MTU size never crossed my mind until MPLS designed network came to the picture. After adding a switch to the network, OSPF got up, but BGP was going up and down and up and down… That led to the MTU mismatch because bigger frames are needed in a MPLS network.
The default MTU frame size is 1500 bytes, but in a MPLS design where you use labels, with every label you need a little more room. For the one label design you need to increase the MTU size to 1504 bytes. MPLS VPN, which is the most common design, uses two labels and thus you have to increase the MTU size to 1508 bytes or by eight bytes. Each label increased the size of the original IP packet to a value bigger than the original MTU size can handle. In a default MTU size, configuration packets that are bigger get fragmented and when such packets are sent with the Do-Not-Fragment (DF) bit set, packets get dropped by the router.
MTU size is configured on the MPLS interface of the device but first you can check the current MTU size that is configured with the show mpls interfaces “interface_name” detail command. To change the size input next command:
1. configure terminal
2. interface “your mpls interface”
3. mpls mtu 1508 or the maximum of 9000 to be sure
Then check your configuration again with the show mpls interfaces “interface_name” detail command.
london#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 = 1500 london#configure terminal Enter configuration commands, one per line. End with CNTL/Z. london(config)#interface FastEthernet2/6 london(config-if)#mpls mtu 1508 london(config-if)#^Z london# london#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 = 1508