Meaning of the BGP config shown...

Unanswered Question
Dec 15th, 2008

Hi All,

My undeerstanding from the below config is, BGP on RTR-A, will inject ionly default route (0.0.0.0) into the neighbour. None of the the other static routes will be advertised under any circumstances.. is that correct.? please suggest.

**********************************

router bgp 65523

no synchronization

bgp log-neighbor-changes

network 0.0.0.0

redistribute static route-map Internet

neighbor 6.9.36.80 remote-as 65523

neighbor 6.9.36.80 timers 10 30

neighbor 6.9.36.80 next-hop-self

no auto-summary

!

route-map Internet permit 10

match ip address 7

!

access-list 7 permit 0.0.0.0

!

ip route 0.0.0.0 0.0.0.0 6.9.31.49

ip route 10.29.150.0 255.255.255.0 10.100.25.1

ip route 10.29.151.0 255.255.255.0 10.100.25.1

ip route 10.100.0.0 255.255.0.0 10.100.25.1

ip route 192.168.0.0 255.255.0.0 10.100.25.1

ip route 192.168.100.0 255.255.255.0 10.100.25.1

***********************************

Thank you in advance

MS

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Jon Marshall Mon, 12/15/2008 - 14:23

MS

If you want to make sure you only send the default-route 0.0.0.0 to the BGP neighbor 6.9.36.80 an easier way would be

1) remove from your BGP router config

network 0.0.0.0

redistribute static route-map Internet

2) add this to your BGP router config

neighbor 6.9.36.80 default-originate

this will just send a default-route of 0.0.0.0 to that specific neighbor -

http://www.cisco.com/en/US/docs/ios/12_2/iproute/command/reference/1rfbgp1.html#wp1158721

Apologies if i have not answered your question.

Jon

Edison Ortiz Mon, 12/15/2008 - 14:58

Actually, his configuration will also send just the Quad 0 over to the neighbor.

Your suggestion will work but has a different behavior.

default-originate will unconditionally send the Quad 0 over to the neighbor.

The network 0.0.0.0 statement will conditionally send the Quad 0 depending on the RIB. If 0.0.0.0 is in the routing table, the route will be sent, else it does not.

The OP may remove the redistribute static as it's redundant for this purpose.

__

Edison.

Jon Marshall Mon, 12/15/2008 - 15:02

"Actually, his configuration will also send just the Quad 0 over to the neighbor"

Yes it would i just thought the "redistribute static" statement was unnecessary as you point out and it could cause problems later if the route-map was accidentally modified.

"Your suggestion will work but has a different behavior" - good point !

Jon

fortis123 Tue, 12/16/2008 - 07:43

Thank you both. So the above configuration just send quad 0s to the neighbour but none of the other static routes on the RTR-A under any circumstances.

Thank you both again.

MS

Edison Ortiz Tue, 12/16/2008 - 07:51

That's correct but Jon and I recommend removing the redistribute static, not needed.

HTH,

__

Edison.

mvsheik123 Tue, 12/16/2008 - 08:14

Great. So the below config itself conditionally advertise quad 0 route to the neighbor (as long as there is route to 0.0.0.0 in RIB)

router bgp 65523

no synchronization

bgp log-neighbor-changes

network 0.0.0.0

Thank you

MS

Jon Marshall Tue, 12/16/2008 - 08:15

Yes it will. Note that if you add any other neighbors to your bgp config it will also advetise the default-route to them as well.

Jon

Edison Ortiz Tue, 12/16/2008 - 09:12

Good point Jon, your recommendation with default-originate would be the most prudent on this scenario where you only want to send the Quad 0 to one neighbor but not the other.

However, I don't like sending unconditional Quad 0s for some reason :)

__

Edison.

fortis123 Tue, 12/16/2008 - 13:33

Thank you All. In the scenario I got, this has to be conditional. This is interent router and the neighbor is Internal router. So in case the RIB on RTR-A lost the ISP connection and so route to 0/0 in RIB, the LAN users takes the longer path injected via OSPF for Internet.

Thank you again

MS

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