OTV Overlay interface MTU

[harindhafdo]

Hi All,

when I checked the overlay interface it shows 1400 as the MTU.

OTV-1# sh int overlay 1
Overlay1 is up
    MTU 1400 bytes, BW 1000000 Kbit
     Encapsulation OTV
     Last link flapped 19:02:22
     Last clearing of "show interface" counters never
     Load-Interval is 5 minute (300 seconds)
     RX
       6045068 unicast packets  0 multicast packets
       0 bytes  0 bits/sec  72 packets/sec
     TX
       0 unicast packets  0 multicast packets
       0 bytes  0 bits/sec  0 packets/sec

OTV-1#

Above is the working OTV device output. Does this means that we can transport only packets of MTU 1400 through OTV ? is this MTU considers only the payload or with headers ? if with headers upto which header it calculates ?

I checked on the configuration and couldn’t find any place where I can change this value, therefore I need to know, on what basis this 1400 is coming the OTV interface.

I tried by changing the isis lsp-mtu under otv isis but it did not make any change in the overlay interface MTU.

Rgds

Harin

[Jerry Ye]

Above is the working OTV device output. Does this means that we can transport only packets of MTU 1400 through OTV ? is this MTU considers only the payload or with headers ? if with headers upto which header it calculates ? 

No it doesn't mean the device is only able to transport 1400 bytes packet. This value is the MTU of ISIS control packet. You can try a very simple test if you can affort an outage:

- change the OTV joint interface's MTU to 1000

- shut, no shut the overlay interface

- at this point, OTV will not come up. You can verify it by using show otv isis adj

- once you change to joint interface's MTU back to 1500, OTV will come up

How big of the MTU can OTV transport? It is depends on the path MTU size.

HTH,

jerry