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UDLD Enable and UDLD Aggressive - What is the difference?

astanislaus
Explorer
Explorer

I have read at least 6 links on Cisco Systems to try and really understand the difference between UDLD Enable and UDLD Aggressive. The more I read the more I am confused. My final understanding is as follows and I wish someone to correct me if my understanding is correct.

UDLD Enable just detects if a link becomes Unidirectional - that is - it can send UDLD frames but cannot receive UDLD frames, but takes no action when this happens.

UDLD Aggressive on the other hand shuts down the interface when it can send UDLD frames but cannot receive them and hence called aggressive because this shuts down the interface.

But then I go and read another URL and I find that UDLD Enable also will shut down the interface when it can transmit UDLD frames but cannnot receive UDLD frames from teh other end.

It would be hard to simulate this in the lab because if we unplug one fibre cable from switch A to switch B the interface state goes to LINE DOWN / PROTOCOL DOWN on both switches and we cannot see UDLD playing any role at all - be it UDLD Enable or UDLD Aggressive.

To simulate it, we would have to some how keep the Interface in LINE UP / PROTOCOL UP state on both switches but somehow stop UDLD frames from switch A being received by switch B.

Or is there any other easy way of simulating this in the lab to see the difference between UDLD Enable and UDLD Aggressive behaviours.

11 REPLIES 11

astanislaus
Explorer
Explorer

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/122sx/swcg/udld.htm#wp1027627

For example look at the above link and you will see it says the following which says UDLD Enable and UDLD Aggressive both will shut the port down and hence my initial understanding is wrong:

=============================================

UDLD Aggressive Mode

UDLD aggressive mode is disabled by default. Configure UDLD aggressive mode only on point-to-point links between network devices that support UDLD aggressive mode. With UDLD aggressive mode enabled, when a port on a bidirectional link that 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 disabled.

To prevent spanning tree loops, nonaggressive UDLD with the default interval of 15 seconds is fast enough to shut down a unidirectional link before a blocking port transitions to the forwarding state (with default spanning tree parameters).

When you enable UDLD aggressive mode, you receive additional benefits in the following situations:

?One side of a link has a port stuck (both Tx and Rx)

?One side of a link remains up while the other side of the link has gone down

In these cases, UDLD aggressive mode disables one of the ports on the link, which prevents traffic from being discarding.

lmcruzhsa
Beginner
Beginner

Hi,

We have the same problem here.

UDLD is not clear for us.

We have deployed "udld enable" and we saw today this problem:

interface GigabitEthernet1/0/19
description A - 00 - PacketShaperEu - Inside
switchport trunk encapsulation dot1q
switchport trunk native vlan 99
switchport trunk allowed vlan 21,61,75,94,95,99
switchport mode trunk
switchport nonegotiate
speed 1000
duplex full
udld port
spanning-tree guard root
end

But today we saw this in the log:

000986: Mar  8 10:16:04.407 GMT: %UDLD-4-UDLD_PORT_DISABLED: UDLD disabled interface Gi1/0/19, unidirectional link detected
000987: Mar  8 10:16:04.407 GMT: %PM-4-ERR_DISABLE: udld error detected on Gi1/0/19, putting Gi1/0/19 in err-disable state
000988: Mar  8 10:16:05.515 GMT: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/19, changed state to down
000989: Mar  8 10:16:06.521 GMT: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/19, changed state to down
000990: Mar  8 10:16:31.050 GMT: %PM-4-ERR_RECOVER: Attempting to recover from udld err-disable state on Gi1/0/19
000991: Mar  8 10:16:34.414 GMT: %SPANTREE-5-TOPOTRAP: Topology Change Trap for vlan 21
000992: Mar  8 10:16:35.504 GMT: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/19, changed state to up
000993: Mar  8 10:16:36.511 GMT: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/19, changed state to up
000994: Mar  8 10:17:03.631 GMT: %SPANTREE-5-TOPOTRAP: Topology Change Trap for vlan 99
000995: Mar  8 10:17:52.412 GMT: %UDLD-4-UDLD_PORT_DISABLED: UDLD disabled interface Gi1/0/19, unidirectional link detected
000996: Mar  8 10:17:52.412 GMT: %PM-4-ERR_DISABLE: udld error detected on Gi1/0/19, putting Gi1/0/19 in err-disable state
000997: Mar  8 10:17:53.519 GMT: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/19, changed state to down
000998: Mar  8 10:17:54.526 GMT: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/19, changed state to down
000999: Mar  8 10:18:19.742 GMT: %PM-4-ERR_RECOVER: Attempting to recover from udld err-disable state on Gi1/0/19
001000: Mar  8 10:18:23.173 GMT: %SPANTREE-5-TOPOTRAP: Topology Change Trap for vlan 21
001001: Mar  8 10:18:24.138 GMT: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/19, changed state to up
001002: Mar  8 10:18:25.144 GMT: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/19, changed state to up

So, it is supposed that "udld enable" dont shutdown the port but... according to the log it put the port in err-disabled!

Platform: 3750G

IOS: 12.2(35)SE5

Any idea?

The CCIE book we have here tells us that both modes can shutdown a port, which is not clear for according to the online web pages from cisco and the lan switching fundamentals book.

Hello,

I faced the same problem today in an interface with the same configuration as yours that has been running fine for years but suddendly that err-disable message due to UDLD appeared.

The solution I did was setting the port speed to AUTO with the "no speed" command. This solved the problem.

Hope this helps.