Understanding MTU given for GRE tunnel

[tommyboay]

Hi all,

I have many point to point GRE over IPsec tunnels for my OSPF routing.

I've been working on correcting MSS and MTU values to prevent issues. But something puzzles me :

agde06rtr03#show int tunnel 1

Tunnel1 is up, line protocol is up

  Hardware is Tunnel

  Description: *** blablabla ***

  Internet address is 10.5.4.46/30

  MTU 17916 bytes, BW 50000 Kbit/sec, DLY 50000 usec,

     reliability 255/255, txload 1/255, rxload 1/255

...

  Tunnel transport MTU 1476 bytes

  Tunnel transmit bandwidth 8000 (kbps)

  Tunnel receive bandwidth 8000 (kbps)

Why is that MTU value so high ? What does it represent ?

If anyone can help, thanks much in advance.

Tom

[Richard Burts]

Tom

Would you post the configuration of tunel1? My guess is that something in the configuration is setting the MTU that high. Your post of show interface is a good start. But without seeing exactly what is configured it is hard to give a good answer.

HTH

Rick

[tommyboay]

Hi Rick,

Here is the complete output :

interface Tunnel1

description *** blablabla ***

dampening

bandwidth 50000

ip address 10.5.4.46 255.255.255.252

no ip redirects

no ip unreachables

no ip proxy-arp

ip mtu 1400

ip pim dense-mode

ip tcp adjust-mss 1360

ip ospf hello-interval 1

ip ospf dead-interval 5

load-interval 30

qos pre-classify

cdp enable

tunnel source GigabitEthernet0/1.606

tunnel destination 10.5.5.77

[Rozsa Illes · Accepted solution]

Hello Thomas,

The MTU value in the show interface command was changed in the later IOS versions  The value is calculated based on the platform buffer sizes. So what you are seeing is the maximum value the router can support for a tunnel interface.

A new field, tunnel transport MTU, was added to check the actual MTU value of the interface.

Warm Regards,

Rose

[Richard Burts]

Rose

Thank you for this very helpful information.

HTH

Rick

[tommyboay]

Thanks you both for the time you took to answer my question !

Tom

[Kishore Chennupati]

Hi Rose,

Thanks for that info. May I quickly ask  what are the advantages of calculating the MTU based on platform buffers.

Thanks in advance

Kishore