BGP multihoming - Incoming traffic issue

Unanswered Question
Aug 25th, 2008

We have two internet link from two ISP , configured in multihoming.Expecting outgoing & incoming from primary link only.But it is observed that traffic is coming via both link.

Secondary link path we have already prepend.Local pref is also configured.

I have this problem too.
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Richard Burts Mon, 08/25/2008 - 04:14

gajanan

first let us clear up your comment about having set local preference. Setting this will influence which peer you use for outgoing traffic, but has no effect on incoming traffic.

then let us address the use of prepending. Let us understand that you have only limited control about how traffic comes to you from the Internet when you are multi homed. You can prepend and for some sources in the Internet it will make the "primary" peer more attractive and they will send traffic the way that you wish. But some sources may continue to view the "secondary" peer as attractive. You might add additional prepending, and it might influence some of that traffic. But you may also need to accept that for some sources the path through the "secondary" peer will continue to be more attractive and that you may have some incoming traffic on the "secondary" peer.

HTH

Rick

gajanangavli Mon, 08/25/2008 - 04:30

Thanks Rbusrts ,

but we have similar setup which is working fine only this location is facing problem.

Diagram is simple , both links are connected on single router.

FYI

router bgp ------

no synchronization

no bgp log-neighbor-changes

network A.B.C.D

neighbor W.X.Y.Z remote-as 4755

neighbor W.X.Y.Z password 7 ------------------

neighbor W.X.Y.Z soft-reconfiguration inbound

neighbor W.X.Y.Z prefix-list Mycompany in

neighbor W.X.Y.Z route-map primary in

neighbor P.Q.R.S remote-as 9498

neighbor P.Q.R.S soft-reconfiguration inbound

neighbor P.Q.R.S prefix-list Mycompany in

neighbor P.Q.R.S route-map Backup out

no auto-summary

ip as-path access-list 2 permit ^9498$

ip as-path access-list 3 permit ^4755$

ip prefix-list Mycompany seq 5 permit0.0.0.0/0

ip prefix-list TM-VSNL seq 6 permit A.B.C.D/24

route-map Backup permit 10

match as-path 2

set local-preference 125

route-map Backup permit 10

set as-path prepend --- ---- ---- ---- --- ---

route-map primary permit 10

match as-path 3

set local-preference 150

tdrais Mon, 08/25/2008 - 04:24

I am assuming you mean you have configured as-prepend outbound and local preference inbound on your secondary ISP. To a point this will work but like you other people are also doing this. If someone has the same 2 isp but want to do the opposite of you then either them or you will get the traffic in the wrong path.

You can never get 100% doing as-prepend. This is all related to business peering relationships between the ISP. They have all kinds of rules about how much traffic from which partner ISP can flow where.

The only other method is to use conditional advertisement. This means you do not advertise your prefixes to ISP 2 until ISP 1 goes down. This method tends to be slower than prepending and does not as well protect against issues that are more distant in the network.

Unless you pay for traffic on the backup or you are overloading the link it should cause no issue using it the way you have it. I generally I let BGP do its thing and only try to force the path when I have a problem.

... Type to slow but at rick says the same

gajanangavli Mon, 08/25/2008 - 21:12

Hi Tim ,

Can anything that i do with configuarion ?

we have similar confugration and set up with no problem but at other location.

I have already raised the ticket with ISP , but they are helpless.

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