ip tcp synwait-time 10

Answered Question
Dec 31st, 2008

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.

http://www.cisco.com/en/US/products/hw/switches/ps718/products_command_reference_chapter09186a00800ed9de.html#12989

I have this problem too.
1 vote
Correct Answer by Richard Burts about 7 years 11 months ago

Said

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.

Thank you for using the rating system to indicate that your question was resolved (and thanks for the rating). It makes the forum more useful when people can read a question and can know that there were responses which did resolve the question.

HTH

Rick

Correct Answer by Richard Burts about 7 years 11 months ago

Said

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.

HTH

Rick

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (3 ratings)
Loading.
Collin Clark Wed, 12/31/2008 - 11:39

From your link if you read a little further down it reads;

If your network contains PSTN DDR, it is possible that the call setup time exceeds 30 seconds. This amount of time is not sufficient in networks that have dialup asynchronous connections because it affects your ability to Telnet over the interface (from the switch) if the interface must be brought up. If you have this type of network, you might want to set this value to the UNIX value of 75.

Because this is a host parameter, it does not pertain to traffic going through the switch, just for traffic originating at the switch. Because UNIX has a fixed 75-second timeout, hosts are unlikely to see this problem.

Does that make sense?

Correct Answer
Richard Burts Wed, 12/31/2008 - 11:56

Said

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.

HTH

Rick

saidfrh Wed, 12/31/2008 - 12:10

Rick,

Thanks for the explanation. Should the ip tcp synwait-time be consistent on the router, firewall and switches? Since the switches are closer to hosts, is the ip tcp synwait-time given more duration?

Correct Answer
Richard Burts Wed, 12/31/2008 - 12:17

Said

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.

Thank you for using the rating system to indicate that your question was resolved (and thanks for the rating). It makes the forum more useful when people can read a question and can know that there were responses which did resolve the question.

HTH

Rick

saidfrh Wed, 12/31/2008 - 13:00

Rick,

On another note, what is the reason for assigning a domain name in the router? ex. ip domain name trewq.com

Thanks.

Said

Richard Burts Thu, 01/01/2009 - 12:02

Said

Configuring the domain name in the router config allows the router to fill out names that you might use. For example lets assume that your domain is trewq.com and that you have a TFTP server named server1. If you have configured domain name to specify trewq.com then you could enter this command:

copy tftp flash

and when prompted for the address of the server you could just type server1. But if the domain name was not configured then you would have to type server1.trewq.com

HTH

Rick

Actions

This Discussion