[c-nsp] Best practice, MPLS and MTU settings

Eric A Louie elouie at yahoo.com
Thu Oct 24 21:32:26 EDT 2013

I'm preparing to recommend how my company should engineer the MPLS connections within our backbone.  I'd like some feedback.

Right now, I have a suggested workaround that reduces the MSS on the upstream Internet interfaces, so that we keep the TCP payload at 1400.  This is due to asymmetrical traffic flows (Internet BGP preferred routes to my ISPs) and DF bits set by servers in web-land.  When one of these 1500 byte packets enters an MPLS interface, the label causes the packet to be 4 bytes too big, and the packets are dropped and the user is left with an incomplete webpage or a blank page.  I tested raising the MTU on the MPLS interfaces and found that it worked as expected.  So my permanent solution is to raise the MTU on the MPLS interfaces to some reasonable number of bytes.

I have routers that have a maximum IP MTU of 1530, and other that have maximums of 9980 and 9216.

The 1530 byte MTU devices are not in the core of the backbone and would not transport any tagged traffic.  I'm not going to create any MPLS tunnels throughout the network - all MPLS traffic will be with MPLS-capable switches point to point.

So my recommendation would be a 1600 byte MTU on MPLS interfaces.  I have packet size value information to support my recommendation.  It also seems to be the value that Cisco recommends.

How have you solved this fragmentation problem?  If you used the method of increasing the MTU, what value did you use?

Thanks, I'll summarize or share the data numbers if you are interested.

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