MTU is being automatically changed by something

[pavelsh_ucs]

Hi,

I've got very strange issue on two of my routers.

I've pretty straightforward tunnel configuration:

interface Tunnel0

ip address 10.5.4.66 255.255.255.252

ip mtu 1390

ip virtual-reassembly

ip tcp adjust-mss 1350

ip ospf authentication message-digest

ip ospf message-digest-key 1 md5 7 123

ip ospf network point-to-point

ip ospf cost 6580

load-interval 30

qos pre-classify

tunnel source x.y.z.a

tunnel destination z.x.a.b

tunnel mode ipsec ipv4

tunnel path-mtu-discovery

tunnel protection ipsec profile ospf-tunnel

All works fine but sometime cisco is changing this tunnel ip mtu to something else. Sometimes it shows in config, sometimes no.

We have a not good time to get idea who (or what) is changing.

SNMP is disabled. tacacs command accounting shows nothing. sh run shows that last config change was two days ago, but mtu was changed in last hour.

Like a ghost change our mtu

ios is

c870-advipservicesk9-mz.124-15.T12.bin

[Peter Paluch · Accepted solution]

Hello,

I have not personally experienced a similar behavior but theoretically, this could be caused by the tunnel path-mtu-discovery command. You may want to remove it and see if the MTU stops being automagically changed.

If this issue is determined to be indeed cause by the tunnel path-mtu-discovery then I suggest you put the command back, as it basically adapts your tunnel MTU to an appropriate size.

Best regards,

Peter

[Jonathan Bayless]

How do you know it is changing and is it changing incorrectly? As Peter mentioned, the MTU discovery feature tells the router to try to figure out the MTU on its own and use what it thinks is best.

[pavelsh_ucs]

Hi Jonothan,

Just check the output of "sh ip int". Also "sh ip ospf nei" shows me that some ospf neighbour can't make adjancency with my router.

Thanks