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mpls mtu command

aretsu
Level 1
Level 1

Hello,

I am deploying a EoMPLS service with a service provider in his proudction network.

The Core routers on this network are configured with the "mpls mtu 1508". This MPLS MTU size does not work with an EoMPLS implementation, I need at least 1530 bytes, so I need to change the MPLS MTU command on all the Core Routers.

My question is: Is there any impact of configuring the "mpls mtu 1530"? Does this affect the LDP sessions I have? I think not but I have read the following:

"All devices on a physical medium must have the same MPLS MTU value in order for MPLS to interoperate"

Obviously, I will configure the "mpls mtu" on one router first and then on the neighbors, but one at a time (it is an ethernet segment) so ... will the LDP session remain up and running for the time it takes to configure all the routers?

Thanks for your responses!!

Alex

6 Replies 6

Harold Ritter
Cisco Employee
Cisco Employee

Configuring "mpls mtu 1530" has no impact. You just need to make sure that the layer 2 devices connecting the routers support ethernet frames of 1530 bytes.

The LDP session will not be affected while you are migrating from 1508 to 1530.

Let me know if I answered your question,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

Hi,

Sorry to barge in,but I have three questions:

1. If you do not configure 'mpls mtu' command on the interface is it true that default cisco behavior is to set mpls mtu to be ip mtu + 8 bytes?

2. when router receives a packet (IP or non IP) which is larger than mtu configured in ingress interface, the packet is dropped (as opposed to fragmenting packet, if allowed, on egress interface).

Is it true that if mpls is enabled on the interface router will fragment the packet on ingress interface as well (based on mpls mtu) and not drop it?

3. when passing labeled packets (1508 bytes and up) over switches interconnecting router, what do you have to enable on switches not to drop the packet as being "baby giants"? do you have to enable jumbo frames support to make it work or maybe something else?

Thanks,

David

1. the default bahavior is that the mpls mtu is the same as interface mtu (default of 1500 for Ethernet interfaces).

2. If "mpls mtu" is set to a higher value than 1500 on the igress interface then the incoming baby giant frames will be accepted if they contain an MPLS information. No fragmentation is required at this point.

3. This is different on just about every switches. Here's a good URL to help you determine what you need to configure on different Cisco switches:

http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml

Let me know if I answered your questions,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

Hi,

Thank you for your answer. Let me reiterate regarding point number 2 above.

My question is if mpls mtu is set for 1500 on the ingress interface and double tagged frame arrives (1508 bytes), will a router fragment a frame to two double tagged tagged frames or will it drop it until mpls mtu of 1508 and above is configured on this ingress interface?

Thanks,

David

In this case, the frame would be dropped. Fragmentation is typically performed on the outbound interface, not on the inbound.

As you indicated, setting the "mpls mtu 1508" on the ingress interface will solve the issue.

Let me know if I answered your question,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

Just to complete on my explanation, if the outbound interface on the ingress router had a mpls mtu of 1500 then fragmentation would take place assuming that an IPv4 packet was received and that the DF bit is not set.

Hope this helps,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
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