BGP - practical example - best AS path selection.

[Filip Knezevic]

Hello guys!

I have a question. Let's say an organization is multi homed and it's peering with 3 different ISP. One ISP has the best line, it is the premium line. This organization has a client across the globe. they want this client to use only the premium line if possible.

How can I achieve this?

Im thinking thats easier to control traffic on the way out. Should I use local preference in the route-map on the premium peering?

Can I control how the packet will return? Maybe MED or prepend? 

I know this sounds a bit vague, but I would just like to hear some ideas. I will probably do a small lab to check all this, but would like to have some guidelines.

Thanks!!!

[Jon Marshall]

If this is internet connectivity then AS path prepending to influence inbound traffic from the remote client. 

And yes you can use local preference with IBGP at your end assuming multiple routers, if you have all ISPs connecting to same router use weight. 

Jon

[Filip Knezevic]

Thanks Jon! Always here to help :).

First of all, one confusing matter: If all three ISPs (multihoming) provide only a default route to the organization, would any route manipulation in the IN direction be possible or not? Thinking about it, how can we manipulate the routes we don't see? 

I suppose this question was completely idiotic, but I had to ask :(.

Assuming they get the whole routing table, and client across the globe has 1.1.1.0/24 and the organization has 2.2.2.0/24 (with AS 111), what would those route maps roughly look like?

Im thinking for AS prepend something like this:

ip prefix-list TEST seq 10 permit 1.1.1.0/24

route-map TEST

match ip address prefix-list TEST

set ip as-path prepend 111 111 111

neighbor 10.10.10.2 route-map TEST in

My understanding is only 1.1.1.0/24 ips would be prepended but all other traffic would be load balanced? Or go normal ways...

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But I'm a bit confused about the preference. There are two routers, with iBGP between. Since local preference is locally significant, it will affect both routers?

Also, this would be the out direction. That means we will put organization's announced prefix 2.2.2.0/24 in the prefix list. Then we can match it in the route map and give different preference. So local network, or the announced IP addresses from 2.2.2.0 range, will have pref 100 for ISP1, pref 110 for ISP2 and pref 300 for ISP3. ISP3 wins, and everything from 2.2.2.0 towards the internet will go out ISP3. Please correct me if I'm wrong. If this is the case, this won't affect the traffic to the client 1.1.1.0 across the globe, but everything from the organization to the internet?

and the config would be something like

ip prefix-list PREF seq 10 permit 2.2.2.0/24

route map PREF

match ip address prefix-list PREF

set local-preference 120

neighbor 10.10.10.2 route-map PREF out

[Jon Marshall]

Only have phone at the moment so answer will be a bit short, i’ll answer in more detail tomorrow. 

You are applying your route maps in the wrong direction ie. you prepend outgoing routes to affect incoming traffic and you apply local preference to incoming routes to affect outgoing traffic. 

As for local preference it is significant between the IBGP peers ie. the local preference is exchanged between the routers unlike weight which is significant only to the router it is configured on. 

By all means come back for further clarification and I will catch up tomorrow unless someone else jumps in. 

Jon

[Filip Knezevic]

Not going anywhere, John. I will patiently wait. :)

[Jon Marshall · Accepted solution]

Using your example subnets ie. client uses 1.1.1.0/24, main site uses 2.2.2.0/24 and main site has connectivity to ISP1, ISP2 and ISP3.

At the main site you would prepend the 2.2.2.0/24 route (not the 1.1.1.0/24 as in your last post) as you advertise that subnet to the ISPs eg. ISP1 is preferred so no prepending, ISP2 is next so prepend maybe once or twice and ISP3 is least preferred so prepend the most. 

That will affect traffic coming from the client to the 2.2.2.0/24 subnet at the main site. 

For traffic to the client you can use local preference and you apply it to the 1.1.1.0/24 route advertisement received from the ISPs. 

Does that make sense ?

Jon

[Filip Knezevic]

Absolutely. I mixed out the directions! :)

Just another thing. If the ISP 1 and ISP2 are just providing a default route to the configuration, and ISP 3 a full or partial table, 1.1.1.0/24 route advertisement will only be received from the ISP3. So fiddling with local preference will be possible only with ISP3 in that case? 

[Jon Marshall]

So the main site receives a default route from ISP1 and ISP2 and gets a specific route for 1.1.1.0/24 from ISP3 ? 

If so the most specific route always wins so it doesn't matter what you do with BGP attributes, ISP3 will always be the preferred route. 

You could however use local preference to choose between ISP1 and ISP2 but still ISP3 would be used first unless the link was down. 

Jon

[Filip Knezevic]

Of course. Shortest prefix rule first.

So basically, my assumption is correct - to be fully able to manipulate with the 1.1.1.0/24 all 3 ISPs should be advertising it. I know this is common sense, but I like to be sure with these things. And moving past the CCNA level is not so easy sometimes :).

Thanks for your support! 

[Jon Marshall]

No problem, glad to have helped. 

Jon