BGP Problem

[Reyad Safi]

Hello Experts

i have a problem on my external routers .

i have 2 internet links from 2 different ISPs , i have also 2 cisco 7200 routers connected to the 2 internet link .

i have also my own AS number configured on both routers , and the first router has ebgp with ISP1 , and the second router has ebgp with ISP2 , and the 2 routers have ibgp in between .

when i disconnect one ISP , the internet rerouted to the second ISP automaticlly , but when i disconnect the link between the 2 routers , the internet goes down .

the 2 routers connected with each other through DS3 link .

any idea to guide me to solve this problem .

thanks in advanced .

Reyad

[Marwan ALshawi]

the only possible cause i can think about is that your internal traffic is going to one router and then re router over the DS3 link to the other router and up to the Internet

once you disconnect this link you getting this issue

however it is not supposed to be like this and again this is assumption

try to disconnect the DS3

check each of your routers routing table and verify if it has a default route point to the directly connected ISP

also check which router you internal LAN sending the traffic to

hope this help

[milan.kulik]

Hi,

what do you exaclty mean by "...the internet goes down"?

All your users are losing access to the Internet totally?

Or just some users to some Internet targets only?

As you say "the 2 routers connected with each other through DS3 link", I suppose each of them being in other site?

IMHO, you might be getting an asymmetric routing in your topology easily:

Users on one site might be using one router to forward traffic to the Internet,  while returning traffic (from some part of the Internet at least) might be taking the second router to enter your AS?

In that case, breaking the connection between your routers might make your users to lose connectivity to a part of the Internet...

(While in a case of diconnecting one ISP, both outgoing and incoming traffic to/from the Internet has to take the other router as an entry point.)

HTH,

Milan

[Reyad Safi]

Dear Marwan & Milan

thank you for reply

i put my topology below.

there's no users connected to the 2nd router . it's used only to connect the 2nd ISP and for ADSL servies for staff internet access .

i notice that , the out going traffic go through the 1st ISP , while the incoming traffic through the 2nd ISP while i put a static routes with equal metric to the both ISPs .

[Marwan ALshawi]

Hi Reyad,

you loosing the Internet connectivity when the DS3 link is down because you have asymmetrical routing issue where the traffic go out one path and come back via another path

not sure if you are using BGP with your ISPs ?

if not you need to ask ISP2 to make some changes from there end to make it less preferred

however if you can use BGP with both ISPs then you will have more control for the inbound and outbound path selection

hope this help

[milan.kulik]

Hi,

you can try to avoid the current asymmetric routing by prepending your AS number several times to your prefixes advertised to ISP2. You can also ask ISP2 to make the prefixes received from your Router2 less preferred.

But still you are not absolutely sure - it might not work within the ISP2 AS and his customers, e.g., or some other part of the Internet.

What I would think additionally is:

How to avoid your Router2 to advertise your prefixes in a case the DS3 line fails?

What about removig static routes from the Router2 and advertise to ISP2 only your prefixes received by iBGP from your Router1? In a case of the DS3 line failure your Router2 would not advertise anything to ISP2 then.

In a case of Router1 failure you would lose your Internet connectivity totally - but that happens in your current topology anyway :-(

HTH,

Milan

[msrohman]

I have a similar design. We load balance across two ISPs and also failover one another.

I agree. See if you can have your ISPs advertise the Internet routes to your 7200s. They can probably advertise default routes too. You could either download the entire internet routing tables or a subset. You could then prefer one default route over the other depending on outbound traffic.

Keep in mind that it's up to the IsP on how to route the traffic back your AS. But sometimes you can manipulate that as well with your public AS.

HTH

M

Sent from Cisco Technical Support iPhone App

[Reyad Safi]

thank you all for your reply

im now out of country , i will check when i back ,and if i face a problem i will disturbe you again , thank you

Reyad

[Reyad Safi]

Dear All

Good Day

i asked my friend ( CCIE expert ) about this problem , we discussed that there's some traffic outgoing from the 1st ISP , and back through the 2nd ISP , so when the internal DS3 link goes down , the 2 routers still recieved the incoming traffic , becouse the BGP still up on both of them .

he advised me to configure a tracking and policy based routing to shut down the 2nd ISP bgp link when the DS3 goes down , to force the traffic to outgoing and incoming from the 1st ISP .

i will try to apply the configuration and hope this solve my problem .

i will update you accordingly

Reyad

[Marwan ALshawi · Accepted solution]

yes this is another option

but in fact this is not fixing your problem in best practice way, what i mean here are you happy that you traffic go out of your network via one link and comes back via another link and also this will be confing into ur network using the DS3 while it can be done in a better way if you want to by either discussing this with your ISP2 and make sure they give it less preference to make ISP1 as the primary

or you can use also IP sla to stop advertise your networks if the ISP1 link is up

hope this help