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BGP Multi Homing

alshamlan
Level 1
Level 1

Dear,

I am an ISP that have multi homing with two upsteam provider. I have BGP configuration with both ISP.

ISP - A : send me full routing BGP table and it worked with me fine when the ISP-B disconnected ( for outgoing and incoming traffic)

ISP-B : send me the defualt route 0.0.0.0 only,

when both are connected the outgoing always from ISP-A and incoming from ISP-B. however, whenever i disconnect ISP-A I cannot receive or send any traffic through ISP-B .

apprciate your help and cooperation.

regards

MHS

36 Replies 36

Danilo Dy
VIP Alumni
VIP Alumni

Since you are multi homing, you need to received full route from both ISP. Ask your ISP B to provide you  full route and remove the default route from the advertisement.

Execute ip route google_ip_address and see if its point to your ISP interface IP. If it is;

- you may be receiving full route from ISPB but you may not be advertising your prefix to ISPB correctly

- your ISPB may have ACL in its interface that block your outgoing or incoming

In the first place, why thus ISPB only advertise default route to you? There might be some misunderstanding.

Thanks Danilo,

my ISP-B have only the defualt route from their upstream provider. how we can send the full route if he doesnt have it?

There is no other way to do it?

regards

MHS

I just edited my reply

Since ISPB can only advertise default route to you from his upstream ISP, check with ISPB whether his upstream ISP  know that you are connected to him and you need internet transit. The ISP above ISPB might not know you are connected to ISPB and they may not have open your access to their ACL to ISPB.

Yes the upstream provider of ISP-B knows and they allow my prifex through their network.

I tested through looking glasss and I can see that the traffic is coming through ISP-B, but why I cannot receive anything from ISP-B ( no incoming and no outgoing ) when ISP-A become down.

Thanks

MHS

This means your BGP advertisement is correct as you can see it in looking-glass thru ISPB (and its upstream). The problem here could be ACL from ISPB and/or ISPB upstream. Check with your ISPB at their network and ask them to check with their upstream ISP as well. Check your ACL also to be sure

Btw, thus the router connected to ISPA and ISPB are the same router or different routers? When you test the connection thru ISPB did you use the router that connected to ISPB (i.e. ping google from router connected to ISPB)?

I am working closely with ISP-B and there is no ACL blocking the outgoing traffic.

How we can confirm that I dont have problem in my configuration.

MY show ip bgp out put is

   Network          Next Hop            Metric LocPrf Weight Path

*> 0.0.0.0          ISP-B interface                           0 ISP-B i

after this route there is BGP routes from ISP-A

Show ip route is

B*    0.0.0.0/0 [20/0] via ISP-B interfecae , 00:46:24

after that the BGP route to ISP-A

      1.0.0.0/8 is variably subnetted, 839 subnets, 14 masks

How I can force my traffic to go through ISP-B, I used to shut ISP-A to do it but still as I inform you it didnt work once I shut down ISP-A

I am using one router for both ISPs

thanks

can you execute "show ip route google_ip_address" if it points to ISP-B interface?

Router#show ip route 209.85.227.105

Routing entry for 209.85.226.0/23, supernet

  Known via "MY_AS", distance 20, metric 0

  Tag AS_ISP-A, type external

  Last update from IP-interface of ISP-A 02:03:01 ago

  Routing Descriptor Blocks:

  *IP-interface of ISP-A, from IP-interface of ISP-A, 02:03:01 ago

      Route metric is 0, traffic share count is 1

      AS Hops 3

      Route tag 39273

      MPLS label: none

See from the above output only to ISP-A

kindly advice

regards

MHS

I thought you shutdown your connection to ISPA?

You want me to shut down ISP-A and send the result?

sry, i thought you are doing testring now. you can only do that during non-production time. can you post your configuration? you can PM me if you have concern to post here.

I can shut down the ISP-A no problem if you want.

I prefer to send you configuration through PM.

regards

send me the configuration first.

sent

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