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New Member

MPLS over GRE tunnles

HI : Are there any MTU issues of running MPLS over GRE tunnels??

what will be the MTU size ?

thnak you

  • MPLS
New Member

Re: MPLS over GRE tunnles

GRE has an overhead of 24 bytes, and can directly interfere with the MPLS overhead. The MTU associated with an MPLS packet is broken down like so:

Ethernet Payload - 1500

802.1q header - 18

AToM Header - 4 (Required for ATM and FR only)

AToM Label - 4

LDP Label - 4

TE Label - 4

MPLS Fast Reroute - 4

Total = 1538

Granted, you may not configure all of those features above into your MPLS network, this is a good baseline to use for the MPLS MTU. You need to configure the core network to accept an MTU of at least 1538 bytes, without GRE.

You need to ensure that your GRE tunnels can support an MTU greater than 1562 if you plan to implement additional MPLS features like TE and AToM.


Re: MPLS over GRE tunnles

If you are using Fastethernet interfaces, 1476 will be the MTU used.

Hope this helps.

New Member

Re: MPLS over GRE tunnles

Also, if you get a chance (and have trouble falling asleep) - check out RFC 4023 regarding MPLS over GRE. Here is a cut from that RFC for starters:

"The MPLS-in-GRE encapsulation encapsulates an MPLS packet in GRE [RFC2784]. The packet then consists of an IP header (either IPv4 or IPv6), followed by a GRE header, followed by an MPLS label stack as specified in [RFC3032]."

Hope that helps.

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