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MTU PMTUD

Posted on 2014-11-23
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Last Modified: 2014-11-24
I know how Routers handle MTU restriction issues using ICMP return messaged to inform the originating host of the packet size issue and for the host to reduce its payload.

Just curious how this is dealt with in the Data Center where the fabric is layer 2 for the most part?
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Question by:sectel
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Garry-G earned 500 total points
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On a pure L2 connection, incoming frames larger than the configured MTU will be dropped. You may get error messages on the device, though that depends on the vendor and configuration.
For incoming frames that are still OK for that port, but too large for the outgoing port (not sure if that can happen on a switch, though - normally the MTU/Jumbo MTU is configured system-wide, not per port, and then left to Layer 3 to complain about), a signal might be sent to L3+ to notify the sending station, though I haven't seen such an option on any device I've come across.
In short - make sure all your switches are configured for large enough frames to allow for any current and future use case (we typically configure 1600 MTU and maximum allowed MTU, which is 9198 on Cisco switches IIRC, for Jumbo MTU ... the first will comfortably take care of any L2 switching and VLAN options like 1Q, as well as MPLS label stacks, the latter for high speed links with server interconnects etc.). This is especially important as some devices may require a reboot to change system-wide MTUs
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