inter-domain link recovery

Joel Jaeggli joelja at
Wed Aug 15 08:32:21 UTC 2007

Chengchen Hu wrote:
> Thank you for your detailed explainaton. 
> Just suppose no business fators (like multiple ASes belongs to a same ISP),  is it always possible for BGP to automatically find an alternative path when failure occurs if exist one? If not, what may be the causes? 

If you have multiple paths to a given prefix in your rib, you're going
to use the shortest one. If it's withdrawn you'll use the next shortest
one. If you have no paths remaining to that prefix, you can't forward
the packet anymore.

I think to look back at your original question. You're asking a specfic
question about the dec 06 earth quake outage... The best people to ask
why to took so long to restore are the operators who were most
dramatically affected.

The fact of the matter is most ISP's are not in the business of buying
more diversity than they think they need in order to insure business
continuity, support sla's and stay in business. The earthquake and
undersea landslide affected a number of fiber paths over a short period
of time.

I think it's fair to assume that a number of operators have updated to
their risk models to account for that sort of threat in the future. it's
to totally anticipate the threat of loosing ~80% of your fiber capacity
in a rather dense and well connected cooridor.

There were two talks on the subject of that particular event at the
first 07 nanog, you can peruse them here:

In particular the second talk discusses the signature of that outage in
the routing table in some detail.

> C. Hu 
> -------------------------------------------------------------
> From: Roland Dobbins
> Data: 2007-08-15 13:21:33
> To: nanog
> CC: 
> Subject: Re: inter-domain link recovery
> On Aug 14, 2007, at 9:06 PM, Chengchen Hu wrote:
>> 1. Why BGP-like protocol failed to recover the path sometimes? Is  
>> it mainly because the policy setting by the ISP and network operators?
> There are an infinitude of possible answers to these questions which  
> have nothing to do with BGP, per se; those answers are very  
> subjective in nature.  Can you provide some specific examples  
> (citing, say, publicly-available historical BGP tables available from  
> route-views, RIPE, et. al.) of an instance in which you believe that  
> the BGP protocol itself is the culprit, along with the supporting  
> data which indicate that the prefixes in question should've remained  
> globally (for some value of 'globally') reachable?
> Or are these questions more to do with the general provisioning of  
> interconnection relationships, and not specific to the routing  
> protocol(s) in question?
> Physical connectivity to a specific point in a geographical region  
> does not equate to logical connectivity to all the various networks  
> in that larger region; SP networks (and customer networks, for that  
> matter) are interconnected and exchange routing information (and, by  
> implication, traffic) based upon various economic/contractual,  
> technical/operational, and policy considerations which vary greatly  
> from one instance to the next.  So, the assertion that there were  
> multiple unaffected physical data links to/from Taiwan in the cited  
> instance - leaving aside for the moment whether this was actually the  
> case, or whether sufficient capacity existed in those links to  
> service traffic to/from the prefixes in question - in and of itself  
> has no bearing on whether or not the appropriate physical and logical  
> connectivity was in place in the form of peering or transit  
> relationships to allow continued global reachability of the prefixes  
> in question.
>> 2. What is the actions a network operator will take when such  
>> failures occures? Is it the case like that, 1)to find (a)  
>> alternative path(s); 2)negotiate with other ISP if need; 3)modify  
>> the policy and reroute the traffic. Which actions may be time  
>> consuming?
> All of the above, and all of the above.  Again, it's very  
> situationally dependent.
>> 3. There may be more than one alternative paths and what is the  
>> criterion for the network operator to finally select one or some of  
>> them?
> Proximate physical connectivity; capacity; economic/contractual,  
> technical/operational, and policy considerations.
>> 4. what infomation is required for a network operator to find the  
>> new route?
> By 'find the new route', do you mean a new physical and logical  
> interconnection to another SP?
> The following references should help shed some light on the general  
> principles involved:
> <>
> <>
> < 
> 0,1144,0321127005,00.html>
> -----------------------------------------------------------------------
> Roland Dobbins <rdobbins at> // 408.527.6376 voice
> 	Culture eats strategy for breakfast.
>             -- Ford Motor Company

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