MPLS TE RSVP

[Ratheesh mv]

By default RSVP uses state refresh technology. It helps to track the state of neighbors. 

What is the benefit of using RSVP hello message on the top of RSVP state refresh technology ?
What is the exact difference between RSVP hellos and  RSVP state refresh ?

Thanks in advance 

[Harold Ritter · Accepted solution]

Hi @Ratheesh mv ,

Here's a good explanation of the difference between the RSVP state refresh and RSVP hello messages.

"So we discussed earlier that how RSVP is a soft-state protocol and needs periodic refresh messages (PATH and RESV) to maintain the session state. Now lets say If a Node  goes down for whatever reason, we have to wait until the refresh messages are timed out before we can release the resources occupied by the session crossing that node. If we want a quicker resource release, then we can decrease the refresh timer which will result in periodic refresh messages sent more frequently and in that way a failure can be detected quickly resulting in a quicker resource release. However, this will consume in large amount of resources. Essentially, with this approach we solve one problem but created another one.

A better way to solve this particular problem was to learn from other protocols like IGP ‘s and implement an Hello extension to detect neighbor router failures and that’s what exactly happened. The Hello extension introduced the concept of RSVP adjacency which didn’t exist before. Earlier the routers were only aware of the status of the interfaces running the RSVP protocol (operationally up or down) and the state of the LSP-Path (or, more accurately, the state of the RSVP sessions). With the introduction of HELLO extension, RSVP started behaving like other protocols and can detect neighbor failures by constantly sending HELLO messages (REQUEST object) and expect an ACKNOLWEDGEMENT object in return. When several HELLO packets are missing, the neighbor is declared down and all the RSVP sessions are cleared. One can set faster interval for RSVP Hello messages as there are only two routers involved in RSVP peering routers compared to reducing the RSVP Refresh timer which will affect all the RSVP sessions crossing the midpoint router.  Though keep in mind this only solves the Problem of detecting a neighbor failure faster."

Source of this information:

https://packetpushers.net/rsvp-te-protocol-deep-dive-part2/

Regards,