Configuring BGP exclude one subnetwork (network command) - ASR1002-X

[martin.germano]

  Hi all, i'm brand new on BGP, so i try to connect to another AS:

router bgp 100
 bgp log-neighbor-changes

 network 192.168.0.0 mask 255.255.0.0
 neighbor 10.0.8.254 remote-as 200

This seem work but i want to exclude for advertising one subnetwork (192.168.8.0/24 for example) for example.

is possible? How?

Thanks!

Martín.

[Meheretab Mengistu · Accepted solution]

Hi Martin,

---

router bgp 100
 bgp log-neighbor-changes

 network 192.168.0.0 mask 255.255.0.0
 neighbor 10.0.8.254 remote-as 200

This seem work but i want to exclude for advertising one subnetwork (192.168.8.0/24 for example) for example.

is possible? How?

---

Yes, it is possible. One way of excluding is using prefix-lists and route-map. Here is an example.

ip prefix-list  EXCLUDE_ADDRESS seq 5 deny 192.168.8.0/24 le 32

ip prefix-list  EXCLUDE_ADDRESS seq10 permit 192.168.0.0/16 le 32

!

route-map ADVERTISE_TO_NEIGHBOR permit 10

match ip address prefix-list EXCLUDE_ADDRESS

!

router bgp AS_NUMBER

neighbor X.X.X.X route-map  ADVERTISE_TO_NEIGHBOR out

!

HTH,

Meheretab

---

[Julio E. Moisa]

Hi

Just adding to the previous post, if you have more networks to advertise, include this line:

ip prefix-list  EXCLUDE_ADDRESS seq 15 permit 0.0.0.0/0 le 32

The line will allow everything else. 

[martin.germano]

  Thanks all for reply,

  Meheretab, i don't understand this:

I must to put this command:
  neighbor 10.0.8.254 remote-as 200

but instead if i make this command :
  neighbor 10.0.8.254 route-map  ADVERTISE_TO_NEIGHBOR out

I don't specify the AS. I'm right?

Thanks

[Meheretab Mengistu]

Martin,

You will need both commands: the first one is to create a neighbor relationship and the latter one is to filter routes to a particular neighbor. The complete command should have something like this:

router bgp 100
neighbor 10.0.8.254 remote-as 200
neighbor 10.0.8.254 route-map ADVERTISE_TO_NEIGHBOR out
!

HTH,
Meheretab

[martin.germano]

  Great! i gonna try it. Thanks for all.

Have a nice day!

Martín

[Julio E. Moisa]

Hi

Yes, this line: neighbor 10.0.8.254 remote-as 200  is mandatory because it is creating the BGP peering otherwise you will lose connection to that peer via BGP. Now this line is a complement for filtering: 

neighbor 10.0.8.254 route-map  ADVERTISE_TO_NEIGHBOR out

:-)

[Georg Pauwen]

Hello,

on a side note, and as an alternative to Julio's and Meheretab's solutions, you could also configure the below. The aggregate address by defaults sends all more specific networks, and the suppress map filters those specified in the respective route map...

router bgp 100
aggregate-address 192.168.0.0 255.255.0.0 suppress-map SUPPRESS
!
ip prefix-list SPECIFIC seq 5 permit 192.168.8.0/24
!
route-map SUPPRESS permit 10
match ip address prefix-list SPECIFIC

[paul driver]

Hello

Caution should be taken when aggregating in BGP as it can lead to loops if you are not careful, due to the fact routing information can be lost on the advertised aggregate, As by default not only are you hiding the more specific routes you are also hiding AS numbers which are use in bgp to prevent loops!

res
Paul