Bgp default route

[CSCO11238553]

Got a change last night to remove the advertisement of default route to Telstra(cqdnsopr01c10) as they have mentioned that they are receiving 0.0.0.0 from (HIDCSR01).

BGP Config of HIDCSR01

router bgp 64633

no synchronization

bgp log-neighbor-changes

timers bgp 7 21

redistribute connected

redistribute static route-map REDIST_STATIC

neighbor 10.88.128.89 remote-as 64632 // BGP Peer Telstra

neighbor 10.88.128.89 description Telstra Managed Router

neighbor 10.88.128.89 soft-reconfiguration inbound

neighbor 10.88.128.89 prefix-list NO_DEFAULT_ROUTE out//Prefix list added to prevent default route advertisement

neighbor 10.88.128.89 route-map LOCAL_PREF_200 in

no auto-summary

p prefix-list NO_DEFAULT_ROUTE seq 1 deny 0.0.0.0/32

ip prefix-list NO_DEFAULT_ROUTE seq 2 permit 0.0.0.0/0 le 32 ·

after doing the above prefix list, Telstra can still see the default route on their end, I tried checking the advertised BGP routes to them but I couldn’t see any 0.0.0.0 being advertised and there’s no default originate configured on that peer. · Redistribute static also has a route map in place which prevent default route from being redistributed.

BGP received routes from Telstra(cqdnsopr01c10) cqdnsopr01c10#show ip bgp neighbor 10.88.128.90 received-route

BGP table version is 127374, local router ID is 10.88.129.25 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale, m multipath, b backup-path, x best-external, f RT-Filter, a additional-path Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path r> 0.0.0.0 10.88.128.90//quuhidcsr01 0 0 64633 i ·

Not sure why it is still advertising even with the prefix list and where is it coming from. ·

Would it be better to have them filter the default route on their end?

[paul driver]

Hello "neighbor 10.88.128.89 soft-reconfiguration inbound//Prefix list added to prevent default route advertisement" Not sure I understand this - the prefix -list look okay but where is it being applied? I would expect it to be defind OUTBOUND in bgp Res Paul

[CSCO11238553]

Hi Paul, Its there i just forgot to add it, i updated the bgp syntax above to show the prefix list applied outbound.

[prajithtr_2]

is there any other routing protocol running between them. "r> 0.0.0.0 10.88.128.90" doesn't these show the default routes are advertised by some other routing protocols.can you do show ip route and confirm it.

regards

Prajithtr

[Vasilii Mikhailovskii]

Hello.

Your prefix list in not precise.

It should be

ip prefix-list NO_DEFAULT_ROUTE seq 1 deny 0.0.0.0/0

instead of ip prefix-list NO_DEFAULT_ROUTE seq 1 deny 0.0.0.0/32

PS: also I would suggest to use route-map to filter outbound updates instead of simple prefix list (just for convinience).

[CSCO11238553]

There could be static default route on their side thats why there's a rib failure on their bgp table. Could someone explain the difference between /0 ans /32 for the prefix list above? And can someone confirm if the below command will work? ip prefix-list NO_DEFAULT_ROUTE seq 1 deny 0.0.0.0/0 ip prefix-list NO_DEFAULT_ROUTE seq 1 permit 0.0.0.0/0 le 32 Route-map no_default_route permit 10 Match ip prefix-list No_default_route neighbor 10.88.128.89 route-map no_default_route out ThNks guys!

[CSCO11238553]

There could be static default route on their side thats why there's a rib failure on their bgp table.

Could someone explain the difference between /0 ans /32 for the prefix list above?

And can someone confirm if the below command will work?

ip prefix-list NO_DEFAULT_ROUTE seq 1 deny 0.0.0.0/0

ip prefix-list NO_DEFAULT_ROUTE seq 1 permit 0.0.0.0/0 le 32

Route-map no_default_route permit 10

Match ip prefix-list No_default_route

neighbor 10.88.128.89 route-map no_default_route out

ThNks guys!

[Vasilii Mikhailovskii]

Hello.

0.0.0.0/32 is a host route to 0.0.0.0 IP-address.

0.0.0.0/0 is a default gateway.

For your configuration you need:

ip prefix-list NO_DEFAULT_ROUTE seq 1 deny 0.0.0.0/0
ip prefix-list NO_DEFAULT_ROUTE seq 10 permit 0.0.0.0/0 le 32

[CSCO11238553]

Hi vasili, 

thanks!

like so?

ip prefix-list NO_DEFAULT_ROUTE seq 1 deny 0.0.0.0/0

ip prefix-list NO_DEFAULT_ROUTE seq 1 permit 0.0.0.0/0 le 32

Route-map no_default_route permit 10

Match ip prefix-list No_default_route

neighbor 10.88.128.89 route-map no_default_route out

But the question still remains, i dont have default route originate enabled, how come its advertising the default route to ebgp peer.

[Vasilii Mikhailovskii]

Hello.

In your case another AS is sending you default gateway (check it - sh ip bgp 0.0.0.0/0). And you AS is just a transit, so you don't need default-originate to send default gateway further.

[CSCO11238553]

Hi vasili,

i dont have 0.0.0.0 in my bgp routing table when i use show ip bgp 0.0.0.0/0.

what i have is just a static default route but not redistributing to bgp so not sure why they are receiving this from us.

[CSCO11238553]

Nn