inter-domain link recovery

Chengchen Hu huc at
Wed Aug 15 07:07:04 UTC 2007

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? 

C. Hu 

From: Roland Dobbins
Data: 2007-08-15 13:21:33
To: nanog
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:




Roland Dobbins <rdobbins at> // 408.527.6376 voice

	Culture eats strategy for breakfast.

            -- Ford Motor Company

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