BGP Network and Static route

[SajeshB]

If i have 2 static route 10.0.0.0/8 and 10.105.0.38/32 in my routing table. And in bgp If I adverstise network 10.105.0.38 255.255.255.255.
What will happen if static route 10.105.0.38/32 is unreachable or down, will bgp still adverstise the network ? as i have backup route 10.0.0.0/8

[Harold Ritter]

Hi @SajeshB ,

BGP requires a perfect match in the routing table in order to originate the route specified via the network statement.

Regards,

[SajeshB]

Hi @Harold Ritter ,

Thanks for replying, However im in similar situation where my exact Static route was unreachable (Nexthop via Gig0/0/0 is down) but BGP was still advertising the network to ISP. However i didnt got the live logs but from checking I see there was an /8 prefix static route and i think that was causing an issue.

[Harold Ritter]

Hi @SajeshB ,

The only way to find out what is really going on is to check what routes are present in the RIB, once the static routes pointing at Gig0/0/0 disappears. You might receiving the /32 route from another source?

Regards,

[SajeshB]

Hi @Harold Ritter ,

To simulate this and tshoot live I requested downtime from them, but looking at configuration I can tell their is no other source that is giving /32 subnet apart from matching that /8 static route.

[Richard Burts]

We do not know how the static route is configured, do not know if there is anything like track that would remove the static route if the next hop becomes unreachable. IOS will remove a static route if the outbound interface is down. But does not remove a static route just because the next hop is not reachable - especially true on Ethernet interfaces. So we need more information about how this is configured.

[Harold Ritter]

Hi @Richard Burts ,

I believe this post is a follow-up to this other post. 

https://community.cisco.com/t5/routing/bgp-route-advertise-issue/m-p/4518564#M359871

Regards,

[SajeshB]

hi @Harold Ritter @Richard Burts ,

You can refer that post. Thankyou

[SajeshB]

hi @Richard Burts  @Richard Burts ,

The static route is configured via 'next-hop' which is reachable via Gig0/0/0 which was down during the outage. so the outbound interface is down. Their is no ip sla and track is configured to monitor next hop.

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[Richard Burts]

You tell us that Gig0/0/0 was down. But what does that really mean? An important question is whether interface Gig0/0/0 goes into line protocol down state. If the next hop device is not responding then the static route would not be working and it feels like Gig0/0/0 is not working. But that does not necessarily mean that Gig0/0/0 is line protocol down.

The fact that the static route is still being advertised by BGP suggests that Gig0/0/0 is not in the line protocol down state. If this situation happens again it would be helpful to get the output of show interface Gig0/0/0.

[SajeshB]

Thanks @Richard Burts , but when I checked in router logging it was clearly logged that line protocol changed state to down and also interface changed state to down for Gig0/0/0 for almost 1 hour then this interface came up.

[Harold Ritter · Accepted solution]

Hi @Richard Burts ,

I think it equally important to "show ip route 10.105.0.38/32" when the interface goes "down". At the end of the day, if this route is present in the RIB then the BGP network statement will originate the route in BGP.

Regards,

[SajeshB]

Yea thanks @Harold Ritter , Its better I will wait and simulate this again and check during the interface goes down. but it was really helpful to clear the confusion of /8 and /24 static route.