From: Tim (ccie2be@nyc.rr.com)
Date: Sat Jul 05 2008 - 15:29:04 ART
Thanks Petr.
That helps but when would it be better to use UDLD in Normal mode? It seems
like aggressive mode would always be better because UDLD doesn't shut the
port which could maybe allow STP loops to occur.
Tim
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Petr
Lapukhov
Sent: Saturday, July 05, 2008 2:08 PM
To: Jason Madsen
Cc: Jonathan Greenwood II; Tim; ccielab@groupstudy.com
Subject: Re: UDLD
UDLD is Cisco proprietary extension for detection of mis-configured
(basically unidirectional) links. The idea is pretty simple - to allow two
switches verify they can both send and receive data on a point-to-point
connection. It's plain to see that an unidirectional link may end up in STP
loop, which is highly undesirable. Note that problems with unidirectional
links usually occur on fiber-optical connections and are not common on
twisted-pair connections, where link pulses are used to monitor the
connection integrity.
The confusion about UDLD is that Cisco provides quite unclear description of
it's operations be it on CatOS or IOS platform. So here is a short overview
of how UDLD works.
1) Both UDLD peers discover each other by exchanging special packets sent to
well-known MAC address 01:00:0C:CC:CC:CC. (Naturally, those frames are only
understood by Cisco switches). Each switch sends it's own device ID, the
originator port ID and timeout value to it's peer. Additionally, a switch
echoes back the ID of it's neighbor (if the switch does see the neighbor).
Since some versions of CatOS and IOS you can change UDLD timers globally.
2) If no echo with our ID has been seen from the peer for a certain amount
of time, the port is suspected to be unidirectional. What happens next
depends on UDLD mode of operations.
3) In "Normal" mode, if the physical state of port (as reported by Layer 1)
is still up, UDLD marks this port as "Undetermined", but does NOT shut down
or disable the port, which continues to operate under it's current STP
status. This mode is iformational, and you can review the "undetermined"
ports using CLI show commands when troubleshooting.
3) If UDLD is set to "Agressive" mode, once the switch loses it's neighbor,
it actively tries to re-establish the relationship by sending a UDLD frame 8
times every 1 second (surpisingly this coincides with TCP keepalives retry
values used by FCIP on Cisco MDS storage switches :). If the neighbor does
not respond after that, port is considered to be unidirectional and brought
to "Errdisable" state. (Note that you can configure "errdisable recovery" to
make switch automatically recover from such issues)
To finish the overview, recall that UDLD is designed to be a STP helper.
Therefore UDLD should be able to detect a unidirectional link before STP
would unblock it due to missed BPDU. Thus, when you configure UDLD timer,
make sure your UDLD timer is set so that unidirectional link is detected
before MaxAge + 2xForwardDelay expires (by default it's 20+2x15 seconds,
though who uses the STP defaults nowdays?!)
HTH
-- Petr Lapukhov, CCIE #16379 (R&S/Security/SP/Voice) petr@internetworkexpert.comInternetwork Expert, Inc. http://www.InternetworkExpert.com
2008/7/5 Jason Madsen <madsen.jason@gmail.com>:
> Yep, here it is per the Command Ref' Uasge Guideline section: > > UDLD supports two modes of operation: normal (the default) and aggressive. > In normal mode, UDLD detects unidirectional links due to misconnected > interfaces on fiber-optic connections. In aggressive mode, UDLD also > detects > unidirectional links due to one-way traffic on fiber-optic and twisted-pair > links and due to misconnected interfaces on fiber-optic links. > > Jason > > On Sat, Jul 5, 2008 at 10:00 AM, Jonathan Greenwood II <gwood83@gmail.com> > wrote: > > > Aggressive mode is used for twisted pair/copper connected interfaces, > udld > > command by its self is used for all fiber connected interfaces. > > > > HTH > > > > Jonathan > > > > On Sat, Jul 5, 2008 at 10:36 AM, Tim <ccie2be@nyc.rr.com> wrote: > > > > > Hi Guys, > > > > > > > > > > > > Can someone help me understand the difference between regular mode and > > > aggressive mode with respect to UDLD? > > > > > > > > > > > > I've read the Cisco doc's but they're about as clear as mud. > > > > > > > > > > > > Thanks, Tim > > > > > > > > >
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