Service Provider Link Flap on Last mile

Unanswered Question
Mar 26th, 2008


Can any one please tell me if the service provider link flaps on the last mile , does the ospf/BGP routing observe any error in that particular device.

this is scenario where service provider connecting to our Fast ethernet port.

Please let me know all possible ways to find out root cause and trouble shoot for this type of issue.

Prompt response will be highly appreciated!



I have this problem too.
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lgijssel Wed, 03/26/2008 - 02:09

Very often, this will not be the case. Fiber links for example are often terminated on a provider-switch and the customer is connected to a copper-port. A link-interruption in the fiber-section will not affect the interface state of the customer's device.

This kind of failure can only be detected by the time-outs on the routing protocol that are occurring as a result of the link-flap.

Short interruptions will not be noticed, slowness and hickups are the most major indicators for this kind of issue.

Running a ping to the first router on the provider-side with a timeout of 1s can show when the packets are failing but icmp is not always returned by the providers who are of course trying to guard their network against attacks. Still you may try to run a ping against a provider's device in the path. This is likely your best indicator.



royalblues Wed, 03/26/2008 - 02:26

I agree with Leo that most of the time a backend link failure will not get reflected on the customer facing interface.

But there is a feature called link down signal retransmit (customer side signal failure) available with EoSDH links which would cause the interface to go down at your end when there is a backend failure. We got this feature enabled on our EoSDH links which improved the BGP convergence as it did not have to wait for the BGP hold time to expire



mirzaakberali Wed, 03/26/2008 - 02:36

Hi Narayan,

In my case the first mile is not facing any flaps and it is only the last mle which is facing flap . due to which out interface is still showing up as the first mile is not down.

So what is happening is traffic is going from router to the first mile device ...but after couple of minutes it is recognising there is no return traffic we manuaaly moving link traffic of a service provider to another one provider.

So this scenario i want some thing to show on my fast ethernet even when our last mile interface is down.

So....were u recommending "link down signal retransmit" for this to resolve this type of issue....can u share what is this and its feature....

Please help.



royalblues Wed, 03/26/2008 - 03:34


The Settings need to be actually done on the SP equipment/muxes.

When you have a P2P leased line but the interface handover is Ethernet, the above configurations brings down the line protocol on the Ethernet interface if there is a failure in the SDH backbone

Try speaking to your SP about this feature.


guruprasadr Wed, 03/26/2008 - 02:35

HI Akber, [Pls Rate if HELPS]

The LAST MILE type could be of Copper / Fiber / Serial type.

When the LAST MILE Fails:


> Check whether any Routing Protocol on LAST MILE.

> Ping to Neigbor IP / the Service Provider WAN IP, in most cases it will be of /30 segment.

> In some cases, the Service Provider will depend on some other provider on taking the LAST MILE until his / customer premesis. In case of MUX Setup, the traffic dip can occur if some MUX fails on the Path.

Try to run PING against the Provider device to see the LINK Status. The Service Provider PE Routers will be connected to a SP CORE Switch. May be if an X-Connect or an primary Up-link itself fails it will not be reflected at Customer end.


Best Regards,

Guru Prasad R

guruprasadr Wed, 03/26/2008 - 02:48

HI Akber,

Reg:link down signal retransmit

> I hope Narayan can add more details.

In this case, if you run BGP at LAST MILE, normally the Neighbor should show DOWN even if the First Mile is available.

So, naturally the BGP Convergence should happen and need to see for next available path. Normally, you can fine tune BGP Timers for faster convergence.

Do RATE ALL Informative POST

Best Regards,

Guru Prasad R

mirzaakberali Wed, 03/26/2008 - 03:39

hi Guru,

How do implement this change!

How can i improve BGP convergense and timer valuse so it will the link as down even in a scenario where first mile is up and last mile is down.



royalblues Wed, 03/26/2008 - 03:45

You can reduce the BGP timers using the following command

router bgp

timers bgp 5 15

you would be required to clear your BGP session after this using clear ip bgp *

What is the router platform you are using. I would recommend using BFD (Bidirectional Forwarding detection) if your router supports it rather than lowering the timers



guruprasadr Wed, 03/26/2008 - 03:48

HI Akber, [Pls Rate if HELPS]

The Simple Option to prefer Secondary Link if the Primary Fails means,

Execute a Static Route with higer AD Value towards the Secondary Link.

In case, if you are running some protocol say: OSPF inside your Local Network and BGP as LAST MILE Network means,

When you redistribute, you can use the BGP Attribute "Local Preferncce / Weight(Cisco Specific)" to prefer the Secondary / Primary.

You can use interesting option as:


## For FAST Convergence ##

router bgp 50000

neighbor remote-as 50000

neighbor fall-over

!!Configures the BGP peering to use fast session deactivation. BGP will remove all routes learned through this peer if the session is deactivated. Enabling this feature improves overall BGP convergence.

Hope I am Informative.

Pls Rate if HELPS

Best Regards,

Guru Prasad R

mirzaakberali Wed, 03/26/2008 - 20:43

Thanks Narayan and Guru for your support!

I need a scenario configured for my network

where in ethernet handover should display the interface down state log when the last mile is down and first mile is still up.

And also we have a secondary link is connected to the router with BGP routing, but the auto failure is not happenin for a while till 10 to 15mins as the primary link is still showing up on the router's ethenet port due to the firstmile up and last mile down issue.

So, Plz let me know wht shoud i do know.

1) whether i suggest link down siganl retransmit to configured for ISP.

2) shld i use Bidirectional Forwarding detection ( If my router support )

3) Along with this i want to my ethernet to show link status as down in logs so i can show to ISP.

4) Finally i need a failure to happned in with in few seconds in this scenario.

Please suggest me the configuration for this.

Pleae help!

If possible please share ur mail id's .

Many Thanks,


mirzaakberali Thu, 03/27/2008 - 03:34


Can any one help me how do i configure the above query using BGP/OSPF protocol.




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