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Why is my Tunnel MTU greater than my serial MTU?

My 4 T1 serial interfaces are all set at 1500 bytes for the MTU. But my Tunnel interfaces are set to 1514 bytes? How did this happen? I thought the Tunnels should match the serial interfaces they use.

1 REPLY
Anonymous
N/A

Re: Why is my Tunnel MTU greater than my serial MTU?

You should use an MTU on your

tunnel interfaces that is below

or equal to the MTU of the

outbound interface, minus the

tunnel protocol overhead, minus the normal IP header size. But this does

not happen automatically; you

need to configure this manually.

If an encapsulated packet of the

tunnel cannot be sent through

the outbound interface, it is

fragmented (even if the DF bit

is set in the payload packet!

This means that applications will

not be able to detect it; the

tunnel is completely transparent

for them, it acts like a serial

line with the given MTU).

For GRE encapsulation

(the default for tunnel

interfaces), this would be 24

bytes of overhead and the MTU of

the tunnel should be set no higher

than the MTU of your outbound

interface minus 24, ie.

1500-24=1476. (The GRE header

uses 4 bytes, the IP header is

20 bytes if there are no options.)

Above this figure, packets will get fragmented and

the performance will be degraded

(but the additional overhead is

only 20 bytes per packet, or 1.3%;

you might be willing to ignore this

as long as your router CPU can

cope with the fragmentation).

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