[CentOS] The most weird thing i've ever seen (networking)

Guy Boisvert boisvert.guy at videotron.ca
Mon Mar 5 14:44:06 UTC 2007

Chris Boyd wrote:
> On Mar 4, 2007, at 7:01 PM, Radu-Cristian FOTESCU wrote:
>> What have a Cisco switch and the DHCP to do
>> with an interface not coming up at all?!
> Some Cisco switches take a long time to negotiate link speed, duplex,
> and bridge forwarding when left at default settings.  This causes the
> initial DHCP requests to time out, leaving you with no IP address as the
> box comes up.  dhclient should finally run again and get an address, but
> it can take a few minutes.
> You can speed this up by using the "portfast" keyword, but do be careful
> and read up on it before enabling it.  Portfast on the wrong port can
> cause a network meltdown.
> --Chris

For those who want to understand the portfast concept:


In resumé, Cisco switches run STA (Spanning Tree Algorithm) on all ports
by default to avoid networking loops that can take the network down.

Portfast is a parameter that immediately transitions the blocked port in
forwarding state upon link up instead of transitioning the port after
STP Calculation that can take between 30 and 50 seconds.  Ports in
Portfast mode still participate in STP.

Portfast is used for ports that connect hosts.

Guy Boisvert, ing.
IngTegration inc.

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