Ethernet MTU size 1500 - Max Frame size 1518 (1522)

Answered Question
Dec 17th, 2008

Hi Guys,

All routers and switches are set to 1500 byte MTUs. With the Ethernet header the frame maximum bigger than this.

If a router received a frame of 1518 (max payload 1500 + ethernet header) does the router fragment this packet?

Thx

Ken

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Correct Answer by Giuseppe Larosa about 7 years 11 months ago

Hello Ken,

the ip MTU is set to 1500 byte this implies an ethernet frame of size 1500+18 can go through.

The router will receive it correctly and will extract the ip packet inside the frame and will forward it

besides this, ethernet frames cannot be fragmented.

Problems can arise with MPLS, double vlan tagging (q in q) that can add more overhead if this happens the error counters of type giants or log messages should be seen.

This can require some tuning on MTU on routers and lan switches.

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Wed, 12/17/2008 - 03:19

Hello Ken,

the ip MTU is set to 1500 byte this implies an ethernet frame of size 1500+18 can go through.

The router will receive it correctly and will extract the ip packet inside the frame and will forward it

besides this, ethernet frames cannot be fragmented.

Problems can arise with MPLS, double vlan tagging (q in q) that can add more overhead if this happens the error counters of type giants or log messages should be seen.

This can require some tuning on MTU on routers and lan switches.

Hope to help

Giuseppe

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