Subject: Re: question on backup facility

From: "Sandeep B" <[email-masked]>
Date: Thu, 31 Oct 2002 03:01:56 +0000

I understand that part. But what I'm saying is if there is a node between 
PLR and MP that does not supports backup facility and is just acting as a 
regular LSR. Topology is like -

ingres --- PLR --- node 1---- MP --- egress

LSP is setup from ingress to egress via PLR--node1---MP.
a backup is setup from PLR to MP via some other available route.

MP is a node with global label space - so there is no need for PLR to send 
PATH message down the backup tunnel and MP has no idea that the main lsp is 
being protected by some incoming backup LSP.

now link between PLR and node1 fails which triggers a PATHTEAR from node1 
(as node1 does not understands backup facility). This PATHTEAR would wipe 
out protected LSP from MP as well as egress.

With this assumption I say that for backup facility to work in a network is 
to have all nodes that would not delete local state on detecting incoming 
link failure (unlike as in this previous case where "node1" triggered a 
PATHTEAR)

-Sandeep

>Hi Sanddep,
>
>At 23:44 30/10/2002 +0000, Sandeep B wrote:
>>As per my understanding for backup path to work with global label space is 
>>to keep all path states intact on a downstream node (MP) when an 
>>intermediate link fails (between PLR and MP)
>>
>>With most of the implementations I see today all path states are removed
>>once a upstream link failure is detected on a router and PATHTEAR is
>>send downstream.
>>
>>So my statement would be correct that "only way for backup facility to 
>>work is to replace all nodes in the network". With even one intermediate 
>>node behaving to not keep its PSB states on upstream node/link failure 
>>would bring down the Lsp and potentially creating a black-hole for a while 
>>(as PLR would think that backup facility is still in use even though MP 
>>has removed its path/resv state on getting a PATHTEAR from NHOP with 
>>respect to PLR)
>
>This case is covered in the draft already:
>
>When a downstream LSR detects a local link failure, for any protected
>LSPs routed over the failed link, Path and Resv state MUST NOT be
>cleared and PathTear and ResvErr messages MUST NOT be sent
>immediately. Further a downstream LSR SHOULD reset the refresh
>timers for these LSPs as if they had just been refreshed. This is to
>allow time for the PLR to begin refreshing state via the bypass
>tunnel. State MUST be removed if it has not been refreshed before
>the refresh timer expires.
>
>JP.
>
>>Comments??
>>
>>-Sandeep
>>
>>
>>4.5.1. Path state
>>
>>   Path state for every locally repaired LSPs is refreshed downstream by
>>   the PLR. These Path messages use a new SENDER_TEMPLATE value (the
>>   IPv4 tunnel sender address is set to a PLR address), and are sent
>>   onto the bypass tunnel with changed PHOP, ERO and RRO.
>>
>>   When a local link fails, there could be some protected LSPs using
>>   this link.
>>>>>At this point, the LSR MUST NOT remove the state (Path
>>   and Resv) and send PathTear and ResvErr messages that are
>>   corresponding to these LSPs immediately.
>>
>>
>>
>>
>>
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