Ip tcp adjust-mss on LAN and BVI

hi all,
just a quick question, we got routers configured with LAN interface and bridged to a BVI interface.
i want to set the ip tcp adjust-mss 1420 but which port will take precedence?
my question, which port do i configure this command?
interface FastEthernet0/0.2
description ### Corp LAN ###
encapsulation dot1Q 2
no ip redirects
ip accounting output-packets
ip nbar protocol-discovery
ip tcp adjust-mss 1420 <<<
interface BVI2
description ### Corp VLAN ###
ip address 192.168.231.1 255.255.255.0
ip flow ingress

Since this command works at the IP layer, you will need to apply it to the routed interface. That will be BVI2 in this case.
Regards,
Mike

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IP TCP Adjust MSS

Hi
We have a network setup where the customers comes via internet to 7600 and from there we for ward this to mpls-vpn cloud
CE -----Internet cloud -------Internet Access router --- 7600-----IP VPN cloud
we use ipsec tunnel from ce to 7600 .Sometimes customer complains of email/other Application not working etc.
Most of the issue are resolved when we put the ip tcp adjust mss command on lan from a higher value to lower value like from 1452 to 1350 etc.
Can somebody clarify abt the working of ip tcp adjust mss and its effect.
Thanks in Advance
Tarun

When a host initiates a TCP session with a server, it negotiates the IP segment size by using the MSS option field in the TCP SYN packet. The value of the MSS field is determined by the maximum transmission unit (MTU) configuration on the host. The default MSS value for a PC is 1500 bytes
Links for Reference:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122newft/122t/122t4/ft_admss.htm
http://cisco.com/en/US/tech/tk827/tk369/technologies_tech_note09186a0080093f1f.shtml
http://cisco.com/en/US/products/hw/routers/ps4081/products_tech_note09186a0080094268.shtml
But the actual MSS between two end points is derived as below.
MSS = MinPathMTU - MinTCLHeadrLen - MinIP HeadrLen = 20 - 20 = MTU - 40.
Now for GRE = GRE header + GRE IP HEader = 4 + 20 = 24
IPSEC = 60 to 72 approx depedning on the encryption used.
Since your internet routers wont be supporting more than 1500 bytes as an MTU, effectively the MSS available for
you host to server session is the actual MTU on the path minus the overhead mentioned above.
which is MinPathMTU - MinTCLHeadrLen - MinIP HeadrLen - (GRE header + GRE IP HEader) - IPSEC overhead
1500 - (40+24+60~72) = 1376~1364.
So a TCP MSS value of 1360 would be safe for your end-to-end TCP sessions over a GRE-IPSEC Tunnel.
If you were not doing a GRE-IPSEC till the 7600 and had a leased circuit to the 7600 then a MSS value of 1460 fits well.
1500-40.
HTH-Cheers,
Swaroop

"ip tcp adjust-mss " command

Hello Everyone,
I wonder "ip tcp adjust-mss " command useage. Basicaly, should i apply this command on routers that are communicating point-to-point ? or there is not must to apply this command on both end ?
I have a IPsec configured router and i can not be sure if i should apply this command on LAN interface or WAN interface ? and Do i have to apply this command on other end ?

Hi,
You can use following configuration instead of former command:
#interface tunnel 0
-if)#mtu 1600
-if)#ip access-group DLP in
-if)#ip address <><>
#ip access-list extended DLP
-acl)#statistics per-entry
-acl)#deny ip any any packet-length gt <adjust value>
-acl)#permit ip any any
I think, it may helps you.
Houtan

Ip tcp adjust-mss unidirection or bidirectional?

If i configure this command on my cisco CPE with a value of 1440, why do i still have packets who has a mss of 1460, while i clearly see the TCP three-way handshake? I'm no wireshark expert, but maybe you guys can tell me what i am doing wrong? I have made a capture between two hosts who are communicating with each other.
Here is the direct link for a more clearer picture http://s16.postimg.org/4vyeqpg91/syn_bit.png

Hi there,
Correct me if i m wrong, is the capture taken from a PC connected to Cisco?
The default MSS is 1460 which MTU 1500 - 40 Header = 1460 which is announced by the PC in syn and as you can see from the second packet which is syn ack received on the PC through the router the MSS is set to 1440, which means the MSS was modified / adjusted by the router.
Please refer below link for more information and testing MSS.
http://www.cisco.com/c/en/us/td/docs/ios/12_2sb/12_2sba/feature/guide/sb_admss.pdf
HTH
Hitesh

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Try these links for more info:
http://cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide09186a00804247fc.html
http://www.cisco.com/warp/public/105/pmtud_ipfrag.html
http://www.cisco.com/warp/public/105/38.shtml

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JohnT66 wrote:
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I am doing a server/client TCP connection between my laptop and another computer running the server program. With my client program I am able to send commands to the server to which it reacts. However I do not know what the commands are (I just press a button in the client GUI and the command is sent), so my initial thought was that, maybe I was able to monitor/capture the strings/bytes/integers commands, which is send over the TCP/IP. There are only these two computers on the network, so no problem in other traffic there. FYI, they are connected with a wireless peer-to-peer connection.
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Matthew Williams wrote:
Wireshark, http://www.wireshark.org , will capture data off the network and sort/display/categorize.
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Matt
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I have just tried Wireshark, I get a lot of data, so I just have to use a bit of time figuring out what is what, but good program to monitor the LAN connection also wireless. I can monitor all the packages between the client and server. It looks like there is a bunch of data just from keeping the connection active, but again I have to dig a bit deeper before I can say anymore:-)
MikeS81 wrote:
Hi SCMAJA,
another way is to build your own TCP/IP Server with LabView and receive with it the commands send with your client program.
Mike
Hi Mike
Yeah I tried that right now using the TCP Communicator - Passive example, unfortunately the connection between the client and server is kept alive using some commands, so the only command I can get to read is the connect command, because the client then refuses the connection because there is no "right" answer/repsonse from the server :-(
edit:
I just got in touch with the support, and got some of the commands to work. I had to make some crazy command with a header first, then size and last my command, all converted from DEC to HEX.. Not easy to figure out :-)
Message Edited by SCMAJA on 02-08-2008 04:29 PM
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Ip tcp adjust-mss on LAN and BVI

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