Multicast MAC addresses

STP - Dispute

STP - UDLD and Fiber Links

STP Portfast Options

STP UDLD and Autonegotiation

STP fine tuning the spanning tree protocol


[Fiber-optic cables](/physical-layer-optical-fiber-medium) consist of strands of glass/plastic that transmit light. A cable typically consists of one strand for sending data and another strand for receiving data on one side; the order is directly opposite at the remote site. Network devices that use fiber for connectivity **can encounter unidirectional traffic flows if one strand is broken.** 

In such scenarios, the interface [may still show a line-protocol up state](/stp-udld-and-fiber-links); however, [BPDUs](/stp-bpdu-types) are not able to be transmitted, and the downstream switch eventually times out the existing [root port](/stp-port-roles) and identifies a different port as the root port. Traffic is then received on the new root port and forwarded out the strand that is still working, thereby creating a forwarding loop.

Unidirectional Link Detection (UDLD) allows for the bidirectional monitoring of fiber-optic cables. UDLD operates by transmitting UDLD packets to a neighbor device that includes the system ID and port ID of the interface transmitting the UDLD packet. The receiving device then repeats that information, including its system ID and port ID, back to the originating device. The process continues indefinitely.

This ensures that in the event that one of the two strands fails, the [STP](/spanning-tree-protocol-stp) topology will remain stable.

Note that UDLD operates in conjunction with [autonegotiation](/stp-udld-and-autonegotiation).

For additional STP fine tuning features, take a look at [STP fine tuning the spanning tree protocol](/stp-fine-tuning-the-spanning-tree-protocol).

## Links

https://forum.networklessons.com/t/spanning-tree-loopguard-and-udld/1145/67?u=lagapidis

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

https://www.ciscopress.com/articles/article.asp?p=2995351&seqNum=3

STP UDLD

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

STP Root Bridge selection

STP Root Guard

STP Topology Change (TC) flag

STP Topology Change Timer

STP bridge priority values

STP changing link cost

STP configuring on EtherChannel

STP configuring portfast

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 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

Security - Diffie-Hellman groups

Security - Extensible Authentication Protocol over LAN (EAPOL)

Security - GETVPN

Security - IEEE 802.1X

Security - IOS Usernames and Passwords

Security - IPsec nonce

Security - Kerberos

Security - MAC Authentication Bypass

Security - Network Access Control (NAC)

Security - Next Generation Firewall

Security - Secure Unique Device Identifier (SUDI)

Security - WPA 4-way handshake

Security - WebVPN

Security - accessing VTY from another VRF

Security - assigning privilege levels, syntax and behavior

Security - bogons

Security - broadcast-multicast storm

Security - configuring TACACS from another VRF

Security - privilege levels and command output

Security - privilege levels and enable keyword

Security - public key exchange

Security - root CAs and private keys

Security - spoofing

Security - storm control algorithm

Security IKE policy hash command

Security Trusted Platform Module (TPM)

Security aaa new-model

Security authentication on VTY lines

STP UDLD

Links

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