MTU - Part 2

Here is the topology that I used. I have marked each link with a letter to mark the points at which the proceeding packet captures were done. (Note that PC1 and PC2 are GNS3 routers with their icons changed).

A] No MSS Change

Before making any changes to the MSS, I'll first initiate a telnet session from PC1 to PC2.


Packet Capture from PC1's F0/0 interface ("A"):

Packet Capture from R2's F0/1 interface ("B"):

Looking at the above captures we can see that the PC1 sent an MSS of 536 in its SYN packet, and PC2responded with an MSS of 536 in its SYN/ACK packet.


B] R2 F0/0 MSS Change

Now that we've done our benchmark test, let's see what happens when we set an MSS of 500 on R2's F0/0interface.

R2(config)#int f0/0
R2(config-if)#ip tcp adjust-mss 500

Now let's repeat the telnet test:

Packet Capture from PC1's F0/0 interface ("A"):

Packet Capture from R2's F0/1 interface ("B"):

Here we can see that when the packet leaves PC1, it is sending its SYN packet with an MSS of 536, but when the packet leaves R2, the MSS has changed to 500.


C] R2 F0/1 MSS Change

Now I'll remove the "ip tcp adjust-mss" command from R2's F0/0 interface and will apply it to R2's F0/1, this time using an MSS of 510.

R2(config)#int f0/1
R2(config-if)#ip tcp adjust-mss 510

Now let's repeat the telnet test:

Packet Capture from PC1's F0/0 interface ("A"):

Packet Capture from R2's F0/1 interface ("B"):

Here we can see the results are the same as the ones seen when the "ip tcp adjust-mss" command was put on R2's F0/0 interface.


D] MSS on Both Interfaces

As you might expect, if you set the  "ip tcp adjust-mss" command on both of the router's interfaces, it will use the lower of the two.