Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

<Some MPLS questions, agian>

I . when I read the "Label Distribution topic" I getting in trouble with this problem:

- What are the differents between PE and P routers? (in Label distribution mechanism).

- When distrubute Label, which Table dose PE (or P) base on ?

And I feel not sure to anwser those questions :

QUESTION NO: 1

How does a PE-router operating in the default frame mode distribute labels?

A. The PE-router distributes a label for every destination in its FIB.

B. The PE-router only distributes labels for destinations inside the MPLS domain.

C. The PE-router only distributes labels for destinations outside the MPLS domain.

D. The PE-router does not distribute labels. Label distribution is only done by the core

LSR.

==> can you explain for me: what is called "default frame-mode"? in the default frame-mode is "PHP" is enabled by default ?

QUESTION NO: 2

How could you check for potential MTU size issues on the path taken by a PE-to-PE

LSP?

A. Because MPLS packets are label switched, MTU problems can only be detected by the

user applications.

B. Use the ping vrf command with packet size set to the largest MTU along the path

and DF bit set from the local PE-router to ping the remote PE-router.

C. Use the ping vrf command with packet size set to the smallest MTU along the path

and DF bit set from the local PE-router to ping the remote PE-router.

D. Because MPLS packets are label switched, packets are automatically fragmented and

reassembled by the PE-routers.

Therefore, there are no potential MTU issues.

==> I choose B in this question, is it correct ?

Thanks.

1 REPLY
New Member

Re: <Some MPLS questions, agian>

MPLS can work on two modes Frame mode and Cell mode.

Routers by default support frame mode.ATM switches and routers with ATM interface support cell mode.

PHP means Penultimate Hop.PHP is a term use to refer to node which is one HOP before the destination.e.g

R1-R2-R3-R4,your destination is on R4 in that case R3 is acting as PHP.It pops the label and sends it out as IP packet.

The PE router assigns the labels to all its destination learned through IGP.However BGP routes gets the label through their next hop e.g If you learn 120K BGP routes through single next hop.All the BGP routes will be having one label.

There is a feature called BGP with labels which overcomes this limitation.

-Waris

359
Views
0
Helpful
1
Replies
CreatePlease login to create content