How to change the default GRE MTU Size in 2900 series router

[ciscoroyzhang]

       Hi there,   we have cisco 3845 router connect to 2920 router via GRE tunnel.  now the size in 3845 is 1500 bytes while 17904 in 2900 serial router. is any command available to reduce the MTU size to 1500 in 2921 router? 

Show Tunnel in 2921 router

Tunnel7 is up, line protocol is up

Hardware is Tunnel

nternet address is XX XX XX XX/24

MTU 17904 bytes, BW 2048 Kbit/sec, DLY 50000 usec,  

     reliability 255/255, txload 24/255, rxload 38/255

Encapsulation TUNNEL, loopback not set

  Keepalive not set

  Tunnel source XXXXX, destination XXXXX

  Tunnel protocol/transport GRE/IP

    Key 0x9A2FA5, Order sequence numbers 91784088/26580970 (tx/rx)

    Checksumming of packets enabled

  Tunnel TTL 255, Fast tunneling disabled

  Path MTU Discovery, ager 10 mins, min MTU 92

  Tunnel transport MTU 1464 bytes                       <<<<<<<<<<    configure tunnel to use packet size 1464 bytes, total MTU size in 2921 is 17904 bytes

  Tunnel transmit bandwidth 8000 (kbps)

  Tunnel receive bandwidth 8000 (kbps)

Show Tunnel in 3845 router

Tunnel7 is up, line protocol is up

Hardware is Tunnel

Description: fausdrw-r25011

Internet address is XX XX XX XX1/24

MTU 1514 bytes, BW 2048 Kbit, DLY 500000 usec,

     reliability 255/255, txload 34/255, rxload 32/255

Encapsulation TUNNEL, loopback not set

[InayathUlla Sharieff]

this should help:

https://supportforums.cisco.com/docs/DOC-2569

HTH

Regards

Inayath

***Plz rate if this info is helpfull.*******

[ciscoroyzhang]

Hi InayathUlla Sharieff,

     thanks for the reply.  you are right, to configure the GRE tunnel we have to reduce the packet size to fit the MTU 1500 bytes, we have mulitple site with 2800 series rouiter, the packet has been reduced and no problem. 

     but here the problem is  that on 2900 series router, the MTU is 17904 bytes, not 1500 bytes.   I need to work it out how to reduce the MTU to 1500 from 17904 bytes in 2900 router,  which compatble with the MTU in 3845 router. any comments are wlecomed.   Roy

[InayathUlla Sharieff]

I dont have the node as of now.

Dont you get the option to chang the mtu under tunnel interface?

[Peter Koltl]

Resolve IP Fragmentation, MTU, MSS, and PMTUD Issues with GRE and IPSEC

you should not always believe the MTU values in show interface output