From: chris Iannacone (iannaconec@optonline.net)
Date: Mon Apr 10 2006 - 10:08:05 GMT-3
According to
http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_guide_chapter09186a008007f32a.html
here is my understanding of the UDLD mode differences
excerpts are in quotes where they enhance understanding
UDLD normal mode works by sending echoes back and forth at layer two
- this goes on top of layer one autonegociation - the FLPs that are
sent between ports on copper media
( Fast Link Pulses ) that determing how fast the communications can
travel - ie port speed { example: I can go 100mph Well I can only do
80mph ok 80 it is }
Seems to me UDLD and ping are quite similar , Id need the RFCs to make sure.
Aggressive mode will shut down within eight tries , it wont wait
fifteen seconds after the eight tries , this can cause STP loops ,
according to the document referenced
and the relevant passages follow
General UDLD:
"The UDLD protocol allows devices connected through fiber-optic or
copper (for example, Category 5 cabling) Ethernet cables to monitor
the physical configuration of the cables and detect when a
unidirectional link exists"
"UDLD is a Layer 2 protocol that works with the Layer 1 mechanisms to
determine the physical status of a link. At Layer 1, autonegotiation
takes care of physical signaling and fault detection. UDLD performs
tasks that auto negotiation cannot perform, such as detecting the
identities of neighbors and shutting down misconnected ports. When
you enable both auto negotiation and UDLD, Layer 1 and Layer 2
detections work together to prevent physical and logical
unidirectional connections and the malfunctioning of other protocols. "
"UDLD at Layer 2 determines whether those fibers are connected
correctly and whether traffic is flowing bidirectionally between the
right neighbors"
Normal Mode
The switch periodically transmits UDLD messages (packets) to neighbor
devices on ports with UDLD enabled. If the messages are echoed back
to the sender within a specific time frame and they are lacking a
specific acknowledgment (echo), the link is flagged as unidirectional
and the port is shut down
aggressive mode:
"With UDLD aggressive mode enabled, when a port on a bidirectional
link which has a UDLD neighbor relationship established stops
receiving UDLD packets, UDLD tries to reestablish the connection with
the neighbor. After eight failed retries, the port is put into
errdisable state."
Heres a diagram that should clear things up
[]
Apologies to cisco for quoting at such length - for those of us
without Eudora , the diagram is Figure 24.
I hope this helps you with your question.
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