Hello
i have DMVPN setup at a customer site...which is largely working fine. However, I have noticed something on one of the hub routers (ISR4451x running IOS-XE 15.4 (3.13.2s) which is easy to explain away as a bug but will appreciate gaining some insight into?
The mGRE interfaces (hub/spoke) are configured with ip mtu 1416. On most routers (largely ISR4xxx, a few 29xx) I have checked, I see the following:
show crypto ipsec sa detail | i mtu
plaintext mtu 1378, path mtu 1416, ip mtu 1416, ip mtu idb Tunnel0
However, on the hub router in question, this is what i see:
show crypto ipsec sa detail | i mtu
plaintext mtu 1458, path mtu 1500, ip mtu 1500, ip mtu idb (none
I'm guessing the ip mtu idb is the key in all of this and that IOS will select this based on what interfaces have crypto maps or tunnel protection ipsec applied but in what scenario will the platform be unable to determine the ip mtu idb and just use the default?
This has recently become "interesting" as there are data transfer issues from one of the remote sites to this hub site and I am eliminating suspects as I look into it.
Thanks in advance
Sayre
