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Help reqd in BGP config


we hve bgp peering with 2 diff isps .ISPA/ISPB.

our main goal is to have load balancing,redudancy.

we have configured the routers to meet the same conditions.but right now the load balancing is happening but reduandcy is not working i.e., if one link goes down out of 2 our browsing strucks up.

i hve pasted the current config out here.

pls take sometime to go thru it and advice..

router bgp selfAS

no synchronization

bgp log-neighbor-changes

network s.s.212.0

network s.s.213.0

network s.s.214.0

network s.s.215.0

network s.s.216.0

network s.s.217.0

network s.s.218.0

network s.s.219.0

network s.s.252.0

network s.s.253.0

network s.s.254.0

network s.s.255.0

neighbor a.a.a.a remote-as isp2AS

neighbor a.a.a.a ebgp-multihop 5

neighbor a.a.a.a update-source Loopback0

neighbor a.a.a.a weight 1000

neighbor a.a.a.a soft-reconfiguration inbound

neighbor a.a.a.a distribute-list 6 in

neighbor a.a.a.a route-map bi out

neighbor remote-as isp3AS

neighbor b.b.b.b remote-as isp3AS

neighbor b.b.b.b soft-reconfiguration inbound

neighbor b.b.b.b distribute-list 20 in

neighbor b.b.b.b route-map bbbb out

no auto-summary


ip default-network b.b.6.0

ip default-network b.b.1.0

ip default-network a.0.0.0


ip access-list standard bbbbb

permit s.s.219.0

ip access-list standard bbbbpill

permit s.s.255.0

permit s.s.254.0

permit s.s.253.0

permit s.s.215.0

permit s.s.214.0

permit s.s.213.0

permit s.s.212.0

permit s.s.218.0

permit s.s.217.0

permit s.s.216.0




access-list 6 permit a.0.0.0

access-list 6 permit a.0.0.0

access-list 20 permit b.0.0.0

access-list 20 permit b.b.1.0

access-list 20 permit b.b.2.0

access-list 20 permit b.b.6.0

access-list 21 permit b.b.b.b

access-list 21 permit b.b.b.b


route-map bbbb permit 40

match ip address bbbbpill


route-map bbbb permit 50

match ip address bbbbb

set as-path prepend selfAS selfAS selfAS


route-map Check permit 25

set as-path prepend selfAS selfAS


route-map bi permit 30

match ip address ispa




Re: Help reqd in BGP config

Your configs look fine--if you're load sharing, then both links, and both BGP sessions, are working fine. When you have one link failed, do new sessions work? Do the routing tables look right with one link failed?

It could be that when you fail one link, a given session (HTML browasing session?) happened to be going across that link, and the browser may not like the lenght of time it's taking for the network to converge on the single link, or TCP may be resetting, or something along those lines. Could you describe what you mean when you say a browser gets "stuck"?


Re: Help reqd in BGP config

Hi russel

c once my ispa fails all the incoming as well as the outgoing shuld go via ispb which is not happening at present.i m unable to login yahoo and some(notall) international sites when ispa fails.

wht may be the problem ??




Re: Help reqd in BGP config

Have you coordinated this configuration with both ISPs to ensure that they're accepting all the prefixes you're announcing? If some sites work via ISP-B but most don't, this could indicate that ISP-B isn't announcing your prefixes to all of its neighbors for some reason.

I'd suggest initiating a failover to ISP-B and then running some tests to get a better idea of what is happening. Tracerouting to various destinations might be helpful to determine where the problems are occuring. (Are the traceroutes stopping at your router or somewhere in ISP-B's network?) Getting ISP-B involved in the troubleshooting may also be helpful. They'll be able to check their routers to see if your routes are propagating correctly.

New Member

Re: Help reqd in BGP config

prem, I am having teh same problem. Were you able to figure out what's wrong? vikrant

New Member

Re: Help reqd in BGP config

what does the network look like when the link to your primary ISP is failed ? Do you still have the full BGP routing table, is the next hop address correct ? If you log onto an internet route-server does your prefix appear in the routing table ?

Re: Help reqd in BGP config

thks all for u r replies.

i checked with ispb with whom we r creating neigbhoship with our WAN IPs(with ispa we r doing with loopback) as per them they r announcing all the routes to us.

but still i m unable to fix it up.i cant do any experiments with the live n/w will do it in off peak and let u know edwards.


Re: Help reqd in BGP config

There are two possibilities here.

1. Your upstream ISP may have problem because you advertise your network as /24 (maybe one of them aggregating it for you, the other did not). Try aggregating them in your core router. Add the following lines in your BGP configuration

aggregate-address s.s.212.0 summary-only

aggregate-address s.s.252.0 summary-only

2. I see that you have eBGP multihop, does the connection problem surface only when you left with your eBGP connection with ISP2AS. eBGP Multihop peering if successful will receive all BGP route but will not be able to reach the destination if the routers between you and the ISP2AS does not have Internal routing protocol to reach those destinations. I have experience this in both Lab and Production.


YourRouter>R1>R2>R3>R4>ISP2AS = 5 eBGP Multihop, right?

If using static route,

R1 to R4 should know that (the Internet) is via ISP2AS.

R4 to R1 should know that s.s.212.0/21 and s.s.252.0/22 is via YourRouter


ip route BGP_IP_OF_ISP2AS R1


ip route R2

ip route s.s.212.0 YourRouter

ip route s.s.252.0 YourRouter


ip route R3

ip route s.s.212.0 R1

ip route s.s.252.0 R1


ip route R4

ip route s.s.212.0 R2

ip route s.s.252.0 R2


ip route ISP2AS

ip route s.s.212.0 R3

ip route s.s.252.0 R3


ip route BGP_IP_OF_YourRouter R4


Re: Help reqd in BGP config

One thing you can do is to check one of the looking glasses to see how your routes look when the first ISP goes down. In fact, I would check the looking glass and see what it looks like normally, then check it in failure mode, to see what the difference is.

Is a place where you can search various exchange point and provider BGP tables to see what your route looks like as they see it.


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