MPLS Questions

Answered Question
Dec 17th, 2008

What triggers an ISP MPLS router to check if the packet received from the client is in the Label table to forward it via Layer 2 mpls instead of Layer 3 routing?

For example if there is a client that is advertising its subnet via BGP and has MPLS with the same ISP, once it sends a request from clientA to clientB over the mpls cloud.

Does every MPLS router check the packet for mpls labels regardless if it is suppose to forward MPLS or not?

Can you correct me if I wrong? MPLS adds an entry to the CEF FIB so that when the router does a lookup, it sees that the fib has an mpls label and then checks the LIB and later LFIB table?

Correct Answer by Harold Ritter about 8 years 2 months ago

Sparky,

Packets received from customers are generally received as IP packets. The FIB will either contain a label (or label stack) or not. This is generally where the decision is made to label switch or not.

Regards

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 4.3 (3 ratings)
Loading.
Correct Answer
Harold Ritter Wed, 12/17/2008 - 08:30

Sparky,

Packets received from customers are generally received as IP packets. The FIB will either contain a label (or label stack) or not. This is generally where the decision is made to label switch or not.

Regards

viyuan700 Wed, 12/17/2008 - 09:01

Does every MPLS router check the packet for mpls labels regardless if it is suppose to forward MPLS or not?

Yes every router check the labels.Well in MPLS cloud you will have router which are MPLS capable.

Only the edge rouetr have ability to take IP and add a label to it and when it leaves MPLS clous to take away that label.

Once edge router adds a lable it can happen that next hop router can give a diffrent label to that same packet.

Mpls router different label so that the packet can reach the destination as quick as possible.

cisconoobie Wed, 12/17/2008 - 10:24

Thanks for the responses, I guess my main question is, since a non-mpls cef router routes a packet using the fib/adj, once you enable mpls, that same fib shows a label (label stack) so the router knows to perform L2 mpls functions on it?

Otherwise, isnt it inefficient to have a router with mpls support, check packets for mpls even if they arent destined for mpls network?

Ahh hritter, i guess you answered this question. Thanks guys.

ajagadee Wed, 12/17/2008 - 10:35

To add to the previous posts...

One important decision that the router looks at before sending it to the appropriate processing engine, is the EtherType.

EtherType of 0x0080 refers to IP

EtherType of 0x8847 refers to MPLS

Based on the ethertype, the packet is handed over to the appropriate processing engine in the router.

Regards,

Arul

Harold Ritter Wed, 12/17/2008 - 11:08

Arul,

Just one small precision. The ipv4 ethertype is actually 0x0800.

Regards

ajagadee Wed, 12/17/2008 - 11:12

My bad, it was typo. Thanks for the correction Harold!

Regards,

Arul

Harold Ritter Wed, 12/17/2008 - 11:17

Arul,

Not a problem. By the way your comment was really relevant. The ethertype dictates whether the router does a FIB lookup (for 0x0800) or LFIB lookup (0x8847).

Regards,

Actions

This Discussion