STP UDLD


A [fiber connection](/fiber-optic-types) using an [SFP transceiver](/fiber-optics-connectors) typically employs two fiber strands: one for transmission (Tx) and one for reception (Rx). This setup can lead to a unidirectional link if one of the strands fails. Historically, the Tx and Rx operations were independent, meaning one strand could fail while the other continued to function, potentially causing [Layer 2](/osi-model) (L2) [loops](/loops-in-layers-2-and-3).

Modern transceivers often include mechanisms to detect such failures and will consider the link down if either strand fails. However, these mechanisms may take several seconds to activate, during which the link might still appear operational despite a failure.

To mitigate these risks, especially in high-speed, mission-critical core networks, protocols like [LoopGuard](/stp-loop-guard) and [Unidirectional Link Detection (UDLD)](/stp-udld) are essential. These protocols help ensure network stability by detecting and preventing unidirectional link failures.

## Links

https://networklessons.com/spanning-tree/spanning-tree-loopguard-udld

STP - UDLD and Fiber Links

SNMP monitoring routing on a Nexus device

SNMP trap vs inform

SNMP walk

SPAN capturing control packets

SPAN multiple destination ports on Cisco IOS device

SPAN using VLANs as a source on a Nexus device

SPAN

SSH - domain-name prerequisite

SSH connectivity - troubleshooting

SSM Multicast Group Verification

STP - Aging time reduction with a TCN

STP - Aging time return to default

STP - BPDU Filter Use Cases

STP - BPDU generation and port roles

STP - BPDUGuard, BPDUFilter, and Portfast

STP - BPDUs on a switch where STP is disabled

STP - BPDUs, VLAN IDs, and PVST+

STP - Cost of last link added by receiving switch

STP - Disabling spanning-tree

STP - Dispute

STP - Factors to Consider When Choosing a Root Bridge

STP - Learning State

STP - MST CIST Root

STP - MST CST and CIST

STP - MST Virtual Bridge and its role in interfacing with other STP topologies

STP - MST and PVST+ selecting root port

STP - MST maps groups of VLANs

STP - MST region attributes

STP - MST viewed as a single switch by PVST+

STP - Max Age vs Message Age

STP - Multiple Spanning Tree (MST)

STP - Multiple types of STP on the same switch

STP - PVST+ and Rapid PVST+ interaction

STP - Per VLAN Spanning Tree plus (PVST+)

STP - RSTP BPDU Flags and their usage

STP - RSTP EdgePort

STP - RSTP Proposal Timer Operation

STP - RSTP convergence process

STP - RSTP port roles

STP - RSTP prerequisites for immediate move to forward state

STP - RSTP proposal-agreement process

STP - RSTP while timer cannot be observed

STP - RSTP while timer

STP - Root Inconsistency State

STP - Root Link Query (RLQ) BPDU

STP - Shortest Path Bridging (SPB)

STP - Time to select root bridge

STP - Topology Change Acknowledgement (TCA)

STP - Understanding MST regional root identifier and proposal-agreement process

STP - UplinkFast and reconvergence

STP - UplinkFast and root port delay timer

STP - UplinkFast

STP - bridge priority

STP - connecting one port to another port on the same switch

STP - determining the blocked port using port ID

STP - port identification

STP - port roles

STP - port states

STP - spanning-tree root primary command

STP - what bridge ID does the MST region use

STP - what is a topology change

STP BPDU Filter

STP BPDU Guard

STP BPDU Types

STP BPDU destination MAC address

STP Cost Calculation Methods

STP Loop Guard

STP Max Age

STP Message Age

STP PortFast Best Practice

STP Portfast Options

STP Root Bridge selection

STP Root Guard

STP Topology Change (TC) flag

STP Topology Change Timer

STP UDLD and Autonegotiation

STP bridge priority values

STP changing link cost

STP configuring on EtherChannel

STP contents of a BPDU

STP cost long mode

STP cost short mode

STP determining blocked port using cost

STP direct vs indirect link failure

STP fine tuning the spanning tree protocol

STP how a switch identifies a BPDU

STP how to find the root bridge

STP interface type

STP prio nbr value

STP topology change process

Security - 802.1x with WLC and AAA server

Security - AAA for RESTCONF

Security - Best Practices for Repurposing Network Devices

Security - Cisco AnyConnect Secure Mobility Client

Security - Cisco Secure Client

Security - Cisco VPN Client

Security - Cisco VPN client software

STP - UDLD and Fiber Links

Links

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