Unidirectional Link Detection UDLD

Unidirectional Link Detection (UDLD)

Last Updated: [last-modified] (UTC)

 

Unidirectional links are more common with fibre, but may also happen with copper

Imagine that you have a dual-core fibre run between two buildings. Somewhere along the run, one of the cores gets damaged. This may leave you in a position where you have a uni-directional link. You have enough of a link to send in one direction, but not the other.
Devices at each end may have trouble noticing the link impairment. Traffic still flows, in one direction at least, so they may think the link is still up.

 

But that’s better than a completely severed link right? Maybe not. Spanning-tree may allow some layer-2 loops through the network. Why? It is unable to send BPDU’s to a connected switch. One way to protect against this specific problem is Loop Guard. If a port expects BPDU’s, but does not receive them, Loop Guard disables the port.

But how do we protect against other problems? This is when you use Unidirectional Link Detection, or UDLD. Cisco designed the UDLD protocol, but it was later adapted to an industry standard in RFC 5171. Two devices with UDLD will send each other hello packets every 15 seconds. If the responses are missing, the switch disabled the port.

Recommendation: Use aggressive mode

Enable UDLD globally or per interface. It can also be set to enable or aggressive mode. Enable mode will take no action other than to generate syslog messages. Aggressive mode will try to reestablish the link. If it is unable to, it will disable the port. The recommendation is to enable UDLD globally, in aggressive mode. Syslog messages are too easy to miss.

 

 

Configuration

Turn on UDLD per interface on with the udld port or udld port aggressive command. The modes do not need to match at both ends.

 

Per Port Configuration
SW-1(config)#interface GigabitEthernet 0/1
SW-1(config-if)#udld port


SW-2(config)#int GigabitEthernet 0/1
SW-2(config-if)#udld port aggressive 


! Alternatively, use 'udld enable' or 'udld aggressive' to enable it globally

 

When it is first enabled, it passes through an unknown state. This prevents it from assuming that the link is down until it is able to get an initial response from it’s partner.

Once it is getting responses, it transitions into the bidirectional state.

 

Verification
SW-1#show udld

Interface Gi0/0
---
Port enable administrative configuration setting: Disabled
Port enable operational state: Disabled
Current bidirectional state: Unknown

Interface Gi0/1
---
Port enable administrative configuration setting: Enabled
Port enable operational state: Enabled
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: 43800 ms
    Cache Device index: 1
    Current neighbor state: Bidirectional
    Device ID: 91I51YKW0UB  
    Port ID: Gi0/1  
    Neighbor echo 1 device: 9MJM4TSXVT7
    Neighbor echo 1 port: Gi0/1

    TLV Message interval: 15 sec
    No TLV fast-hello interval
    TLV Time out interval: 5
    TLV CDP Device name: SW-2  

Interface Gi0/2
---
Port enable administrative configuration setting: Disabled
Port enable operational state: Disabled
Current bidirectional state: Unknown



SW-2#show udld gigabitEthernet 0/1

Interface Gi0/1
---
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: 32100 ms
    Cache Device index: 1
    Current neighbor state: Bidirectional
    Device ID: 9MJM4TSXVT7  
    Port ID: Gi0/1  
    Neighbor echo 1 device: 91I51YKW0UB
    Neighbor echo 1 port: Gi0/1

    TLV Message interval: 15 sec
    No TLV fast-hello interval
    TLV Time out interval: 5
    TLV CDP Device name: SW-1
 

References

Packet Life – UDLD

 

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