Asymmetric BGP Setup

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Oct 27th, 2003
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Hi,

I am wondering if there is anybody who can give me some pointers on how to setup BGP asymmetrically.

I need to do it as my company is providing DVB services to our customers and they wanted to do BGP with us. As BGP is symmetric in nature, meaning , coming and going via the same interface while DVB is only a simplex connection, therefore I am wondering how cna I go about doing it.

I am thinking of using route-map to state the next hop ip but just have no idea how is the other end going to communicate BGP with me.

Any comment would be appreciated.

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rais Mon, 10/27/2003 - 08:25
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BGP turns out to be asymmetric most of the time. We force it symmetric by advertising prepended AS. You can go symmetric/asymmetic by tuning your AS-path lengths.


Hope this helps.

t.baranski Mon, 10/27/2003 - 14:24
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We'll probably need some more information on what you're trying to do, how the two networks connect, etc. Standard routing will work regardless of whether or not the traffic is unidirectional. If your concern is that only the DVB traffic flow over a given interface, you can possibly use policy routing with a route-map that matches on a port number and destination network. But again, it depends on what the requirements are.

crisisman Mon, 10/27/2003 - 17:57
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Dear All,

Thanks for the interest in my question.

Let me illustrate it with this diagram:

My_BGP_router<->My_layer_3_switch->DVB_encapsulator->customer_downstream_provider_router<->customer_BGP_router<->customer_upstream_provider_router<->internet

What is happening here is that my router, My_BGP_router, is suppose to do BGP with customer_BGP_router. Both customer_downstream_provider_router and customer_upstream_provider_router are 2 different ISPs and customer_BGP_router wanted to do BGP using private AS number. Please note the arrow sign. <-> indicate there is a return path, while -> means there is only 1 path in that direction.

As can be seen, the no. of hops from my_BGP_router is about 3 but I have no idea how many hops are there from customer_BGP_router to me, as I need to get his advertisement of IP address. I can use a route-map to set the next hop ip to point to customer_BGP_router but how can I get his advertisement is one question that is puzzling me. Also I am wondering would his private AS number affect the advertisement process.

Any assistance would be appreciated.


ruwhite Tue, 10/28/2003 - 05:28
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If you're talking about running BGP over a unidirectional link, you can't.... It requires a two way connection to make it work (BGP is based on a TCP session, like an HTTP connection is). The only way to resolve this would be to have some sort of "backchannel" link that the BGP session could run over, and set the next hop in the advertisements so they point down the unidirectional link for routing out towards the customer.


The second issue is that proviate as number. I assume they aren't running BGP with their service provider, or if they are, they are using a private as number that the service provider then strips out of the as path. It shouldn't make any difference what their as number is, as long as you are not advertising their routes back out to anyone else, or you are stripping out the private as numbers from the as path before readvertising the routes.


You should be able to use eBGP multihop to peer with them through another service provider, if you need to, although that can get a little messy.


Russ.W

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