MPLS over default MTU

[jethro.w.tambeana]

Hi there, I have MPLS running over our terrestrial microwave link to a remote site. our microwave link is operating at MTU 9000 and L2 Switches in the path all have jumbo frame enabled. we now need to setup a backup link via another ISP however the issue is that the ISP cannot provide a link with MTU value greater than default 1500. My question is can MPLS be tweaked so that it operates over a link with mtu 1500?

What other option do i have?

[MHM Cisco World]

can you elaborate more, 
if I understand correctly you can config the interface connect to ISP with MTU 1500.

[jethro.w.tambeana]

Hi there, thank you for your response. see below the traffic path from HQ MPLS to remote MPLS Site. If the ISP can only provide a link that has PMTU 1500, is it possible to make MPLS work over that link?

MPLS Router(HQ) <--> Backup link via another ISP Network (PTMU 1500) <--> MPLS Router (Remote) <--> Remote LAN

[MHM Cisco World]

R(config)#interface FastEthernet2/6
R(config-if)#mpls mtu YYYY
!
R#show mpls interfaces fastEthernet 2/6 detail
Interface FastEthernet2/6:
        IP labeling enabled
        LSP Tunnel labeling not enabled
        BGP labeling not enabled
        MPLS not operational
        MTU = YYYY 

So for the MPLS interface you can change the MTU according to your WAN requirement, the Command use is MPLS  MTU YYYY and you can check the new value via show mpls interface.
hope this what you looking for.

[jethro.w.tambeana]

Thank you for that. so my MPLS router connects via the ISP backup link via Ethernet with MTU 1500. what is the value to set for mpls MTU on my router ethernet interface which connects to the ISP link?

[MHM Cisco World]

“Although you can set the MPLS MTU to a value greater than the MPLS MTU, it is recommended that you keep the MPLS MTU less than or equal to the interface MTU to prevent the hardware from dropping packets.”

this cisco recommend for MPLS MTU,

so you can config mpls mtu to Be equal ip mtu which is 1500 this prevent the Packet from drop.

Note:- with mpls mtu the router will recalculated the mtu with consider the label that will be add to Packet. 

[jethro.w.tambeana]

Hi there, further to my previous email, see below the MPLS config on my router on the HQ side and the remote site. these configs below are for when the link is using our primary link which supports 9000bytes MTU value. If we switch to backup link which only supports 1500bytes MTU value, what would be the value to set for MTU on both HQ router and remote router?

HQ MPLS Router interface

interface GigabitEthernet0/0/2
mtu 9216
ip address 172.30.x.x 255.255.255.252
negotiation auto
mpls ip
mpls mtu 9162

REMOTE MPLS ROUTER

interface TenGigabitEthernet0/0/2
mtu 9216
ip address 172.30.x.x 255.255.255.252
no negotiation auto
mpls ip
mpls mtu 9162

[MHM Cisco World]

Hi Friend, 
please can you contact cisco and open TAC case for this issue, 
why? because I worry about the buffer and CPU in your HQ router and remote Site Router, they will do huge work to fragments the packet from jumbo to 1500 packet.
please mention the HW and version of IOS you use in both side.

note:-you do check and see if the packet more than 1600 is drop.

[MHM Cisco World]

https://www.networkworld.com/article/2345240/how-to-avoid-fragmentation-of-mpls-packets-over-fast-ethernet-interfaces.html

make review for this link.