I have a DMVPN setup using simple GRE and VRF's with one Hub and a few spokes. At one client we are having MTU issues where we have had to manually change the MTU of the servers on the hub side and the clients on the spoke side to 1476 or lower. I realize this is due to GRE overhead but I dont understand where the negotiation is failing and how to resolve it without manually changing MTU on all of the devices (some devices have no option to set MTU).
I have read about writing a policy map to clear the DF bit and adjust the MTU size...is this the proper method? I have also read about the tunnel path-mtu-discovery command but I dont have a test router handy and I am unsure if this command is on the tunnel by default. Anyone have any experiance with GRE tunnels and possible MTU issues that could shed some light on this for me? Below are some of my config if it helps.
Hub:
interface Tunnel1
ip vrf forwarding xxxxx
ip address x.x.x.x x.x.x.x
no ip redirects
ip mtu 1400
ip nhrp authentication dmvpn
ip nhrp map multicast dynamic
ip nhrp network-id XXX
delay 1000
tunnel source GigabitEthernet0/0
tunnel mode gre multipoint
tunnel key XXX
tunnel protection ipsec profile XXXX shared
Spoke:
interface Tunnel1
ip address x.x.x.x x.x.x.x
no ip redirects
ip mtu 1400
ip nhrp authentication dmvpn
ip nhrp map multicast x.x.x.x
ip nhrp map x.x.x.x x.x.x.x
ip nhrp network-id xxx
ip nhrp holdtime 300
ip nhrp nhs x.x.x.x
no ip split-horizon eigrp 2
delay 1000
tunnel source GigabitEthernet0
tunnel mode gre multipoint
tunnel key xxx
tunnel protection ipsec profile xxxxx
