Layer 2 protocols

Layer 2 protocols

• CDP
• 
  
• LLDP

UDLD

• 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 should support UDLD to be able to identify and disable unidirectional links.
• A UDLD-capable port cannot detect a unidirectional link if it is connected to a UDLD-incapable port of another switch.
• When configuring the mode (normal or aggressive), make sure that the same mode is configured on both sides of the link.
• It supports two modes of operation
• Normal which is the default
•  In normal mode, UDLD can detect unidirectional links due to misconnected ports on fiber-optic connections
• Aggressive
•  In aggressive mode, UDLD can also detect unidirectional links due to one-way traffic on fiber-optic and twisted-pair links and to misconnected ports on fiber-optic links.
• In normal mode, UDLD detects a unidirectional link when fiber strands in a fiber-optic port are misconnected and the Layer 1 mechanisms do not detect this misconnection. If the ports are connected correctly but the traffic is one way, UDLD does not detect the unidirectional link because the Layer 1 mechanism, which is supposed to detect this condition, does not do so. In this case, the logical link is considered undetermined, and UDLD does not disable the port.
  
  
• When UDLD is in normal mode, if one of the fiber strands in a pair is disconnected, as long as autonegotiation is active, the link does not stay up because the Layer 1 mechanisms detect a physical problem with the link. In this case, UDLD does not take any action, and the logical link is considered undetermined.

• In aggressive mode, UDLD detects a unidirectional link by using the previous detection methods. UDLD in aggressive mode can also detect a unidirectional link on a point-to-point link on which no failure between the two devices is allowed. It can also detect a unidirectional link when one of these problems exists:

• One of the fiber strands in the cable is disconnected.
• On fiber-optic or twisted-pair links, one of the ports is down while the other is up.
• On fiber-optic or twisted-pair links, one of the ports cannot send or receive traffic.
•    In these cases, UDLD disables the affected port.
• The message-timer-interval range is from 1 to 90 seconds. The default value is 15.

Per interface configuration 

Switch#configure terminal 
Switch(config)#interface gigabitEthernet1/0/1
Switch(config-if)#udld port aggressive 
Switch(config-if)#end
Switch#

Verification

Switch#show udld GigabitEthernet1/0/1

Interface Gi1/0/1
---
Port enable administrative configuration setting: Enabled / in aggressive mode
Port enable operational state: Enabled / in aggressive mode
Current bidirectional state: Bidirectional
Current operational state: Advertisement - Single neighbor detected
Message interval: 15000 ms
Time out interval: 5000 ms

Port fast-hello configuration setting: Disabled
Port fast-hello interval: 0 ms
Port fast-hello operational state: Disabled
Neighbor fast-hello configuration setting: Disabled
Neighbor fast-hello interval: Unknown


Entry 1
---
Expiration time: 31600 ms
Cache Device index: 1
Current neighbor state: Bidirectional
Device ID: 346288238580 
Port ID: Gi4/0/1 
Neighbor echo 1 device: 70B4F35F080
Neighbor echo 1 port: Gi1/0/1

TLV Message interval: 15 sec
No TLV fast-hello interval
TLV Time out interval: 5
TLV CDP Device name: MXC.TAC.M.02-3850-01

Switch#show udld neighbors 
Port      Device Name    Device ID   Port ID     Neighbor State
----      -----------    ---------   -------     --------------
Gi1/0/1   346288238580   1           Gi4/0/1     Bidirectional

Total number of bidirectional entries displayed: 1

Use the  udld message time  command to change the message interval:

Switch(config)#udld message time 10
UDLD message interval set to 10 seconds