[j-nsp] Bypass LSP functionality question

[Stefan Fouant]

On 7/6/2011 11:50 AM, David Ball wrote:
>    Just looking for confirmation of a suspicion here.
>
>    If I have an LSP configured with link-protection on every interface
> along the way (creating many-to-one Bypass LSPs, as opposed to 1:1
> detours), no secondary standby path defined, and a protected interface
> fails, the ingress node will have no ability to perform a
> make-before-break, right?  Because the Path Tear messages will be sent
> both upstream and downstream from the failed interface?  The bypass
> will only help me up until the upstream nodes process the Path Tears
> and a new LSP is signalled from the ingress node....or am I missing
> something?

Hi David,

The bypass is only used temporarily to save the traffic that is already 
traversing the LSP.  At the same time that the bypass LSP is being used, 
the node which established the Bypass around the failure will will send 
the Path Tear message upstream towards the Ingress LSR.

Once the Ingress LSR receives the Path Tear it may signal an alternate 
path assuming some Secondary paths have been configured.  You could have 
the Secondary LSP set up as a Standby in which case it is pre-signaled 
but will require double the amount of reservation state in the network. 
  This is inherently make-before-break because the Secondary Standby is 
already established by the time your Primary has failed.

Another option is to configure the Adaptive option which will force the 
Ingress LSR to continue using the Primary LSP (traversing a Bypass) 
until it has signaled the Secondary path and only once the secondary has 
been established will it cease using the Primary.  This has the benefit 
of also reducing the amount of reservation state in the network due to 
the fact that Adaptive option signals LSPs using the Shared Explicit style.

HTHs,

Stefan Fouant
JNCIE-ER #70, JNCIE-M #513, JNCI
Technical Trainer, Juniper Networks
http://www.shortestpathfirst.net
http://www.twitter.com/sfouant