11-16-2021 05:16 PM
I have an MTU issue I can’t figure out, on a Cisco 1100 when I configure a GRE tunnel and specify the MTU I get and fragmentation error
Int tun 650
ip mtu 1330
%Warning: IP MTU value set 1330 is greater than the current transport value 1306, fragmentation may occur
%TUN-4-MTUCONFIGEXCEEDSTRMTU_IPV4: Tunnel650 IPv4 MTU configured 1330 exceeds tunnel transport MTU 1306
Now when looking at the tunnel interface you can clearly see below that the transport MTU is set to 1476, which is the default MTU minus the 24-byte GRE header. Note that all I did was configure the tunnel interface without any other configuration, as soon as I set the MTU I get a warning.
So why am I getting a complaint about MTU fragmentation and where is this 1306 coming from? I should be able to set the MTU at 1330 since it’s below the transport MTU of 1476. I get the same error at 1400 MTU, it only goes away if its below 1306 and I have no idea where the router is getting 1306 from.
Could this be a cosmetic bug?
#sh int tun 650 | in MTU
MTU 9976 bytes, BW 100 Kbit/sec, DLY 50000 usec,
Tunnel transport MTU 1476 bytes
Cisco IOS XE Software, Version 17.03.03
Cisco IOS Software [Amsterdam], ISR Software (ARMV8EL_LINUX_IOSD-UNIVERSALK9-M), Version 17.3.3, RELEASE SOFTWARE (fc7)
cisco C1161X-8PLTEP (1RU) processor with 1987967K/6147K bytes of memory.
11-16-2021 10:40 PM - edited 11-16-2021 10:41 PM
what is the IOS code running ?
Both sides need to be the same MTU setup ?. check the below output to see if you can find that anywhere configured?
show ip interface tunnel --> show you MTU for tunnel
show interface tunnel ---> show you MTU of physical interface
show interface gix/x | include MTU
show ip interface gix/x | include MTU
11-17-2021 09:30 AM
int tun 650
sh int tun 650
Tunnel transport MTU 1476 bytes
All other interfaces have the default MTU of 1500, as does tunnel 650 when created and shows 1476 which is -24 for GRE header.
I'm assuming this has to be a bug, When I create the tunnel and add the ip MTU right away I get a warning
#int tun 650
#ip mtu 1400
%Warning: IP MTU value set 1400 is greater than the current transport value 1306, fragmentation may occur
But if I create the tunnel, exit then come back and add the ip mtu I get no warning.
#no int tun 650
#int tun 650
#exit
#int tun 650
#ip mtu 1400
no warning!!
I searched on the ISO released caveats but didnt see anything that would cause this.
Cisco IOS XE Software, Version 17.03.03
Cisco IOS Software [Amsterdam], ISR Software (ARMV8EL_LINUX_IOSD-UNIVERSALK9-M), Version 17.3.3, RELEASE SOFTWARE (fc7)
Any ideas.
Paul
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