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Cisco Router - ip tcp adjust-mss command

Posted on 2013-12-11
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Last Modified: 2013-12-26
I recently replaced an old Cisco 2621 Router with a new Cisco 2911 Router for one of my customers.  It was easy enough, but when I started doing speed tests and downloads I noticed that all tests were running very slow.  I would plug my laptop directly into the 50mbps circuit, bypassing the 2911, and all my speed tests and downloads came back good.  I was getting 50mbps, which is my circuit speed.  As soon as I would plug the router back in speed tests were back down under 1Mbps.  Speed and Duplex were fine, there were no interface errors, so I called Cisco TAC.  They ended up issuing the command "ip tcp adjust-mss 1360" on the inside interface of the router, and like magic I was able to achieve 50Mbps downloads and speeds through the 2911 router again.   Now I know the purpose of the command is to avoid packet fragmentation during the initial TCP handshake, but this is a command I have never used before.  I am wondering if this is best practice, and something I should be doing on all projects in the future?  Please share any thoughts you may have on this.  Thanks.
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Question by:denver218
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Jan Springer earned 250 total points
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This is not best practice.

However, in certain situations (as with GRE tunnels), the mss needs to be lower to account for overhead.
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by:Jody Lemoine
Jody Lemoine earned 250 total points
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The best practice is to adjust the MSS on interfaces that have lower than a 1500-byte MTU in order to avoid unnecessary fragmentation of packets. Fragmentation leads to multiple packets being sent and additional processing overhead, particularly if NAT is being used.

If your ISP is using PPPoE, for example, this is going to reduce the MTU of your interface by 8 bytes, leaving you with a MTU of 1492. If you attempt to slam 1500-byte packets into this, fragmentation will occur. Setting the MTU to 1452 (MTU - 40 bytes for TCP/IP header overhead) will tell the router to signal clients to send packets no larger than the MTU, avoiding the problem entirely.

For efficiency, setting the MSS should be done on the same interface that has the lower MTU. This will ensure that the router only signals clients to use a lower MTU when the traffic is actually traversing the affected link.
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by:denver218
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