2960XR TCP MSS adjust

[russell.sage]

Can anyone categorically confirm that the 2960XR will support ip tcp mss adjust? the Feature Navigator would indicate it doesn't but there are posts in the community which talk about changing the SDM profile to routing but I don't see how that is that relevant.

[David72]

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@russell.sage wrote: usps tracking

Can anyone categorically confirm that the 2960XR will support ip tcp mss adjust? the Feature Navigator would indicate it doesn't but there are posts in the community which talk about changing the SDM profile to routing but I don't see how that is that relevant.

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Yes, the Cisco 2960XR does support the ip tcp adjust-mss command. This command is used to adjust the Maximum Segment Size (MSS) value of TCP SYN packets going through the router. It’s effective only for TCP connections passing through the router. In most cases, the optimum value for the max-segment-size argument of the ip tcp adjust-mss command is 1452 bytes.

https://www.cisco.com/en/US/docs/ios-xml/ios/ipapp/command/ip_tcp_adjust-mss_through_ip_wccp_web-cache_accelerated.html

Thanks

[Joseph W. Doherty · Accepted solution]

"Yes, the Cisco 2960XR does support the ip tcp adjust-mss command."

You're sure?  (As the 2960XR is an enhanced, low-end, L2 switch.  Also, if this feature supported, no requirements for particular IOS versions?)

"In most cases, the optimum value for the max-segment-size argument of the ip tcp adjust-mss command is 1452 bytes."

What's the actual most-cases?  For 8 bytes, PPPoE?

Personally I would consider some form of tunneling to common usage scenario, but, of course, those usually require a larger adjustment.

[russell.sage]

So I have server on a remote site that needs to communicate with MS Defender. The server initiates an outbound connection but sets it TCP connections with an MSS set at 1460 but has to traverse a Meraki Site to Site VPN which reduces the MTU down to 1432 so the MSS can't exceed 1392. The 2960XR does support L3 routing but when I checked there are no options after ip tcp under the VLAN interface. Thanks for the response

[Joseph W. Doherty]

Cannot say for sure, but think it unlikely to be supported on any 2960 series switch.  (For your sake, hopefully I'm wrong.). Also agree, would seem odd if SDM template would make such a difference, but if one were, I could see the routing template possibly being the key to activate such a feature.

Do you have a switch to test with?  Have you looked at command references for 2960XR IOS versions?

[liviu.gheorghe]

I just looked at a IOS virtual switch in CML with IOS version 15.2 and the command, ip tcp adjust-mss, is not available:

sw2(config)#ip tcp ?
RST-count Configure RST throttle count
async-mobility Configure async-mobility
chunk-size TCP chunk size
ecn Enable Explicit Congestion Notification
intercept Enable TCP intercepting
keepalive Configure TCP Keepalive parameters
mss TCP initial maximum segment size
path-mtu-discovery Enable path-MTU discovery on new TCP connections
queuemax Maximum queue of outgoing TCP packets
selective-ack Enable TCP selective-ACK
synwait-time Set time to wait on new TCP connections
timestamp Enable TCP timestamp option
window-size TCP window size

sw2(config)#ip tcp mss ?
<68-10000> MSS

I agree with @Joseph W. Doherty as to the availability of the feature on a L2 switch and what would you use it at? ssh?

Hope this helps.