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Community Member

SP does not support MTU > 1496

Hi,

I am facing an issue in an environment where the SP infra connecting our MPLS P/PE is not supporting MTU size greater than 1496 because of limitation in their infrastructure.

The scenario is more like this

                 SITE - A                                                                                                                                                 SITE - B

   CE [  ] ---------------------   P/PE [  ]   --------    SP METRO CLOUD (MAX MTU SUPPORTED = 1496)  --------   P/PE [ ]  ----------------------  CE  [  ]

The interface type of P/PE & CE is GigabitEthernet.

I am going to deploy MPLS VPN services and in future TE services too. My question is that the size of the packet with a VPN LAbel will 1508 bytes and if TE services are enabled it will 1512 bytes.

To avoid fragmentation of packets sent from CE cloud of Site A to Site B  what is the best option ?

1. Activate ip tcp path mtu discovery command in CE + PE in both Sites ?

2. Reduce the mpls mtu lower than 1496 bytes ?

Which will be the best option and why ?

Thanks and Regards,

Kas

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: SP does not support MTU > 1496

Hello Kas,

the best option would be to change the provider getting one using modern equipment in their network.

your best choice could be that of reducing MTU on VRF interfaces (facing CE) in order to avoid packet fragmentation

Hope to help

Giuseppe

2 REPLIES
Hall of Fame Super Silver

Re: SP does not support MTU > 1496

Hello Kas,

the best option would be to change the provider getting one using modern equipment in their network.

your best choice could be that of reducing MTU on VRF interfaces (facing CE) in order to avoid packet fragmentation

Hope to help

Giuseppe

Community Member

Re: SP does not support MTU > 1496

Hi Giuseppe,

Thanks for your reply. Further to the second option you mentioned so that means that the VRF interfaces of the MPLS P/PE facing the CE cloud is configured with say

1. ip mtu 1496

2. ip tcp adjust-mss  1430      where 1496 - ( 20B IP header + 20B TCP header + 26 byte Eth Add)

Do you find the above logical ? I guess  that takes care of the tcp issues and for udp 1496 should take care.

Regs

Kas

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