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Community Member

Problem with the MTU size

On a router with Tunnel interface configured when I issue 'sho interface tunnel' command the MTU size shown is 1514, but when I issue 'sho ip interface tunnel' the MTU size is 1476. What is the actual MTU applicable to the interface.

5 REPLIES
Cisco Employee

Re: Problem with the MTU size

The effective mtu on the GRE tunnel is the MTU size of the physical outbound interface minus 24 bytes (additional IP and GRE headers).

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
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Silver

Re: Problem with the MTU size

If you are experiencing problems with the MTU you can increase the IP MTU by using the ip mtu 1500 command. The proper way to correct this is to delete the tunnels then create the tunnels again with the ip mtu 1500 command

Community Member

Re: Problem with the MTU size

Use wireshark and pingmaster ( the flagshipsoft one ) to change the ping mtu real time and see what the threshold is for the ping getting broken up.

Super Bronze

Re: Problem with the MTU size

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Posting

You might be confusing interface MTU, 1514, with a protocol (IP) specific MTU, 1476.

As noted by Harold, the tunnel has defaulted to an IP MTU of 1476 (1500 less 24 GRE overhead), which can be the ideal size for a GRE tunnel.  (Is any thing else shown for show interface #, like "Tunnel protocol/transport GRE/IP"?)

Since your IP MTU showed as 1476, then tunnel is probably using GRE and is providing a IP MTU that's 1500 less GRE header of 24, i.e. 1500 - 24 = 1476.

The 1514 MTU would be the MTU the tunnel is providing.  i.e. "What is the actual MTU applicable to the interface."

I haven't tried this myself, but since the tunnel's MTU isn't a real physical MTU, I would assume it can be increased using the MTU command.  NB: having the MTU larger than what the path's physical MTU can support will require the packet to be fragmented to forwarded.

Problem with the MTU size

Hi,

this problem has been discussed here already, see

https://supportforums.cisco.com/message/3375541#3375541

e.g.

HTH,

Milan

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