BGP advertisement question

am9791 · ‎11-09-2011

Currently I am advertising an address of x.x.x.x/23 however the customer is complaining that they are not seeing the routes propagated to us. They are advertising out x.x.x.x/24 and x.x.x.y/24. Will this cause an issue with route propagation? Do I need to advertise the two /24's, rather than the /23 for proper internet routing?

Reza Sharifi · ‎11-09-2011

Should not make any difference. One /23 is equal 2 /24s

HTH

darren.g · ‎11-09-2011

am9791 wrote:

Currently I am advertising an address of x.x.x.x/23 however the customer is complaining that they are not seeing the routes propagated to us. They are advertising out x.x.x.x/24 and x.x.x.y/24. Will this cause an issue with route propagation? Do I need to advertise the two /24's, rather than the /23 for proper internet routing?

In most cases, the larger the subnet you can advertise the better. It makes route summation easier.

Provided the two /24's are contiguous and form into a /23, there's no difference between advertising the /23 or the two /24's - except in maybe keeping your upstream happy by reducing the number of routes you advertise.

I'm a bit confused with what you're saying, though - the customer is not seeing the routes propagated to you - are you forwarding them to the customer? If so, how - another BGP peer, or some form of other routing protocol?

Cheers.

Konstantin Dunaev · ‎11-10-2011

Most ISP checks RIPE (I'm from Europe, in USA it's ARIN database I suppose) information to "validate" the recieving BGP prefixes. If there is any inconsistency between DB and announcements - those prefixes will be not accepted. But it happens only on a "customer" faced BGP sessions. If e.g. in Database there is information about two /24 and you advertise a single /23 then I'd say it will be not accespted by your upstream ISP.

guyhlupi · ‎11-10-2011

I'm not sure that I understand the issue entirely, but here's some feedback based on what I think the scenario is. It sounds to me like you have a customer that is announcing 2 /24s to you. You are somehow advertising those two /24s as a /23. If that is the case then I understand their concern. For example, let's say you are advertising the /23 to your upstreams using a null route to get the reachability information required to make the announcment valid. In normal operation you will see your customer's two /24 announcements and when people route traffic to you based on the /23 announcement you will route the traffic you receive to them since the two /24s are more specific. So in a situation where everything is fine this works well.

The problem comes in when your connection to the customer announcing the 2 /24s goes down. If you are not relying on their /24 advertisements to advertise your /23 you will continue to announce the /23. When people route traffic to you for that /23 there will be nowhere to send it since you are no longer seeing their /24s.

If this is the case you should simply accept what the customer is sending you and use that to advertise to your upstreams. If you do that when you lose connectivity to the customer you will stop announcing the routes and the alternate provider they have will take over all traffic.

Konstantin Dunaev · ‎11-11-2011

I don't think it a real issue, because if customers stops announcing both /24 it doesn't really metter , in both cases they are are not available, in case with two /24 the world simply doesn't see those prefixes and the trafic to them will drop in "default-free" routing zone (Tier-1 ISPs ) in case /23 the traffic will be send to the last ISP and droped there. It's more the question of unnecessary traffic flow and costs for ISP itself, because he should pay for that traffic which will be simply dropped.

But! BGP has a possibility to do a summarization and anounce the summary only if at least one small prifix exists in BGP table

guyhlupi · ‎11-11-2011

Perhaps I shouldn't have used the words stops announcing. What I meant to convey was that if the customer is running BGP it is assumed that they have two service providers. If the circuit to this provider goes down and the provider continues to announce the /23 without having access to the customer's network they will blackhole traffic destined for the blocks.

BGP does have the ability to do summarization and announce the summary if only a small prefix exists in the routing table, but I'm not aware of any provider that does this when dealing with customer routes. It is correct to say that if the provider is using conditional advertisement they will stop announcing the /23 when they stop seeing the customer's advertisements. am9791 if you could clarify whether or not you are using conditional advertisement that would be helpful.

There is another reason, separate from the ones already pointed out, that the customer would want to see the 2 /24s instead of the /23. For example the customer may want to split their inbound traffic so that the traffic for one /24 comes in via provider A and the traffic for the other comes in via provider B. This is relatively simple to do by adding AS path prepends to the routes, but you can't do that if one of the providers is summarizing the blocks for you.