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Community Member

BGP routing Question.

I have a simple question. I'm using BGP with two different ISP's. ISPA and ISPB. Because of problems with ISPB, I have a filter in place that forces all traffic to ISPA even though I can still recive BGP updates from ISPB, and it's the one advertised on the internet. For example if I go to an internet router and traceroute to my site, and shows that it is going thru ISPB. But when I go out to the internet, it goes thru ISPA.

Here is the question: I know that when traffic originates from our site, by design (because of the filter.) it goes thru ISPA. But how is that traffic routed back to us? Does it uses the path that it originated from, or the path that's advertised on the internet.

Thanks for the help, and sorry for the long post.

Community Member

Re: BGP routing Question.


u need to give more info on your setup. There could be lot of reasons whereby traffic is influenced to take particular path for reaching ur network.

The simplest explanation may be - you are using IP block from ISPb (assuming you do not have your own regiestered IP block)... and thus all incoming traffic comes in via ISPb from internet.




Community Member

Re: BGP routing Question.

We have our own registered ISP block. I have a BGP filter in place on ISPB that block traffic from routing out of that interface, even though the interface is up and is receiving BGP updates. If a packet originates form somewhere else, I know it takes ISPB's route, because it is the one advertised on the internet. Basically my question was: If I type in my browser, right now it goes out thru ISPA. Because I'm the one initiating that traffic. Does CISCO's router use that same route to get back to us? Basically those same hops in reverse? or does it send it back based on what's in their routers routing table?

Hope that clarified things a little.

Community Member

Re: BGP routing Question.

just out of curiousity, what kind of filter are you using, weight, local preference. But as for your question, return traffic uses its shortest path back, if it can get back to you via isp b quicker it will take isp b, to fix this try as-path prepending to your route when advertising to isp b, this will make isp b's path back look longer to other internet routers


Re: BGP routing Question.

The traffic would be asymmetrical, meaning the forward path wouldn't be the same as the return path. If ISP B is the one advertising your address space onto the Internet, the routers will use ISP B to get back to you. If you wanted to have ISP A take the return traffic, set up MED.

More info on BGP path selection:



Re: BGP routing Question.

Sorry, forgot you had two connections with two *different* ISPs...don't use MED...use the previous posters suggestion-AS path prepending.


Re: BGP routing Question.

Routing is hop by hop--so every ISP along the path chooses the next hop baced on what's in their local routing tables. The traffic could be (and probably is) taking a completely different path there and back.

When you say you have your own registered ISP block, I assume you mean your own address space from one of the registries, not a block you've received from one of your upstream ISPs. If it's a block you've received from one of your upstream ISPs, it could be aggregated through one ISP, and not through the other. Since routing is always longest prefix length match wins, the ISP not aggregating your address space would receive all of your inbound traffic.

So, if your address space isn't being aggregated, you should try as path prepend. It won't always work, but sometimes it will. If one ISP is aggregating, you'll need them to punch a hole in their aggregate to make the as path prepend even start to work.


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