ip next hop unreacheable , routes still advertised ..

[thomas.fayet]

Hello community .

I have a strange behavior  and a simple proble . I configured the following static route

ip route 10.84.22.0 255.255.254.0 10.84.23.254

That I advertsied in eBGP :

router bgp 65000

network 10.84.22.0 mask 255.255.254.0

the IP next hop 10.84.23.254 is cascaded on my customer LAN . At nominal time the router advertsed the route in BGP

pjnb1376#sh ip bgp nei 57.213.169.169 ad

           Next Hop            Metric LocPrf Weight Path

*> 10.84.22.0/23    10.84.23.254             0         32768 i

My issue :

When the lan interface of the router goes down , the router still advertise the route !!! Even if the IP next hop

10.84.23.254  is not reacheable anymore ....

The box is a Cisco 1941 using

1900-universalk9-mz.SPA.151-4.M1

Is this a bug a normal behavior present in Cisco 15.x Mainline now ?

Thanks

Da

[Peter Paluch]

Thomas,

You are injecting this static route into the BGP using the network command. As a result, this network will be present in the BGP as long as it is present in your own routing table. So, when the next hop or the interface towards it goes down, does the static route disappear from the routing table? If it remains there, it will also remain in BGP.

Also note that BGP may require up to 60 seconds to advertise a change in the routing table so even if the route is being removed from the routing table, allow for a minute or so for BGP to take notice.

One comment: the route appears to recursively close into itself - the next hop towards this network is within the network itself. Why is that? Is there a more specific route towards the 10.84.23.254?

Best regards,

Peter

[thomas.fayet]

Hello peter .

No , the route does not dissepears , even if the IP next hop is down .........and we wait 5 mns , at least .

The setup is very simple :

my router --------- customer fw ------------ the subnet I mentioned above .

Before I am pretty sure that is we were pointing a static route towrds a certain ip next hop , when the ip next hop goes down , the router should stop advertsing the route ........

Could this be a bug ?

rgds

[Peter Paluch]

Hello Thomas,

Before I am pretty sure that is we were pointing a static route towrds a  certain ip next hop , when the ip next hop goes down , the router  should stop advertsing the route

It depends on what you mean by "next hop goes down". If just the next hop stops responding but the interface towards it remains in the up/up state, the static route will remain placed in the routing table. Only if the interface goes down, the static route will be removed.

Which of these two kinds of next hop failures have you encountered? In particular, did the outgoing interface go down?

Can you post your entire sanitized configuration and the output of the show ip route command? Thank you!

Best regards,

Peter

[thomas.fayet]

Unfortunately I can't  shut down the LAN interface right now as the client is working as we speak .

Back in a days , under ios version 12.x I am 99.9% sure that if the IP next hop you were using to "talk" to the cascaded subnet was down (Ie not replying to ping anymore from the router ) then , network command or not in BGP , the router would have stoped adversising the route . This would make sense as it would mean the cascaded subnet will not be reacheable via this very next hop so the behavior seemed to be logical ....

Here is the conf :

router bgp 65000

network 10.84.22.0 mask 255.255.254.0

neighbor 57.213.169.169 remote-as 13879

!

ip route 10.84.22.0 255.255.254.0 10.84.23.254

so you see , it 's very simple ..... But my router keep on advertising

10.84.22.0 /23 even if 10.84.23.254 is down ..... and even if the LAN (outgoing interface) is also down/down ....very weird .

Thanks

[thomas.fayet]

And the show ip route , sorry.

jnb1376#   sh ip route sta

S        10.84.22.0/23 [1/0] via 10.84.23.254

[dontluvuhnomor]

Hello Thomas and Peter,

Thank you for this informative conversation of you guys.

I am encountering the same issue as Thomas however on my case, I already added the exit interface and the next hop ip address. Now to test, I've manually shut down the interface of the neighboring router where my next hop ip is configured. Expecting that the static route will be removed from my routing table, but it has not been removed, interestingly, it is still in the routing table. I have no way of trying to do this on real equipment as I do not have one. Is this just a GNS3 bug?

Any explanation will be so much appreciated.

Thank you in advance.

Jovel

[Richard Burts]

Jovel

It is not a bug. It seems that perhaps you have not fully understood the points being made in the discussion about static routes. Assuming that we are talking about router 1  as long as its interface toward the next hop is in the up state then it keeps the static route in its routing table. Router 1 has no way to know what is the state of the interface on router 3 that is the next hop and router 1 does not actively test reachability of the next hop so the fact that router 3 interface is admin down does not affect anything on router 1.

If you want to be sure that the static route is removed when the next hop is not reachable then you would need to use something like IP SLA and tracking applied to the static route.

HTH

Rick

[Jon Marshall]

Thomas

Just to add to Peter's posts. If the link is a point-to-point link then it should indeed remove the route from the routing table. So you may be remembering an example where you had a serial connection for example and the remote end went down. Then you would have seen the route removed from the routing table and BGP would have then stopped advertising it.

But if your interface is ethernet for example and your interface is connected into a switch and the next-hop device is also connected into the switch, the next-hop interface can go down but that won't affect your LAN interface, it will still be up because it is connected to the switch. So, as Peter says, the route will not be removed.

In these sort of cases you need to check the availability of the next-hop device and to do this you would use something like IP SLA where you can track the availability of the next-hop and if it goes down you then remove the static route from your routing table.

Of course your interface may not be ethernet in which case i have just told you something you didn't need to know

Jon

[thomas.fayet]

John.

Thanks for the reply . Indeed the interface is an ehternet interface and when making the test this morning the lan interface was down/down .

We can also see this in the router's log :

Sep 12 08:51:38: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to down

Sep 12 08:51:39: %LINK-3-UPDOWN: Interface GigabitEthernet0/0, changed state to down

But still the router keeps on advertising the route .......

There was also this meesage popping up in the log ...so far I have not been able to figure out what this message means.

Sep 12 08:54:39: %IPRT-3-RIB_LOOP: Resolution loop formed by routes in RIB

Very weird .

Rgds

Da

[Peter Paluch]

Hi Thomas,

I have a distinct feeling that this must be somehow related to the next hop of the route pointing towards the route itself. The logging message %IPRT-3-RIB_LOOP confirms that there is an infinite recursion created in your routing table.

Can you please enter the following command (it has been introduced recently in 15.0(1)M IOSes) and post the output here?

show ip route loops

Thank you!

Best regards,

Peter

[skarthic]

John,

Is there a reason why the static route is setup as  - ip route 10.84.22.0 255.255.254.0 10.84.23.254?

As per Peter's suggestion, check how do we get into a loop when the interface goes down.

Thanks.

[thomas.fayet]

well we use the /29 as interco between the router and the fw .

The /23 is cascaded behind the customer fw .

I found it strange the router create a rib failure when the int goes down .....

I will try to do another test today and post the result of the sh ip route loops

thanks !

[Peter Paluch]

Hello Thomas,

Any news on this?

Best regards,

Peter

[thomas.fayet]

Hello Peter .

I am trying to get my client to do a test . Once I have done it I will post the results here .

Rgds

Da