MTU mismatch in switch interfaces

[from88]

Hello,

If the switch gets large packet and accepts it, but the egress port is configured with lower MTU number, than ingress port. The router would fragment it, but what the switch would do? Disacrd on egress ?

[Mark Malone]

It can depend on the hardware what it supports l2/l3 ,older switches may not support certain types of frame sizes or the manipulation of mtu and only standard packet size,  it also depends if the DF flag is set if it is traffic gets dropped as it specifies don't fragment  , routers can also drop the packet if this flag is set , certain endpoints can send the packet set like that from source.

If its a pure layer 2 switch I don't think they have any concept of ip fragmentation so the layer 3 device will  make the decision not the switch in that case

[from88]

so the pure l2 switch could forward out for example jumbo frame out the port which have lower mtu configured ?

[Mark Malone · Accepted solution]

I would think the router would fragment it first as it would know a port in path may not support it before it gets to layer 2 and theres also a feature endpoints use to try prevent this happening PMTUD it basically dynamically discovers between endpoints whats the minimum allowed mtu so I would say that may prevent your scenario from happening or a least should in theory