Simple Network & Aggregate address command

[news2010a]

Imagine someone tells me "I want to advertise network 150.33.36.0/23" via BGP.

Then I am given the following config regarding this:

(...)

router bgp 100

network 150.33.36.0 mask 255.255.255.0

aggregate-address 150.33.36.0 255.255.254.0 summary-only

(...)

My conclusion:

I do not believe that the above is right.

I think this should be:

router bgp 100

network 150.33.36.0 mask 255.255.254.0

aggregate-address 150.33.36.0 255.255.254.0 summary-only

Can someone confirm if the first configuration could work?

[Giuseppe Larosa · Accepted solution]

Hello Marlon,

the right configuration is the first one because the aggregate command creates a /23 if one component route, more specific route is present in the BGP table. the network command may provide that component route if prefix 150.33.36.0/24 is known in some way in the IP routing table.

note a component more specific route has to be in the BGP table by any means: passed by another BGP peer or locally injected by network command

One command that can influence BGP behaviour for network command  is auto-summary

with auto-summary disabled ( it should be the default nowdays) an exact match is required for a network command to be effective

for this reason configuration 2 may be  wrong:

the network command would match only if 150.33.36.0/23 exists in the IP routing table, but if it already exists the aggregate command would be useless!

when auto-summary is enabled the network command can match more specific routes, but major networks are advertised in this case 150.33/16 would be advertised to a neighbor in another major network.

Hope to help

Giuseppe