GRE Tunneling performance issue

Unanswered Question
Jun 20th, 2008

hi guys,

is there a way to avoid fragmentation caused by gre tunneling (that is normal packet + gre header addition causes fragmentation)

cpu cycles hits huge due to this fragmentation and defragmentation on tunnel destination

anyone have idea how to avoid this ?

i have a one solution applying ip mtu command on router to avoid fragmentation

any one who has better experience on GRE tunneling

how to measure the performance issues by GRE ?

Any datasheet or white paper saying Which router can support amount of tunneling traffic ?


I have this problem too.
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Paolo Bevilacqua Sat, 06/21/2008 - 04:02


Actually if you apply IP MTU command, fragmentation will occur anyway. Simply said, there is no way to avoid it.

Attached a document with router performances. It is not relative to tunneling however should give you an idea of what you can expect.

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shiva_ial Sat, 06/21/2008 - 05:14

thanks for the reply

actually i have already come across tcp mtu adjustment command and the performance sheet you have attached.

i am curious to know is there any other way

some one has to put as best solution.


Paolo Bevilacqua Sat, 06/21/2008 - 06:30

Hi, if you routers suffer when fragmenting, you should look at buffers failures, in show buffers. Also what routers is your and what is the circuit speed.

Most people use GRE tunneling without issues.

Richard Burts Sat, 06/21/2008 - 08:52


I have used ip tcp adjust-mss with good success when using GRE tunnels. I would consider it the best solution to the fragmentation issue. I am puzzled at your reluctance to use it.



shiva_ial Mon, 06/23/2008 - 21:45

Hi rick,

pls explain how adjusting TCP MSS value will solve fragmentation issue.

what are all the advantages ?


Richard Burts Tue, 06/24/2008 - 03:53


If you use the ip tcp adjust-mss then the router will look for the TCP SYN packet that is used to initiate a TCP session. In this packet is the mss which determines the size that a TCP packet can be during this TCP session. The router will make sure that this value is no larger than a size that you specify. And you specify a size that will fit in your tunnel without requiring fragmentation. So all the packets in this TCP session go through the tunnel without requiring fragmentation.



shiva_ial Tue, 07/01/2008 - 21:25

yes,i understood on tcp front but what would happen in case of udp packets......



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