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BGP Path Selection

Greetings, 

Long time reader first time poster, my question for the group is that of a could you and if you can how do you this? Below is a quick abstract illustration of what I'm working with, so:

  1. I need to reach the host AS60 from AS10
  2. AS60 has manipulated best path selection via as-prepend for the AS60 network in order to favor the AS30 link over AS40/50.(AS50 being the default best path) 
    1. So, the path selection would be from my AS10 host> AS10>AS20>AS30>AS60> AS60 Host
  3. For performance reasons I need to be able to use send my packets down the AS50 link to AS60 
    1. Path would be my AS10 host> AS10>AS20>AS50>AS60> AS60 Host

That all said, AS60 will not be able to change their as-prepend, so my question is there anyway I can work with my AS20 carrier in this case, to move my packets down AS50 rather than AS30?

Thanks, Phil

2 Accepted Solutions

Accepted Solutions

chrihussey
VIP Alumni
VIP Alumni

So if AS20 were to set the local preference of the AS60 route coming from AS50 to a higher value, it would prefer the path through AS50. The pecking order for choosing a BGP path is:

1- Highest weight

2- Highest local preference

3- Prefer the path that was locally originated via a network or aggregate BGP subcommand or through redistribution from an IGP.

4- Shortest AS-Path

View solution in original post

if your AS are public AS's....

Any good carrier has bgp policy.

for example

https://apps.db.ripe.net/search/lookup.html?source=ripe&key=AS12389&type=aut-num 

so you  can add described community and "ask"  carrier to do some as-path modifications for you.

View solution in original post

16 Replies 16

Jon Marshall
Hall of Fame
Hall of Fame

Phil

Someone else may have a solution but as far as I am aware there is no way to do this because all traffic to the remote host has to go through AS 20 before a decision is made as to which path to use and there is no way you can modify any attributes etc. to tell that carrier which path you want to use.

That is not to say they could not do something for you, because they may be able to, just that there is no way with BGP to inform them which way you want the traffic to go.

Like I say perhaps someone else has a possible solution.

Jon

chrihussey
VIP Alumni
VIP Alumni

So if AS20 were to set the local preference of the AS60 route coming from AS50 to a higher value, it would prefer the path through AS50. The pecking order for choosing a BGP path is:

1- Highest weight

2- Highest local preference

3- Prefer the path that was locally originated via a network or aggregate BGP subcommand or through redistribution from an IGP.

4- Shortest AS-Path

They could also just use weight assuming it was a Cisco router I suppose.

The key point is though that there is no way to dynamically tell the carrier to do this ie. you would have to arrange for it and if you then wanted to switch back to the other path again you would need to talk to them about changing it back.

I think Phil was looking for a way with BGP to inform the carrier which path to use but I could well be wrong :)

Jon

I chose local preference since it implied an AS as opposed to a single router. However, if it was a single AS20 router that connected to both AS10 and AS50, then weight would be best as it would not affect the rest of the AS.

Agree about the lack of dynamic nature and requiring having to work with the AS if you wanted it implemented and to  switch it back.The AS may not also agree either as it would affect all routing and not just the one user.

It's just a way to accomplish what was being asked.

Good point about why to use local preference as it makes no assumptions about the carrier setup.

Jon

local preference is not an option in this case.

AS20 will choose the best path to AS60 and will announce this route to AS10.

probably, you can build a tunnel from AS10  to AS60, and for tunnel's destination ip use ip address on the link between AS60 - AS50

It is an option for AS 20 which is what is being suggested.

Jon

a.alekseev/Jon, 

I'm trying to become familiar with public ISP community policies, some carrier have them clearly stated while other don't, or at least don't have it available for public viewing. 

These guys seem to have the closest summary of Regional/National/Global Carrier community policies.

https://onestep.net/communities/

The list seems to be complete, a few the names like Qwest is now Century Link, TW is now part of Level3. But still seems like a good starting point. Any other suggestions?

whois -h whois.radb.net asXXXX
or you can try to ask them about their policy by e-mail which you can get from from AS query on http://radb.net/

if your AS are public AS's....

Any good carrier has bgp policy.

for example

https://apps.db.ripe.net/search/lookup.html?source=ripe&key=AS12389&type=aut-num 

so you  can add described community and "ask"  carrier to do some as-path modifications for you.

It's funny you should mention the tunnel option because I have done this in order to reach AS60 via the AS50 carrier. The tunnel originates from AS10 host gw router and is backhauled via a private line that travels a few thousand miles and then exit's out the AS50 carrier connection. It works,however, the private line is 100mb/s where the illustrated paths above have speeds ranging from 10gb/s,40gb/s, and yes 100gb/s. So, I'd like it to go out the at least he 10gig link :-) 

Thanks for the replies, 

Jon, yes I was looking for a way of achieving this transparently, but Chrishussey's suggest maybe the only choice I have. Since AS20 is my carrier I believe I can make the request, however one other item I forget to mention, and is not too big of a deal for me to adjust, but that  is that the AS10 host sits on a IP subnet that is a /29 or /27 which is statically routed. So, correct me if I'm wrong here, but I'll have to move that host over to a /24 network which I advertise via BGP. 

Thanks, Phil

All the ASs in the path are going to need to know about the /29 subnet so unless your carrier is using a static route pointing to your  AS for that route and advertising it to the other ASs then yes you would need to advertise it yourself.

Again difficult to say without knowing the full setup.

Jon

Yeah, the other ASs will only hear minimum of a /24, no biggie, I just have to move the host over to the /24. 

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