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UDLD Questions

November 25th, 2019 in SWITCH 300-115 Go to comments

UDLD quick overview:
+ Detect the unidirectional links (one-way communication) and helps preventing Layer 2 loops
+ Cisco proprietary
+ Layer 2 protocol (but works with Layer 1 mechanisms)
+ UDLD is disabled by default on all ports
+ Switch usually sends UDLD message (with the UDLD well-known MAC address 01:00:0C:CC:CC:CC) to the far end device every 15 seconds and expects UDLD messages to be returned. If it is not returned, we may have a unidirectional link
+ Two modes:
Normal mode: If UDLD detects a unidirectional link, it just marks this port as “Undetermined”, but does NOT shut down or disable the port. A syslog message is also generated.
Aggressive mode (recommended mode): If UDLD detects a unidirectional link, it is going to send a UDLD message every one second for eight seconds. If UDLD does not see any of these messages returned to itself, it would put that port into error-disabled state.
+ Can be used along with Etherchannel. If UDLD detects unidirectional link in one of the physical connections in the Etherchannel bundle, UDLD only puts that faulty physical link into error-disabled state (not the whole Etherchannel bundle)
+ Can be enabled globally or per-port basis.

Note: A unidirectional link occurs whenever the traffic transmitted by a local device over a link is received by a neighbor, but traffic transmitted from the neighbor is not received by the local device

Question 1

Explanation

UDLD is a Layer 2 protocol that enables devices connected through fiber-optic or twisted-pair Ethernet cables to monitor the physical configuration of the cables and detect when a unidirectional link exists. All connected devices must support UDLD for the protocol to successfully identify and disable unidirectional links. When UDLD detects a unidirectional link, it administratively shuts down the affected port and alerts you. Unidirectional links can cause a variety of problems, including spanning-tree topology loops.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3550/software/release/12-1_19_ea1/configuration/guide/3550scg/swudld.html#wp1019932

Question 2

Explanation

A unidirectional link occurs whenever traffic sent by a local device is received by its neighbor but traffic from the neighbor is not received by the local device.

UDLD supports two modes of operation: normal (the default) and aggressive. In normal mode, UDLD can detect unidirectional links due to misconnected interfaces on fiber-optic connections. In aggressive mode, UDLD can also detect unidirectional links due to one-way traffic on fiber-optic and twisted-pair links and to misconnected interfaces on fiber-optic links.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3550/software/release/12-1_19_ea1/configuration/guide/3550scg/swudld.html

Question 3

Explanation

When unidirectional link occurs, UDLD can put that port into errdisable state (same as shutdown). The administrator must manually shut/no shut to bring that interface up. If we want the interface to automatically recover then configure the “errdisable autorecovery”. For example:

errdisable recovery cause udld
errdisable recovery interval 30

By doing so, the port will be place back in up state (no err-disabled state) after 30 seconds, if the port still has violation it will be placed again in “err-disabled” state, otherwise it will remain in up state.

Question 4

Explanation

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.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/15-0SY/configuration/guide/15_0_sy_swcg/udld.html#wp1027627

Question 5

Question 6

Question 7

Question 8

Question 9

Explanation

The Cisco-proprietary UDLD protocol monitors the physical configuration of the links between devices and ports that support UDLD. UDLD detects the existence of unidirectional links. When a unidirectional link is detected, UDLD puts the affected port into the errdisabled state and alerts the user. UDLD can operate in either normal or aggressive mode.

UDLD is a Layer 2 protocol that works with the Layer 1 protocols 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 LAN ports. When you enable both autonegotiation and UDLD, Layer 1 and Layer 2 detections work together to prevent physical and logical unidirectional connections and the malfunctioning of other protocols.

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 -> C is correct.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/udld.html

Question 10a

Explanation

With this configuration when the damage takes place, the link will be put into err-disabled state but after 15 seconds it will be brought to up state to check if the damage has been fixed. If not it will be brought back to err-disabled state again.

Question 10b

Explanation

When UDLD detects a unidirectional link, it administratively shuts down the affected port and alerts you. Unidirectional links can cause a variety of problems, including spanning-tree topology loops -> A is correct.

With the two first commands, the port will be place back in up state (no err-disabled state) after 15 seconds, if the port still has violation it will be placed again in “err-disabled” state, otherwise it will remain in up state -> D is correct.

Reference: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9500/software/release/16-8/configuration_guide/ha/b_168_ha_9500_cg/b_168_ha_9500_cg_chapter_011.html#concept_xbc_5xx_31b

Comments
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  1. RARU
    December 4th, 2019

    After analyzing once again the operation of udld, I can say that Q8´s answer is correct.
    A. UDLD attempts to re-establish … before declaring the link down. looks correct but it is not. UDLD by default detect the unidirectional link after three fails (45 seconds), and only after detect the unidirectional link, UDLD attempts to re-establish. So, first, the port going to err-disabled state and after the attempt to re-establish.

  2. Raspasoti
    January 28th, 2020

    For Q10b answer D makes absolutely no sense. How can UDLD repair a broken fiber? If anything it will continue recover and go back to err-disable until the fiber has been replaced, but this has nothing to do with repairing a fiber. Certprepare, can you give us a more logical explanation.

  3. Bez
    February 8th, 2020

    Yep you are correct Q10b is really badly worded, if a port could splice fiber it would be a genius.

  4. King in the Castle
    February 11th, 2020

    Complete Q6 options are

    A. Other interface will recover.
    B. All links go down
    C. link stays up
    D. Reset interface

    @certprepare please update the choices.

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