Configuring UDLD Verify before forwarding

When an UDLD enabled port transitions to link-up, the port will begin with a UDLD blocking state. UDLD will probe via protocol packet exchange to determine the bidirectional state of the link. Until UDLD has completed the probe, all data traffic will be blocked. If the link is found to be bidirectional, UDLD will unblock the port for data traffic to pass. Once UDLD unblocks the port, other protocols will see the port as up and data traffic can be safely forwarded.

The default mode of a switch is “forward first then verify’’. Enabling UDLD link-up will default to “forward first then verify”. To change the mode to “verify then forward”, you need to configure using the commands found in section 6.72.


[NOTE: ]

NOTE: Link-UP data traffic will resumed after probing the link partner completes. All other protocols running will see the port as down.


UDLD time delay

UDLD protocol informs the link partner simultaneously as it detects a state change from unidirectional to bidirectional traffic. Additional packet exchanges will be carried out by UDLD in addition to the existing UDLD exchanges whenever state changes from unidirectional to bidirectional.

Peer state transition timings

Interval Time Interval 1 Interval 1 + delta Interval 2 Interval 3
  5 sec 5+(<5) sec* 10 sec 15 sec
With triggered updates State = blockedPeer State = blocked Inform PeerState = unblockedPeer State = unblocked Regular UDLD TX Regular UDLD TX
Without triggered updates State = blockedPeer State = blocked State = unblockedPeer State = blocked Inform PeerState = unblockedPeer State = unblocked Regular UDLD TX
*delta is the time when the unblock event occurs on local side

Restrictions

  • There is no support available when configuring this mode from the web and menu interface.

  • There are no new packet types are introduced with UDLD.

  • There are no new UDLD timers being introduced.

UDLD configuration commands

Syntax:

HP Switch(config)# link-keepalive mode [verify-then-forward | forward-then-verify]

This command configures the link-keepalive mode.

Link-keepalive provides two modes of operation; verify-then-forward and forward-then-verify.

When using the verify-then-forward mode, the port is in a blocking state until the link configured for UDLD establishes bidirectional communication. When using the forward-then-verify mode, the port forwards the data then verifies the status of the link-in state.

When a unidirectional state is detected, the port is moved to a blocked state.

When a bidirectional state is detected, the data is forwarded without interruption.

Syntax:

HP Switch(config)# link-keepalive mode verify-then-forward

Keeps the port in a logically blocked state until the link configured for UDLD has been successfully established in bi-directional communication.

Syntax:

HP Switch(config)# link-keepalive mode forward-then-verify

Forwards the data then verifies the status of the link. If a unidirectional state is detected, the port is then moved to a blocked state.

Syntax:

HP Switch(config)# link-keepalive interval <deciseconds>

Configure the interval for link-keepalive. The link-keepalive interval is the time between sending two UDLD packets. The time interval is entered in deciseconds (1/10 sec). The default keepalive interval is 50 deciseconds.

Example:

A value of 10 is 1 sec., 11 is 1.1 sec.

Syntax:

HP Switch(config)# link-keepalive retries <number>

Maximum number of sending attempts for UDLD packets before declaring the link as faulty.

Default keepalive attempt is 4.

Show commands

Syntax:

HP Switch(config)# show link-keepalive

Sample output:

Total link-keepalive enabled ports: 8
Keepalive Retries : 4
Keepalive Interval: 5 sec
Keepalive Mode : verify-then-forward
Physical Keepalive Adjacent UDLD

Port  Enabled Status           Status    Switch       VLAN
----- ------- ---------------- --------- ------------- ----------
1     Yes     down             off-line  000000-000000 untagged
2     Yes     down             off-line  000000-000000 untagged
3     Yes     down             off-line  000000-000000 untagged
4     Yes     down             off-line  000000-000000 untagged
5     Yes     down             off-line  000000-000000 untagged
6     Yes     down             off-line  000000-000000 untagged
7     Yes     down             off-line  000000-000000 untagged
8     Yes     down             off-line  000000-000000 untagged

RMON generated when user changes UDLD mode

RMON events are generated when UDLD is configured. The first time you configure the mode, the UDLD states will be re-initialized. An event log entry is initiated to include the reason for the initial UDLD blocking state during link up.

Example:

UDLD mode [verify-then-forward | forward-then-verify] is configured

Severity: - Info.