[c-nsp] ios tcp defaults

Nikolay Shopik shopik+lists at nvcube.net
Fri Apr 22 11:41:55 EDT 2016

If no ip tcp path-mtu-discovery set, TCP MSS will be 538, unless you set
ip tcp mss.


It seems it was always was disabled since its introduction.

On 22/04/16 16:59, Sebastian Beutel wrote:
> Hi List,
>     in some kind of spring-cleaning of our configuration collection, i
> encountered some lines that differ from Ciscos defaults in many of our
> switches. The Cisco default for the lines in question is like this:
> no ip tcp selective-ack
> no ip tcp path-mtu-discovery
> This makes me wonder because i believe that pmtu discovery and selective ack
> are good things. Furthermore, in our heritage config defaults selective-ack
> and path-mtu-discovery are explicitly enabled.
> The question i like to ask is therefore: Does anyone know why Cisco chose to
> disable this by default and am i right that it's safe these days to enable it?
> Best,
>       Sebastian.
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