Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

Bronze

MPLS MTU Issue

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

3 REPLIES

Re: MPLS MTU Issue

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

Bronze

Re: MPLS MTU Issue

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

Re: MPLS MTU Issue

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

452
Views
0
Helpful
3
Replies
CreatePlease to create content