BGP prefix-list

[crivera]

i want to do basic inbound load sharing in BGP. I have an aggregate address configured in this form:

network 195.160.124.0 mask 255.255.255.252

network 195.160.124.0 mask 255.255.255.254

network 195.160.125.0 mask 255.255.255.254

network 195.160.126.0 mask 255.255.255.254

network 195.160.127.0 mask 255.255.255.254

aggregate-address 192.168.124.0 255.255.252.0

This sumarizes the 124.0,125.0,126.0 and 127.0 class c blocks.

if i add to the aggregate-address entry the summary-only option is this equivalent to using:

network 195.160.124.0 mask 255.255.255.252

network 195.160.124.0 mask 255.255.255.254

network 195.160.125.0 mask 255.255.255.254

network 195.160.126.0 mask 255.255.255.254

network 195.160.127.0 mask 255.255.255.254

neighbor 172.1.1.1 prefix-list blockname out

ip prefix-list blockname permit 192.168.124.0 /22

ip prefix-list blockname permit 192.168.124.0 /23

ip prefix-list blockname permit 192.168.125.0 /23

ip prefix-list blockname permit 192.168.126.0 /23

ip prefix-list blockname permit 192.168.127.0 /23

[Harold Ritter]

First, your subnet masks are wrong.

/22 is 255.255.252.0 and /23 is 255.255.254.0

Second, the aggregate with summary-only would only advertise the summary 192.168.124.0/22 and not the more specifics. Your prefix-list is different since it allows the aggregate as well as the more specifics.

Hope this helps,

[crivera]

Yes you are right, i made a mistake writing the netmasks it should be 255.255.252.0 and 255.255.254.0

So can i also aggreate with prefix-list?

My idea is: i have to BGP routers, connecting to different ASs, i want to publish the summary trough both, and specifics in each one. so i can load balance the incoming traffic to diferent networks trough diferent neighbor ASs

[Harold Ritter]

You are still advertising the more spefics from this router, so I suppose that on the other BGP router you wouldn't be advertising the more specifics.

Is the block you want to advertise limited to 192.168.124/22? If so you should only be advertising half of the more specifics on this BGP router and the other half on the other BGP router.

By the way,

192.168.125.0/23 is wrong, so is 192.168.127.0/23 since the 24th bit is set.

Hope this helps,

[johansens]

Hi there,

First of all, the ip prefix-list command is an "exact-match" command unless you specify a "gt" or "le" parameter.

Second; the "network x.x.x.x mask n.n.n.n" command in the BGP-process is also an "exact-match" command which also implies the route must exist in the internal routing table.

So when you see these two "constraints" together, your second configuration would not allow any of the configured networks to be announced to the 172.1.1.1 BGP peer. This is because of your netmasks (which I suspect should be 255.255.252.0 and 255.255.254.0).

If you corrected the netmasks, you would be announcing the /22 network AND the specific /23 networks as well to the 172.1.1.1 peer.

The summary-only option of the "aggregate-address" command will remove the more specific networks from the announcement. It would be equivalent to this config:

network 195.160.124.0 mask 255.255.255.252

network 195.160.124.0 mask 255.255.255.254

network 195.160.125.0 mask 255.255.255.254

network 195.160.126.0 mask 255.255.255.254

network 195.160.127.0 mask 255.255.255.254

neighbor 172.1.1.1 prefix-list blockname out

ip prefix-list blockname permit 192.168.124.0/22

!

ip route 195.160.124.0 255.255.252.0 Null0

!

The static route would be important (but could point to another place) as the network MUST exist in the routingtable for this configuration (unlike when the aggregate-address generates it itself).

Did it help? If so, please rate it.

[crivera]

yes the netmasks are wrong

would this configuration work?:

network 195.160.124.0 mask 255.255.252.0

network 195.160.124.0 mask 255.255.254.0

network 195.160.125.0 mask 255.255.254.0

network 195.160.126.0 mask 255.255.254.0

network 195.160.127.0 mask 255.255.254.0

neighbor 172.1.1.1 prefix-list blockname out

ip prefix-list blockname permit 192.168.124.0/22

ip prefix-list blockname permit 192.168.124.0/23

ip prefix-list blockname permit 192.168.125.0/23

ip prefix-list blockname permit 192.168.126.0/23

ip prefix-list blockname permit 192.168.127.0/23

[johansens]

Hi again,

Well.. some of it could work, but you have some errors..

This would be the needed configs:

network 195.160.124.0 mask 255.255.252.0

network 195.160.124.0 mask 255.255.254.0

network 195.160.126.0 mask 255.255.254.0

neighbor 172.1.1.1 prefix-list blockname out

ip prefix-list blockname permit 192.168.124.0/22

ip prefix-list blockname permit 192.168.124.0/23

ip prefix-list blockname permit 192.168.126.0/23

And you would need to have specific routes in your IGP corresponding to the following networks:

192.168.124.0/22

192.168.124.0/23

192.168.126.0/23

This would then announce ALL of the latter networks to the 172.1.1.1 peer.

Or if you really have /24 networks and want a /22 "aggregate" as well, this is the config:

network 195.160.124.0 mask 255.255.252.0

network 195.160.124.0 mask 255.255.255.0

network 195.160.125.0 mask 255.255.255.0

network 195.160.126.0 mask 255.255.255.0

network 195.160.127.0 mask 255.255.255.0

neighbor 172.1.1.1 prefix-list blockname out

ip prefix-list blockname permit 192.168.124.0/22

ip prefix-list blockname permit 192.168.124.0/24

ip prefix-list blockname permit 192.168.125.0/24

ip prefix-list blockname permit 192.168.126.0/24

ip prefix-list blockname permit 192.168.127.0/24

Which then needs the following networks present in the IGP:

192.168.124.0/22

192.168.124.0/24

192.168.125.0/24

192.168.126.0/24

192.168.127.0/24

And the latter config would then announce ALL of them to the 172.1.1.1 peer.

Did it help? If so, please rate it.

[crivera]

Could i put in one router

network 195.160.124.0 mask 255.255.252.0

network 195.160.126.0 mask 255.255.255.0

network 195.160.127.0 mask 255.255.255.0

and in my other bgp router

network 195.160.124.0 mask 255.255.252.0

network 195.160.124.0 mask 255.255.255.0

network 195.160.125.0 mask 255.255.255.0

and still mantain redundancy in case one of the routers or the links fail, the routers connect to different ASs

[pkhatri]

Yes, that will work. That will allow you to split incoming traffic between the two links. In addition, it will give you the ability to draw all traffic through the other link if either link fails.

Paresh.