MPLS MTU Issue

Haris P · ‎02-18-2009

Dear experts ,

I have an ATM link between two branches and we are running MPLS between them .

I still didn't changed the MTU size on any of my interfaces (means I'm using all default value ) . But when I'm pinging with a packet size greater than 1496 , I'm getting error . Please see the below outputs

Router1#ping vrf mpls-test 200.0.6.1 source 192.168.6.1 size 1497

Type escape sequence to abort.

Sending 5, 1497-byte ICMP Echos to 200.0.6.1, timeout is 2 seconds:

Packet sent with a source address of 192.168.6.1

.....

Success rate is 0 percent (0/5)

Router1#ping vrf mpls-test 200.0.6.1 source 192.168.6.1 size 1496

Type escape sequence to abort.

Sending 5, 1496-byte ICMP Echos to 200.0.6.1, timeout is 2 seconds:

Packet sent with a source address of 192.168.6.1

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 8/9/12 ms

Router1#

Regards

Haris

Mohamed Sobair · ‎02-18-2009

Haris,

you should change the MTU value for all enabled MPLS Atm Interfaces.

The layer-2 header field in the ATM, precisely the (VPI,VCI) fields whcich are used as the MPLS label should be added to the MPLS MTU + 4 byte control word.

The layer-2 header equals 16+12 = 18 Byte

The Control Word equal 4 Byte

The result (sum of both) = 22

If you add the value 22 to the MPLS MTU in all Mpls enabled ATM interfaces, you should be able to ping.

HTH

Mohamed

Haris P · ‎02-18-2009

Dear Mohamed ,

Do you mean to change the vale "mpls mtu " to 1500 + 22 = 1522 ??

What about doing the below config ? Or whether I have to decrese the value for MTU again ?

ip mtu 1492

ip tcp-adjust mss 1470

Mohamed Sobair · ‎02-18-2009

Haris,

Why you want to decrease the MTU size?

These values are arbituary values!!!

well, The "ip tcp-adjust-mss" takes into consideration the TCP header "TCP window size".

Normally implemented in GRE Tunneling. you should consider the 20 byte TCP header + 20 Byte IP header + 4 Bytes GRE header , The total comes to 44 Byte.

The result should be "IP tcp-adjust-mss 1466" , this if its configured on a gre interface., However if configured in different type of interfaces , you have to include the 20byte TCP header.

HTH

Mohamed