BGP:advertising networks

[Wassim Aouadi]

hi,

According to the diagram and the config, why isn't there a "network 172.16.20.0 mask 255.255.255.0" on router B? Does the "network" statement advertise only connected networks?

please check the attached files (taken from Cisco Student Guide).

[rais]

172.16.20.0 is not owned by routerB. It belongs to A. network statement on B would advertise connected routes from B.

Thanks.

[milan.kulik]

Hi,

it's not necessary as the 172.16.20.0/24 is received via BGP session from router A.

The "network" statement is necessary to advertise to BGP routes received from IGP, static and connected subnets.

See http://www.cisco.com/en/US/docs/ios/12_0/np1/command/reference/1rbgp.html#wp1020532

for details.

BR,

Milan

[hasmurizal]

Hi,

The network statement does necessarily advertised connected networks, it is just to advertise the router self network into bgp.

If you wanted to advertise the connected network, just include in the router bgp config "redistribute connected"

[Giuseppe Larosa]

Hello Wassim,

as Milan has correctly noted is RA that is advertising net 172.16.20.0/24 on its iBGP session to RB.

RB can then advertise to RC over an eBGP session the network 172.16.20.0/24 and will change the BGP next-hop attribute to ip 10.1.1.x (BGP next-hop recalculation at EBGP session).

From the point of view of RC there is no difference between the two subnets:

172.16.10.0/24

172.16.20.0/24

both comes over eBGP with AS path= 65000 i

BGP next hop= 10.1.1.x

Hope to help

Giuseppe