Multicast MAC addresses

STP - Dispute

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 still shows a line-protocol up state; however, BPDUs are not able to be transmitted, and the downstream switch eventually times out the existing root port 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://networklessons.com/spanning-tree/spanning-tree-loopguard-udld

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

STP UDLD

SPAN using VLANs as a source on a Nexus device

SPAN

SSH - domain-name prerequisite

SSH connectivity - troubleshooting

STP - Aging time reduction with a TCN

STP - BPDU generation and port roles

STP - BPDUGuard, BPDUFilter, and Portfast

STP - BPDUs on a switch where STP is disabled

STP - Disabling spanning-tree

STP - Learning State

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 - Multiple Spanning Tree (MST)

STP - PVST+ and Rapid PVST+ interaction

STP - Per VLAN Spanning Tree plus (PVST+)

STP - RSTP EdgePort

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

STP - Shortest Path Bridging (SPB)

STP - Topology Change Acknowledgement (TCA)

STP - bridge priority

STP - determining the blocked port using port ID

STP - port identification

STP - port roles

STP - port states

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 - 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

Security keyrings

STP UDLD

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