First Hop Redundancy Protocols are the Layer 3 answer to resiliency. It adds multiple Layer 3 paths to allow devices to reach their destinations.
First Hop Redundancy Protocols by creating a virtual gateway for the network that is shared between Layer 3 devices. There are many different protocols but three are relevant for the CCNP exam:
Unidirectional Link Detection allows for monitoring of physical media that separates its send and receive media, like fibre optic cables.
UDLD operates by transmitting a packet to the neighbour device that contains a system and port ID of the originating interface. The receiving interface gets this information and mirrors it back including its own system and port ID. This process continues to ensure that both send and receive parts of the link have communication between them.
Unidirectional Link Detection has two operating modes, normal and aggressive.
Unidirectional Link Detection can be enabled globally with the command udld enable with the optional additional suffix keyword of aggressive to enable aggressive mode. Enabling UDLD globally will switch on the UDLD mechanisms on all SFP ports.
Individual ports can have UDLD capabilities disabled in interface configuration mode with the command udld port disable
In order for UDLD to function correctly, both ends of the switch must have UDLD enabled to communicate packets correctly. The status of UDLD can be checked with the command show udld neighbors
The new TSHOOT exam requires you to isolate issues on a defined network topology. The purpose of the exam is to evaluate troubleshooting skills, not how quickly you can grasp a network topology you have never seen. To this end, it is Cisco’s intent that the exam network topology be available for review before the candidate attempt the exam.
https://learningnetwork.cisco.com/thread/10965
As practice I mirrored the topology in the linked thread to an easy to read topology below.
A - VLAN 10 - 10.2.1.0/24DSW1 - 10.2.1.1DSW2 - 10.2.1.2 B - VLAN 20 - 10.2.2.0/24 DSW1 - 10.2.2.2 DSW2 - 10.2.2.1 C - DSW1 <-> R4 - 10.1.4.4/30 DSW1 - 10.1.4.6 R4 - 10.1.4.5 D - DSW2 <-> R4 - 10.1.4.8/30 DSW2 - 10.1.4.10 R4 - 10.1.4.9 E - R4 <-> R3 - 10.1.1.8/30 R4 - 10.1.1.10 R3 - 10.1.1.9 F - R3 <-> R2 - 10.1.1.4/30 R3 - 10.1.1.6 R2 - 10.1.1.5 G - R2 <-> R1 - 10.1.1.0/30 R2 - 10.1.1.2 R1 - 10.1.1.1 H - R1 <-> WAN - 209.65.200.224/30 R1 - 209.65.200.225 ISP Router - 209.65.200.226 X - Link between DSW1 and DSW2 (not shown) - 10.2.4.12/30 DSW1 - 10.2.4.13 DSW2 - 10.2.4.14