RE: PortFast Question

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Date: Mon, 27 Sep 2004 15:28:27 -0400
To: "Stephen W. Corey - 5535" <swc@wardandsmith.com>, <security-basics@securityfocus.com>, <secnews@gmail.com>

STP (Spanning Tree Protocol) is enabled by default on VLAN 1 and on all
newly created. While STP goes through its four steps
(blocking,listening,learning,forwarding) which can take between 30-50
seconds no user data will pass and some applications may time out. By
enabling portfast you are forcing the switchport into forwarding mode
immediately. The port still participates in STP in the event that if
the port is to be a part of the loop, it will eventually transition into
STP blocking mode.

In regards to security, as long as the port is participating in STP,
there is a possibility that some device attached to that port and also
running STP with lower bridge priority than that of the current root
bridge, will assume the root bridge function and affect active STP
topology, thus rendering the network suboptimal. Permanent STP
recalculation caused by the temporary introduction and subsequent
removal of STP devices with low (zero) bridge priority represent a
simple form of Denial of Service (DoS) attack on the network.

The STP portfast BPDU guard enhancement is designed to allow network
designers to enforce the STP domain borders and keep the active topology
predictable. The devices behind the ports with STP portfast enabled are
not allowed to influence the STP topology. This is achieved by disabling
the port with portfast configured upon reception of BPDU. The port is
transitioned into errdisable state, and a message is printed on the
console. The following is an example of the message printed out as a
result of BPDU guard operation:

2000 May 12 15:13:32 %SPANTREE-2-RX_PORTFAST:Received BPDU on PortFast
enable port.
Disabling 2/1
2000 May 12 15:13:32 %PAGP-5-PORTFROMSTP:Port 2/1 left bridge port 2/1

-----Original Message-----
From: Stephen W. Corey - 5535 [mailto:swc@wardandsmith.com]
Sent: Monday, September 27, 2004 9:03 AM
To: security-basics@securityfocus.com
Subject: RE: PortFast Question

We run portfast on all Catalyst ports that connect to a "non-switch"
device, like PCs, servers, routers, etc. From what I saw, it works by
not listening for MAC addresses as long before going to "active" state.
I have never heard of any security issues by doing this. I believe Cisco
still recommends this mode for optimum performance. You can always use
Nessus (or some other up to date vuln scanner) to see if anything can be
exploited.

As for why it happens, here's my thought. Because it's speeding up a
"natural" switch port process, weird things can happen. Depending on how
the device (i.e. PC hardware) acts on layer 2, it may need the "full"
startup procedure to be run. To me, portfast is a non-standard shortcut,
and it may not work in every situation. As you probably read, you can't
plug a portfast port into a switch, so there could easily be other
devices it's incompatible with (Cisco can't test everything).
  

-----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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• Previous message: Steve Fletcher: "RE: PortFast Question"
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