What is the command ip tcp synwait-time for? Cisco's defination is the following. Switches operate at L2, L2+L3. TCP operates at L4. How does a switch eatablishes a TCP connection?
To set a period of time the switch waits while attempting to establish a TCP connection before it times out, use the ip tcp synwait-time global configuration command. To restore the default time, use the no form of this command.
In general I make the synwait-time consistent on all devices (because I generally leave it at the default value). If I were going to change it, I would change it to make it larger on devices that were further away (and therefore more subject to network transmission delay) and shorter on devices that were closer.
The main time that I do manipulate the synwait-time is when I am working in the lab (where I generally have good response times) and I make it quite short. This way my attempt to telnet or SSH will succeeed or it will time out quickly.
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The response from Collin is a very good explanation of why you might want to manipulate the tcp synwait-time. But as I read your original post I believe that you are asking a somewhat different set of questions. You ask what is synwait-time for, and you ask:" How does a switch eatablishes a TCP connection?" So let me answer them in this way:
- what is it for? it is for specifying how long a device (a switch or a router) will wait while it is attempting to establish a TCP connection. The device attempts to establish a TCP connection by sending a SYN request and waiting for a SYN-ACK response. It will wait for the duration of the synwait timer and if it receives no responses it times out the request.
- how does a switch establish a TCP connection? If you are on the switch and attempt to make a telnet connection to another switch, that will attempt to establish a TCP connection since telnet is based on TCP. Or if you are on the switch and attempt to send an image file using FTP it will attempt to establish a TCP connection since FTP is based on TCP.