MPLS tag-switching

Unanswered Question

I'm trying to bootstrap myself on MPLS and ran into an unusual problem in the field.


We've got a mesh of routers setup to do tag switching amongst each other to support a few pseudo-wire MPLS point to point links for customers.


Our typical conf:

ip cef

mpls label protocol ldp

tag-switching tdp router-id Loopback0 force

!

int FastEthernet0/0

description WAN link

ip address 1.2.3.1 255.255.255.252

mpls label protocol ldp

tag-switching mtu 1532

tag-switching ip

!

int POS1/0

description Another WAN link

ip address 1.2.3.5 255.255.255.252

mpls label protocol ldp

tag-switching ip

!

int FastEthernet 2/0

description Non MPLS Cust Facing

ip address 10.10.0.1 255.255.255.0

!

int FastEthernet 2/1

description MPLS Pseudowire Cust Facing

mpls l2transport route 1.2.3.253 10101


Now, it was my understanding that the tag-switching mtu statement is only for tag-switched packets, untagged IP uses the interface's default mtu of 1500 if not otherwise adjusted. Someone removed the tag-switched mtu 1532 statement off a couple WAN links, and both MPLS and non pseudowire tagged traffic were affected. There was still basic L2/L3 connectivity, but larger IP packets were being dropped.


The observed behavior implied to me that all packets were being tagged, and reading up on the tag-switching command reference re-enforces that assumption. Dropping down to the lab, doing a ping between two routers with tag-switching setup and ldp neighborship established, neither echo requests or replies are tagged.


Where is the hole in my understanding of how this works?

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Harold Ritter Wed, 02/04/2009 - 12:56

Your understanding is correct. By default LDP will allocate and advertise label for all prefixes and all traffic will be therefore label switched. You need to configure label filtering if you only need certain prefixes to have labels assigned and advertised by LDP.


Here's a brief example where only loopback addresses for the routers hosting the pseudowires are advertised.


no tag-switching advertise-tags

tag-switching advertise-tags for 1

access-list 1 permit 192.168.100.0 0.0.0.255


This same configuration need to be applied for all LDP enabled routers. It will only allow label advertisement for prefixes in the 192.168.100.0-192.168.100.255 range.


Regards

shivlu jain Thu, 02/05/2009 - 06:43

In addition to hritter post,


You always advertise labels for your loopbacks which are using for peering with BGP. Because on everyhop the next hop will always be the PE loopback address and every router in path is having label for that loopback address.

If you are using the same technique then mind one thing not to do the summarization for the loopbacks. If you do it your all mpls traffic will be dropped on abr.


regadrs

shivlu jain



Mohamed Sobair Thu, 02/05/2009 - 07:37

Hi,


The New LDP is developed to support up to 4 labels in the label stack.


MTU differs, you have:

1- Layer-2 MTU.

2- IP MTU.

3- MPLS MTU.


Why MPLS MTU should be modified? cause the label is inserted/imposed between the layer2 and layer 3 headers. bear in mind the Controlword is also inserted between the layer-2 and layer-3 header.

so the sum will be as follows:


1- a single label of 4 Byte.

2- Control Word of 4 Byte.

3- Pseudowire would include l2 header 4 byte + vlan header of 4 byte (If tagging is configured).


The sum of (MPLS MTU) should be 1512 , without ethernet tagging.


HTH

Mohamed

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