UDLD Questions

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:

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