UDLD - One side

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Feb 24th, 2010
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Hi All


Have went through lots of post on UDLD, and wanted a quick confirmation..


what will happen when UDLD aggressive mode is enabled only one side of the link - I presume it wouldnt cause any issues.. is that right ?

We are planning to role UDLD aggressive and have core-edge switch design.. once i enable on core switch, would it cause any issues with the trunk to edge , if it is not enabled on the edge switch ?


I also read that UDLD normal mode is enabled default on the switch ? so, would the switch already know to detect Unidirectional link by default, and just that it doesnt shut the port (as aggressive does) ?


Thanks in advance

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sblavanya Wed, 02/24/2010 - 14:33
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Went thro some docs, and found this very useful doc:


http://blog.internetworkexpert.com/2008/07/05/udld-modes-of-operation/


the author clearly explains the need for the switch to hear back from the edge switch about its own ID..when it is enabled only one side, the switch wouldnt establish a two way communication, and the UDLD state would be "unknown"..


when you have UDLD enabled on one side, with aggressive on other, it would still work, with err disabled only one one switch...


Nice doc..


Thanks..

Giuseppe Larosa Thu, 02/25/2010 - 00:54
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Hello Sblavanya,


>> We are planning to role UDLD aggressive and have core-edge switch design.. once i enable on core switch, would it cause any issues with the trunk to edge , if it is not enabled on the edge switch ?


UDLD has a basic neighbor state machine, so it will react to missing messages, but only if a neighbor was present on that link.

If other side is not enabled for UDLD, UDLD will not do anything.


Hope to help

Giuseppe

sblavanya Tue, 03/02/2010 - 08:20
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Hi Guiseppe


Thanks for the response..


I have one more doubt here..


I came across saying UDLD is enabled by default on fiber interfaces, and disabled globally... have a look at this table..


Feature
Default Value
UDLD global enable state
Globally disabled
UDLD aggressive mode
Disabled
UDLD per-port enable state for fiber-optic media
Enabled on all Ethernet fiber-optic LAN ports.
UDLD per-port enable state for twisted-pair (copper) media
Disabled on all Ethernet 10/100 and 1000BASE-TX LAN ports.



Now, with fiber ports, does it enable UDLD normal mode or aggressive mode by default.. I presume it is normal mode.. right ? In that case , even if i have UDLD aggressive enabled globally, the port would have UDLD normal ? because interface paramters over write global ? Do we need to enable aggressive specific on all ports ?


Thanks

Giuseppe Larosa Wed, 03/03/2010 - 01:11
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Hello Sblavanya,

my understanding is that fiber optics ports inherit global configuration settings if it is configured for UDLD aggressive it will be enabled for UDLD aggressive on the port unless you configure that specific port for UDLD normal mode.


Specifies the UDLD mode of operation:

aggressive Enables UDLD in aggressive  mode on all fiber-optic interfaces.


see

http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/46sg/configuration/guide/udld.html#wp1043856


to be noted that if you decide to use UDLD in your network you are likely going to use UDLD as a tool that helps in containing the effects of bridging loops.


We use UDLD, spanning-tree loop guard and broadcast storm-control.

Time is a key  parameter: having the time to access devices involved in a bridging loop and to break the loop by shutting down some inter-switch links can make the difference.


Hope to help

Giuseppe



sblavanya Wed, 03/03/2010 - 06:28
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Hi Giuseppe


Thanks again.. that was really useful...


one last question.. on set based switches.. we have set udld aggressive port mod/port command... i didnt see any global configuration (like in IOS) which can be enabled to enable UDLD on all interfaces ? Are you aware of this ? Is it that we gotta enable UDLD aggressive on set based switches on individual ports ? Issue is, we have numerous cat os switches, with hundreds of ports and deploying configs thro NCM as a part of standardization.. NCM checks for base policies only, and not port level info.. so, we gotta manually check this on all switches and configure it manually ?


Thanks

Giuseppe Larosa Wed, 03/03/2010 - 08:40
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Hello Sblavanya,


on CatOS switches should be the same: you can enable UDLD at global level


see

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/catos/8.x/configuration/guide/udld.html#wp1019913


Console> (enable) set udld enable
UDLD enabled globally
Console> (enable) show udld
UDLD      : enabled
Console> (enable)


So you can check with sh udld the state of UDLD on your devices


But you may be right that UDLD aggressive can be enabled only at the port level as I see reading the link above.


set udld aggressive-mode enable mod/port


be aware the command accepts range of ports like

set udld aggressive-mode enable 4/1-5


! this enables UDLD aggressive on ports 4/1 - 4/5


So it may be easier, you need to get a list of ports where other Cisco switches are connected and this is simple if you have CDP enabled on your network

Then you can build a single command to enable UDLD aggressive only on those ports towards other switches



Hope to help

Giuseppe


sblavanya Wed, 03/03/2010 - 09:01
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yeps.. udld enable on setbased enables only normal mode.. for aggressive, i guess we need more work, to manually check the ports before implementation.. and with huge no of switches on the network, it becomes even tougher !


ill come back with anymore questions before the implementation..


Thanks for all your answers Giusippe...

Ganesh Hariharan Thu, 02/25/2010 - 03:08
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Hi All


Have went through lots of post on UDLD, and wanted a quick confirmation..


what will happen when UDLD aggressive mode is enabled only one side of the link - I presume it wouldnt cause any issues.. is that right ?

We are planning to role UDLD aggressive and have core-edge switch design.. once i enable on core switch, would it cause any issues with the trunk to edge , if it is not enabled on the edge switch ?


I also read that UDLD normal mode is enabled default on the switch ? so, would the switch already know to detect Unidirectional link by default, and just that it doesnt shut the port (as aggressive does) ?


Thanks in advance



Hi,


As per the defination of UDLD, UDLD is a Layer 2 protocol that works with 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 autonegotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected ports. When you enable both autonegotiation and UDLD, Layer 1 and 2 detections work together to prevent physical and logical unidirectional connections and the malfunctioning of other protocols.



In aggressive mode, if the link state of the port is determined to be bi-directional and the UDLD information times out while the link on the port is still up, UDLD tries to re-establish the state of the port. If not successful, the port is put into the errdisable state.


In normal mode, if the link state of the port was determined to be bi-directional and the UDLD information times out, no action is taken by UDLD. The port state for UDLD is marked as undetermined. The port behaves according to its STP state.


Hope to Help


Ganesh.H

danrya Wed, 11/28/2012 - 17:04
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One more clarification:

In normal mode, if the link state of the port was determined to be bi-directional and UDLD time's out, an empty echo is sent to the neighbor, no other action is taken by UDLD on the neighbor that timed out, BUT if the empty echo is received by the other neighbor the port will be err-disabled and you will see this log message:


%UDLD-4-UDLD_PORT_DISABLED: UDLD disabled interface Ethernet2/6, empty echo detected


So to correct some misconception :

UDLD normal and UDLD aggressive must create a bi-directional state before any action is taken

UDLD normal and UDLD aggressive can create a bi-directional state with a peer in either mode.

UDLD normal and UDLD aggressive WILL err-disable if an empty echo is received.

UDLD aggressive will err-disable if NO echo is received.

UDLD normal will take no action if NO echo is received.


Dan Ryan, NCE

Cisco Systems, Inc

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