RE: PortFast Question

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• Maybe in reply to: Josh Sukol: "PortFast Question"
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To: 'Josh Sukol' <secnews@gmail.com>, security-basics@securityfocus.com
Date: Tue, 28 Sep 2004 09:38:28 -0400

For some reason I had a slight brain fart when describing portfast....

It is not auto-negotiation with regards to port speed/duplex, it (portfast)
is part of STP (spanning tree protocol). When you connect a device to a
catalyst port, the port will begin the learning phase (basically trying to
determine if the device is another switch or a device) and what state it
should place the port it (forwarding, blocking or trunk). Enabling PortFast
on a specific port removes the learning phase and places the port
immediately into a forwarding state. (PortFast should not be used when
connecting switch port to switch port, only for devices connected to a
specific switch port)

The port "still" must be manually set to the speed/duplex as I was
describing earlier, which is 'not' a function of portfast. Both sides (the
port and the device) should be set manually to insure the proper
speed/duplex.

I apologize for the confusion and my lack of thought when writing my
original post......

LordInfidel

-----Original Message-----
From: LordInfidel@directionweb.com [mailto:LordInfidel@directionweb.com]
Sent: Monday, September 27, 2004 10:16 AM
To: 'Josh Sukol'; security-basics@securityfocus.com
Subject: RE: PortFast Question

If I had to guess..... the proprietary hardware box is having a hard time
using auto-negotiation.

Here's what happens when you connect a device to a switch/hub, and both
sides are set to auto-negotiate.

The connecting device will try to connect at it's maximum speed and duplex.
If the other side(in this case the switch) can understand the connecting
device and hence agree at the speed and duplex, the connection is made. If
it can not understand the connecting device, it says Hey I can't understand
that connection request, try another...

And they both go back and forth until a connection is made. Now there are
times when a connection, "appears" to be made but you can not ping or it
seems like the connection is really slow. That is because there are
transmission errors due to the way each connection is expecting to receive
the data.

Now with portfast, you are removing auto-negotiation from the switch and you
are telling the switch port "Do not attempt to auto-negotiate, assume the
port is 100/Full and bring the port up as such".

As far as protecting that port, you can lock that port down to the MAC
address of the connecting device.

Typically, for any static network device that you are using, (servers,
routers, firewalls, etc), the network adapter on the device should be
manually set for speed/duplex. Never leave it set to auto.

-----Original Message-----
From: Josh Sukol [mailto:secnews@gmail.com]
Sent: Friday, September 24, 2004 10:05 AM
To: security-basics@securityfocus.com
Subject: PortFast Question

I am running a small network using four Cisco Catalyst 2950 switches.
I am in the process of configuring a new software package that uses
some proprietary hardware that connects to the network via Ethernet.
When plugged into the network the device would connect for a minute or
two and than connectivity would drop (i.e. ping would fail, and the
light on the switch would turn from green to amber) This pattern
continued for as long as the device was plugged into the network. The
cabling was checked and tested with other equipment and there were no
other problems.

After trying several other things I eventually started changing the
ethernet port settings on the switch itself and found that by enabling
portfast the device functioned fine. I have found very little
information about port fast security issues. I was able to find and
did read up on PortFast BPDU guard and potential DoS using malformed
packets. Are there any other security issues that effect me enabling
Portfast on specific ports that connect back to a single device? Are
there any other ways to solve this problem that might allow me to
sidestep this potential security issues all together?

- Slightly Off Topic -
If anyone knows why this behavior occurs and why enabling portfast
fixes the connectivity issue I would be very interested to a hear an
explanation.

Thanks in advance for the wisdom!

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