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how can RSVP-TE use IGP TE extension for LSP computation?

What relation between them.for example, ospf opaque LSA and RSVP-TE.

Could somebody give me a detailed description?

1 ACCEPTED SOLUTION

Accepted Solutions

Re: how can RSVP-TE use IGP TE extension for LSP computation?

Hi Ricky,

Step 1) IGP - Gives us the status of all the links with respect to TE. That is all routers will tell each other about the resource availability like the bandwitdh available for each type of priority reservation from 0-7, max reservation possible, if any TE specific attributes or metrics assigned to the links etc.

The IGP behaves just like it would for normal IP routing that is it will give information about the current status. Now the way we run SPF on the database to get our routing table, in TE we run CSPF to get the table for RSVP Path messages to traverse.

Step 2) Now based on the Tunnel tailend IP the path is selected after running CSPF on the LSA/TLV received. This path is your ERO.

Step 3) Now the RSVP kicks in it sends PATH messages on this ERO path and the path is recorded using RRO.

Step 4) The PATH messages traverse till the Tail End. And each hop when it receives the PATH message send a RESV messages back saying I hav the requested resource and blocks the resources for the setup. If the RESV messages are received from all the hops in between till the tail end router. The TUnnel setup completes.

Now if you see the IGP did not use any ERO, for path calculation. But when you run CSPF on the Opaque LSA/Wide TLV received from the IGP you get the best path. This path is the ERO, which consists of the IP addresses of the devices till the tail end.

And when you use explicit routing in your configuration you dont want to use the CSPF path but want to use a manually decided path which is not the ERO.

HTH-Cheers,

Swaroop

8 REPLIES

Re: how can RSVP-TE use IGP TE extension for LSP computation?

Hi,

there are two things to be done by a MPLS TE tunnel head end:

1) Path computation

2) signaling of newly calculated path

For 1) the information learned through the IGP TE extensions will be used. So OSPF or ISIS will calculate the path taking into account the constraints (bandwidth, affinity bits, etc.) imposed. RSVP is not involved at all.

When the path is calculated in step 1), then all the reservations need to be made and also tunnel labels need to be exchanged - the LSP needs to be implemented. This is achieved through RSVP - Resource Reservation Protocol.

Once the reservation succeeds RSVP hellos are used f.e. to check for path errors. And the new bandwidth availability on the links will be announced through IGP TE extensions, i.e. OSPF opaque LSAs or ISIS LSPs.

So each of the protocols takes care of part of the picture: IGP topology database for path computation, RSVP for LSP signaling and resource reservation.

Hope this helps! please rate all posts.

Regards, Martin

New Member

Re: how can RSVP-TE use IGP TE extension for LSP computation?

hi,martin:

Thanks for your quickly response.As you know,ERO in path message is used to create and control the LSP(path).

If RSVP is not involved in path computation at all,how can RSVP control the path creation via ERO ?

Re: how can RSVP-TE use IGP TE extension for LSP computation?

Hi Ricky,

From the information received from IGP extensions for path calculaltion, and the ERO object, the RSVP-TE generates Labels for that specific traffic engineered path. Which is the Tunnel label, which is used to switch traffic on an engineered path.

Thats how RSVP-TE controls the LSP setup.

HTH-Cheers!

Swaroop

New Member

Re: how can RSVP-TE use IGP TE extension for LSP computation?

Hi,swaroop: Thanks for your reply, Can you explain how to control path via ERO?

Re: how can RSVP-TE use IGP TE extension for LSP computation?

Hi,

IGP calculates the path, i.e. a sequence of interface IP addresses. This sequence is then handed over to RSVP to signal the path setup.

You can control ERO IP addess sequence by configuring it explicitly for a specific tunnel through "path-option explicit " and a path-list containing the IP addesses.

IGP calculation can be configured through path-option dynamic.

For the detailed commands have a look at the MPLS TE configuration guide of your specific IOS version or the command reference.

Regards, Martin

New Member

Re: how can RSVP-TE use IGP TE extension for LSP computation?

Does it mean IGP use ERO for path calcultaion ? right? IGP must consider the ERO in path selection?

Re: how can RSVP-TE use IGP TE extension for LSP computation?

Hi Ricky,

Step 1) IGP - Gives us the status of all the links with respect to TE. That is all routers will tell each other about the resource availability like the bandwitdh available for each type of priority reservation from 0-7, max reservation possible, if any TE specific attributes or metrics assigned to the links etc.

The IGP behaves just like it would for normal IP routing that is it will give information about the current status. Now the way we run SPF on the database to get our routing table, in TE we run CSPF to get the table for RSVP Path messages to traverse.

Step 2) Now based on the Tunnel tailend IP the path is selected after running CSPF on the LSA/TLV received. This path is your ERO.

Step 3) Now the RSVP kicks in it sends PATH messages on this ERO path and the path is recorded using RRO.

Step 4) The PATH messages traverse till the Tail End. And each hop when it receives the PATH message send a RESV messages back saying I hav the requested resource and blocks the resources for the setup. If the RESV messages are received from all the hops in between till the tail end router. The TUnnel setup completes.

Now if you see the IGP did not use any ERO, for path calculation. But when you run CSPF on the Opaque LSA/Wide TLV received from the IGP you get the best path. This path is the ERO, which consists of the IP addresses of the devices till the tail end.

And when you use explicit routing in your configuration you dont want to use the CSPF path but want to use a manually decided path which is not the ERO.

HTH-Cheers,

Swaroop

New Member

Re: how can RSVP-TE use IGP TE extension for LSP computation?

all,I really appreciate your help. I'll continue to learn MPLS-TE.

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