Task 1.1 - Why is system MTU increase not required?

When configuring dot1q tunnels I thought it was always necessary to increase the system MTU on the transit switches.  The SG says it is not required in this case.  Can anyone tell me why?



  • It's not just through one switch either, its through 2.  Confusing!  Guess the safe bet is to always just increase the size but I'd like to know why they said this...

  • I agree with you,  raising the MTU is neccessary.


  • To be safe I usually raise it to 1504 myself.  One thing to keep in mind about this lab is that since I did do that my OSPF adj wouldn't form over area 38 between SW3 and SW4 because of mtu mismatch.  I used the command ip os mtu-ignore on Po1 of Sw4 and the adj came up.   

     I had to do the same thing for Area 38 adj as well between Sw 2 and Sw3.

  • Either use ip ospf mtu-ignore or change so that the MTU matches. It is not a must to raise the MTU but packets could be dropped if they are too large on transit switches.

  • I agree with you.  Just out of habit and best practice I raise it to 1504 and since I had done that I had to use ospf mtu ignore.  Otherwise I would of had to raise the MTU on the other switches and I didn't want to wait on a reload nor did it require it. 

    The reason I mentioned this was in case anyone else had raised their MTU to 1504 and then had trouble forming adjacency.

    on a side note, i can't get vlan 1111 R1 and SW4 to communicate.  Any idea what that might me?  MY solution is very similar in what the SG used to configure their dot1q tunnel with the exception of my raising my mtu on 2 of the switches.   I spent an hour trying to figure it out and I couldn't so my work around was to create a tunnel between Sw4 and R3 and advertise that to OSPF just so I could at least get the other parts of my lab to work but i'm really annoyed that i lost points (3) beacuse i cant' get that part to work.  Thanks.



  • Not sure, post your config if you want me to look at it.

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