MTU Size in GRE

[wasiimcisco]

Hi,

I am running GRE Tunnel in Hub and Spoke with OSPF. All branches connected to Hub with MPLS layer 3 cloud. But few times users complain about the network disconnection for outlook and other applications. Now one branch users continously complain for outlook disconnection and we raised the call with microsoft as per their saying we need to increase the MTU size on the router to 1500. Currently on the tunnel MTU size is 1476. Both on the Hub and spoke.

Brach side configuration

Branch has 8 Mbps Bandwidth

interface Tunnel560
description connected to the HO 
bandwidth 2048
ip address 172.27.5.78 255.255.255.252
ip mtu 1476
ip ospf network point-to-point
ip ospf cost 1
keepalive 5 3
tunnel source FastEthernet0/1
tunnel destination 192.168.253.37

Hub has 30 Mbps Bandwidth

interface Tunnel560
description connected to the Branch 
bandwidth 1024000
ip address 172.27.5.77 255.255.255.252
ip mtu 1476
ip ospf network point-to-point
ip ospf cost 1
keepalive 5 3
tunnel source GigabitEthernet0/0
tunnel destination 192.168.253.9

What will be the impact if i will change the MTU to 1500 and do i need to change anything like PMTU.

Secondly I have 8 Mbps bandwidth on the Brnach Do i need to change the bandwidth to actual or let it be 2 Mbps.

I have already 1476 +24 GRE= 1500 byte then why i need to specially configure 1500 MTU. Looking for the help from the experts.

[Laurent Aubert]

Hi,

If you follow Microsoft advise, fragmentation will happen after encapsulation and will kill the perf of your hub router ;-). What is happening is outlook IP packet are sent with DF bit set so they are dropped by the spoke router as they are bigger than 1476B.

Two solution:

1- Apply a route-map on all your spoke router LAN interface which clear the DF bit so fragmentation will occur before encapsulation. Drawback is it will increase the load on the server as they have to rebuild the packet.

2- Spoof the TCP mss so the correct size is negociated and no fragmentation will occur. This is the best solution. To implement it, apply the following command on your HUB LAN interface(s):

interface

ip tcp adjust-mss 1400

!

HTH

Laurent.