostly will select the blocked route and yes there may be issues.

Actually we have two ISPs to route our traffic, is it possible to configure Mexico dest ( to use second ISP provider with BGP? I know it is separate question that the original..
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You can set up a route map to have your voice network prefer 1 ISP over another using BGP.  Here is a example.

Lets say this.
Your Voice network (your asn 55)
ISP Neigh 1 asn 1
ISP Neigh 2 asn 2

First you have to build a prefix list.

ip prefix-list <name> seq <#> permit

Now you have to build a route map for each ISP

for ISP 1
Route-map <name> per 10
 match ip address prefix-list <name>

For ISP 2
Route-map <name> seq 20
 match ip address prefix-list <name>
 prepend 55 55 55 55 55 55 (your asn)

Now in your BGP put the following.

Conf t
router bgp <asn>
neigh <ISP 1 ip> route-map <name> out
neigh <isp 2 ip> route-map <name> out

What that route map does is that it is advertising your voice network to both of your ISP's still.  However on the one that you want the traffic to go it is still logically 1 hop away.  On the one you want to use as backup you are adding logical hops for each time you type your asn in that prepend statement.  In my example it added 6 hops.  Once this is done your traffic for that network will prefer the path with the least amout of hops as its primary and if it goes down then it will default over to the other path out.

Hopefully that is what you are looking for.


I checked the other US provider also goes thro' Alestra (lossy ISP) and has ping of over 200ms. I am out of ideas at this point. Any suggestions???
Is your first hop outside of your network 200ms return?  If so what kinda of connection do you have.  If it is point to point or tropospheric scatter shot that would be normal.  But if you are running fiber or t1/t3 it should be a lot faster.  

That time is most likely a layer 1 problem and not in your BGP.  If you are cabled in whomever is in charge of that infrastructure needs to looks at their gear.  The only other thing is if they put a delay statement on their router on the interface that you plug into.  But both ISP's doing that doesnt sound right.


Thanks, we may have to go in for change in ISP.

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