Update BGP routing table after advertised new subnet?

[tinhnho123]

Hi Guys,

After I add a subnet 192.168.20.0/24 into BGP, should I use

clear ip bgp *

or

clear ip bgp * soft

to update the bgp route to the neighbors?

router bgp xxxx
  bgp log-neighbor-changes
  neighbor 10.0.0.1 remote-as 123
  neighbor 10.0.0.1 version 4
!
address-family ipv4
   network 192.168.20.0 mask 255.255.255.0
   network 172.16.0.0 mask 255.255.255.0
exit-address-family

Thanks.

[MHM Cisco World]

I use usually 

clear ip bgp <123>  <<- I use 123 as your sharing config

 

this force bgp to re-establish and exchange the new BGP session and advertise new update. 

[Harold Ritter]

Hi @tinhnho123 ,

You should not have to. Just make sure that the subnet 192.168.20.0/24 is present in the RIB. This is a requirement for BGP to originate the route when you configure a network statement.

Regards,

[tinhnho123]

Sorry for the newbie question; how do I ensure it is present in RIB?

Thanks.

[Harold Ritter]

Hi @tinhnho123 ,

show ip route 192.168.20.0

Regards,

[tinhnho123]

I did a quick

show ip route

from the local router and it looks like the subnet 192.168.20.0/24 is advertised from my neighbor, i checked the neighbor's router, it doesn't advertise subnet 192.168.20.0/24.

R1#sh ip route 192.168.20.0/24
Routing entry for 192.168.20.0/24
Known via "bgp xxx", distance 20, metric 0
Tag 123, type external
Last update from 10.0.0.1 1w1d ago
Routing Descriptor Blocks:
* 10.0.0.1, from 10.0.0.1, 1w1d ago
opaque_ptr 0xxxxxxxxx
Route metric is 0, traffic share count is 1
AS Hops 1
Route tag 123
MPLS label: none

I want my local router to advertise this subnet 192.168.20.0/24, shall I run 'clear ip bgp 123' on my local router?

[MHM Cisco World · Accepted solution]

the bgp is slow so you need to force bgp to advertise the new network 
do 

clear ip bgp 123 

and check the neighbor 

I run small lab show you that how bgp is slow, I add network but still neighbor is not receive it 
immeidate after clear the neighbor get the network. 

[tinhnho123]

Thanks!

[Harold Ritter]

Hi @tinhnho123 ,

Looking at the

show ip route

its appears this route is being learnt from neighbor 10.0.0.1 (as123) already. So why would you want to advertise to that same neighbor?

Regards, 

[tinhnho123]

I just checked, the neighbor doesn't advertise that subnet.  My coworker probably advertised the subnet 192.168.20.0/24 from the neighbor to my local router and then removed it from the neighbor's router. 

I want to advertise this subnet 192.168.20.0/24 from my local router to the neighbor.

[Harold Ritter]

Can you post the

show ip route 192.168.20.0

output again?

Regards,

[tinhnho123]

R1#sh ip route 192.168.20.0
Routing entry for 192.168.0.0/16
Known via "bgp xxx", distance 20, metric 0
Tag 123, type external
Last update from 10.0.0.1 1w1d ago
Routing Descriptor Blocks:
* 10.0.0.1, from 10.0.0.1, 1w1d ago
opaque_ptr 0xxxxxxx
Route metric is 0, traffic share count is 1
AS Hops 1
Route tag 123
MPLS label: none
R1#

[Harold Ritter]

Hi @tinhnho123 ,

192.168.20.0/24 is resolvable via 192.16.0.0. The reason you are not advertising the subnet 192.168.20.0/24 to the neighbor is that you do not have the /24 present in the local RIB. For the BGP network statement to work, you need a matching route in the RIB. Do you have that subnet defined locally? If not how are you supposed to forward traffic to that subnet?

Regards,

[MHM Cisco World]

@Harold Ritter @tinhnho123 
why I see two route one with /24 and other with /16 are you using redistribute ?

R1#sh ip route 192.168.20.0/24
Routing entry for 192.168.20.0/24

R1#sh ip route 192.168.20.0
Routing entry for 192.168.0.0/16

[Harold Ritter]

Hi @MHM Cisco World ,

The /24 was seen when his colleague advertised from the peer. After his colleague removed it from the remote peer, they are left with the /16 in the local RIB. This is the reason his BGP network statement does not work.

Regards,