04-29-2020 02:57 AM
Hi team,
IN our network we have to change the mtu size due to GRE overhead ( 12 bytes ). For that i have come with 2 commands Do i need to enter both commands or one if enough ?
Current IP MTU(1500) = IP Header ( 20 ) + TCP header ( 20 ) + MSS ( 1460 )
New IP MTU (1488) = IP Header ( 20 ) + TCP header ( 20 ) + MSS ( 1448 )
Ethernet MTU ( 1514 ) =Ethernet Header (14) + GRE Header ( 12 ) + IP Header ( 20 ) + TCP header ( 20 ) + MSS ( 1448 )
Option-1
Change IP MTU to 1488
Router1(config)# int ethernet 1/0
Router1(config-if)# ip mtu 1488
Option-2
Change MSS to 1448
Router1(config)# int ethernet 1/0
Router1(config-if)# ip tcp adjust-mss 1448
Do we need to go with both options or either one of them is enough ?
04-29-2020 03:26 AM
Hello @jonk34567 ,
both options because not all traffic is based on TCP for example a video stream would use UDP.
Hope to help
Giuseppe
04-29-2020 08:51 AM - edited 04-29-2020 08:51 AM
BTW, GRE adds 24 bytes, as the GRE tag, itself is 4 byte, but another IP header (20 bytes) is added.
Also BTW, IP TCP adjust-mss only needs to actually be placed on one router, it deals with traffic both in and out (although I too generally place in on both routers).
You might also consider enabling PMTUD on the router, to deal with a link between the GRE end points that doesn't support 1500 MTU. (Effectively, the GRE packets get DF set.)
Also keep in mind, the 20 byte sizes for IP header and TCP headers are minimums. Either or both can be larger (another reason for PMTUD).
A Cisco paper on dealing with encapsulation fragmentation is: https://www.cisco.com/c/en/us/support/docs/ip/generic-routing-encapsulation-gre/25885-pmtud-ipfrag.html.
04-30-2020 04:16 AM
Hi,
If two routers are communicating with diffrent MTU sizes what will happen ? will they choose the lowest amoung them or they will communicate with their own vaules.
04-30-2020 07:56 AM
04-30-2020 09:17 PM
Hi Joseph,
Are suggesting to configure PMTUD with below commands. Also if i execute below command does it affect only tcp traffic and ICMP traffic still be frangmented ?
ip tcp path-mtu-discovery
05-01-2020 11:05 AM
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