Split BGP network

[rutekp123]

Hi,

             I have BGP AS and one class network 187.45.45.0/24. I need to split it to two block /25 between two datecenter.My question is there any difference between configuration like:

"router bgp 100
network 187.45.45.0 mask 255.255.255.128
neighbor 169.254.10.3 remote-as 101"

or should i do it like this:

"ip prefix-list ip-export seq 5 187.45.45.0/25
route-map ip-export-map permit 10
match ip address prefix-list ip-export
router bgp 100
network 187.45.45.0 mask 255.255.255.128
network 187.45.45.0 mask 255.255.255.0
neighbor 169.254.10.3 remote-as 101
neighbor 169.254.10.3 ip-export-map out

"
of course appropriate configuration on second router in second DC for ....129-255 network part.

In first example my AS doesnt known second part of my class is it correct ? In this DC i will only use first part of C class.

[rais]

If /25 is the only network to be advertised to the neighbor, then network statement is fine/sufficient.

However if you want to apply a filter and prevent other routes from going out to the neighbor then route-map is required. 

HTH.

[Harold Ritter]

Hi @rutekp123 ,

Advertising both the /25 and /24 is better if you want the DCs to be the backup for each other. Also, bear in mind that if these advertisements are propagated towards the Internet, the /25 will not be propagated, as a /24 is the longest prefixes that will be permitted on the Internet.

Regards,

[rutekp123]

Yes this class /25 will be sent to eBGP - Internet. If i want to split it and advertise both /24 and /25 i should use longer as-path prepend in backup DC ?

Maybe somone have examples configuration or doc for that situation ?