02-18-2009 01:36 AM
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
02-18-2009 02:21 AM
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
02-18-2009 05:11 AM
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
02-18-2009 07:56 AM
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
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