Re: [spring] Beyond SRv6.

[Dirk Steinberg <[e-mail]>]

There seems to be some confusion regarding TI-LFA.
A couple of comments:

- Remote LFA tunnel is not used with SR, only TI-LFA which 
  only operates on the node that is the PLR (point of local repair).

- Any encapsulation on the ingress PE with or without EH has nothing 
  to do with TI-LFA except for the special case where the ingress PE
  itself is the PLR.

- TI-LFA is not an IGP extension and does not require one. 
  It is a purely local computation based on IGP topology.

- The PLR for TI-LFA may need to insert some SIDs into the SID
  list to steer the packet around the failure. For the LFA base case
  no SIDs are needed at all. If SID insertion is needed, the PLR 
  will push the required number of labels in the MPLS case. 

  For SRv6, the equivalent operation to the label push is to 
  insert an EH with the required SID list. The packet will already 
  have been encapsulated on the ingress PE and in the most 
  common Internet or VPN base use case it will not even have 
  an EH so that this EH insertion will result only in a single EH.

  Alternatively, the PLR could also be configured to perform
  encapsulation with a new IPv6 header using the repair SID
  as IPv6 destination address, without needing any EH.
  This will work for the vast majority of cases. 
  Remember that one 128-bit SID in SRv6 is in most cases
  equivalent to 2 MPLS labels, i.e. a node label plus an
  adjacency SID can be encoded in a single SRv6 SID.

  Only in extreme cases would the PLR need to add an 
  EH to the new IPv6 header with more SIDs.

- EH insertion for TI-LFA has nothing to do with stitching SRv6 domains.

Hope it helps.

Cheers
Dirk

> Am 08.09.2019 um 09:19 schrieb Gyan Mishra <[e-mail]>:
> 
> From reading through all the discussion threads the SR insertion is two fold one being for FRR capabilities using Ti-LFA or remote LFA tunnel so end up requiring double EH insertions on the Ingress PE tunnel head end SRv6 source node and then second scenario being a possible EH insertions occurrence on intermediate nodes.  I have not read through the drafts or RFC regarding Ti-LFA with SR but since that is an IGP extension I am guessing an opaque LSA and is not the traditional MPLS FRR link/node/path protection that adds an additional mpls shim so not sure why an EH insertion needs to occur for Ti-LFA.  Can someone clarify that use case for me.  Also the EH insertion on intermediate node what is the use case or reason for that.  My guess is it’s for special use case of stitching SRv6 domains together.  Please clarify..
>