BGP advertise /25 then aggregate /24 summary only

[Will Phinney]

Hello everyone and thanks for the assistance! Can someone point me in the right direction as to why you would want to advertise a /25 in BGP, but then do an aggregrate /24 summary only in the same process? I'm thrown off by this:

router bgp XXXX
network 10.11.66.0 mask 255.255.255.128
aggregate-address 10.11.66.0 255.255.255.0 summary-only

Why would you want to advertise a summary only when you're adv /25 out? 

[Philip D'Ath]

An aggregate address is usually used to say "I am responsible for this address space", even if it is not all used.  Commonly this is because the prefix is allocated to a site to use how it wants.

 It is common to null route the unused space, and just keep the bit in use.

[Will Phinney]

Thanks Philip! The aggregate would supersede the regular network statement , correct? They just seem like they would do the same thing in my case. Especially knowing the /25 is the web servers that are advertised.

thanks!

[Philip D'Ath]

Actually I'm not sure if just the aggregate address would be advertised in this case.

[Jon Marshall]

For BGP to advertise an aggregate address at least one of the more specific prefixes must be in the BGP table ie. "sh ip bgp".

When you use a "network ..." statement that puts an entry in the BGP table.

So if none of the more specific prefixes are being learnt via BGP and you are not redistributing an IGP into BGP then that network statement is probably there just to make sure there is an entry in the BGP table which means the router can then advertise the summary address.

Only the summary address will be advertised with the configuration you have shown.

Jon

[Will Phinney]

Jon,

I think I see now. It's odd to me though that one would advertise the /25 when in all reality they want to advertise the entire /24. Wouldn't it make more sense to just do:

network 10.11.66.x 255.255.255.0 ?

Thanks, Jon!

[Philip D'Ath]

It would be great if you could rate helpful answers.

[Jon Marshall · Accepted solution]

They are not advertising the /25, that is used simply to get a more specific prefix into the BGP table.

They are only advertising the /24.

They could simply use a network statement with the /24 mask but when you use a network statement for it to be advertised out there must be a matching route in the IP routing table.

So it may be that you don't have a /24 entry in the routing table.

It is difficult to say exactly why it has been done this way without a full understanding of the BGP and IP routing tables.

Jon