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Community Member

Rerouting BGP connections

Hi All,

I'm kinda new to BGP and still learning the kinks of the technology, hopefully you can help me with a little problem I'm having at the moment.

Currently my connection is using BGP through a DS3 link that is running at 45 Mbps. I have a Metro-E connection that is 30 Mbps that I want to use as the primary link going out from my AS. My DS3 link will function as a backup BGP link from then.

Currently I have both links configured for BGP and set them as BGP neighbors but the traffic won't swing over to the Metro-E link (both links are from the same provider, hence both are from the same AS).

Do I need to change the weights or local preference to get the traffic to swing over? I haven't had the chance to try this out yet since I don't have a proper lab to test this out, but any suggestions are definitely welcome.

Thank you all for your kind help!

WabukiSensei

2 ACCEPTED SOLUTIONS

Accepted Solutions
Community Member

Re: Rerouting BGP connections

Hi WabukiSensei ,

In order to filter the incoming routes , you may prefer to use localpreference in order to influence the outbound traffic.

In order to use neighbor X as the first outbound link , you can use the following configuration.

route-map X-INBOUND

set local-preference 120

router bgp MY_ASN

neighbor X route-map X-INBOUND in

After using this config , you can remove the inbound distribute-list from neighbor Y .

This will influence your outbound traffic only. If you experience some problem for the inbound traffic , you may think of using the MED option , too.

HTH,

Kerem

Hall of Fame Super Gold

Re: Rerouting BGP connections

WabukiSensei

Kerem and I have both suggested using local preference to favor the routes learned over the Metro Ethernet. But that will do no good until you fix the issue with your distribute list. The significant parts of the config are:

neighbor Y distribute-list bgp-from-ISP in

ip access-list standard bgp-from-ISP

permit 0.0.0.0

deny any

This filters out all routes except the default route. You need to remove the distributre list to have the local preference make any difference.

note: now that we know that both links terminate on the same router we know that it would be possible to use weight to differentiate paths learned from x and from y. So local preference or weight could either work for you.

HTH

Rick

5 REPLIES
Community Member

Re: Rerouting BGP connections

Hi WabukiSensei,

I would like to clarify if you are having both of your uplinks to the Service provider from one router or not ?

In theory , (assuming that both of your uplinks are present on the same box) once you lose your neighborship , the other link must takeover and traffic should flow. If you announce your networks correctly and receive the correct routes from the service provider , everything should be fine.

For the inbound traffic coming towards your network , please check that you are advertising the correct networks to your Service provider by using " sh ip bgp nei X advertised-routes " . For both of your eBGP neighbors , you must see that you are advertising correct networks.

For the outbound traffic leaving your network , please verify that you are receiving the correct networks from both of your uplinks by using the command " sh ip bgp nei X routes " . Once you see that you are receiving correct routes , please check the local preference and metric of the incoming routes.

If you see that you are advertising correct networks , and you are receiving correct networks , but you are still not sending/receiving any traffic after the switchover , please talk to the administrators of the Service provider to check for any filters applied towards your network.

HTH

Hall of Fame Super Gold

Re: Rerouting BGP connections

WabukiSensei

We might be able to give better advice if we knew more detail about your topology and the deatil of the router configuration. But based on the description provided I would advise that you should configure local preference on the neighbor on the MetroE link to prefer this neighbor.

HTH

Rick

Community Member

Re: Rerouting BGP connections

Hi all,

Thanks for the advice thus far. Yes, both the DS3 and MetroE are on the same router.

Let X = MetroE neighbor

Y = DS3 neighbor

I did a show ip bgp nei X advertised-routes and I am indeed advertising all my routes to neighbor X.

When I did a show ip bgp nei X routes, all incoming routes are coming from the MetroE but when I did it for the DS3 neighbor, my bgp default route appears, which I think is why outbound traffic is going through DS3 instead of MetroE though I could be wrong on this.

router bgp MY_ASN

no synchronization

redistribute static

neighbor X remote-as ISP_ASN

neighbor X password 7 password

neighbor X version 4

neighbor X soft-reconfiguration inbound

neighbor Y remote-as ISP_ASN

neighbor Y password 7 password

neighbor Y version 4

neighbor Y soft-reconfiguration inbound

neighbor Y distribute-list bgp-from-ISP in

no auto-summary

ip access-list standard bgp-from-ISP

permit 0.0.0.0

deny any

Do I need to change my distribute-list in anyway as well since it only applies to the DS3 neighbor at the moment?

Oh and both neighbors are from the same AS.

No local preferences have been set on either side of the connection yet.

Thanks again and sorry if I've confused you folks.

Community Member

Re: Rerouting BGP connections

Hi WabukiSensei ,

In order to filter the incoming routes , you may prefer to use localpreference in order to influence the outbound traffic.

In order to use neighbor X as the first outbound link , you can use the following configuration.

route-map X-INBOUND

set local-preference 120

router bgp MY_ASN

neighbor X route-map X-INBOUND in

After using this config , you can remove the inbound distribute-list from neighbor Y .

This will influence your outbound traffic only. If you experience some problem for the inbound traffic , you may think of using the MED option , too.

HTH,

Kerem

Hall of Fame Super Gold

Re: Rerouting BGP connections

WabukiSensei

Kerem and I have both suggested using local preference to favor the routes learned over the Metro Ethernet. But that will do no good until you fix the issue with your distribute list. The significant parts of the config are:

neighbor Y distribute-list bgp-from-ISP in

ip access-list standard bgp-from-ISP

permit 0.0.0.0

deny any

This filters out all routes except the default route. You need to remove the distributre list to have the local preference make any difference.

note: now that we know that both links terminate on the same router we know that it would be possible to use weight to differentiate paths learned from x and from y. So local preference or weight could either work for you.

HTH

Rick

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