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: ]](images/note.gif) | 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 | ||||
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-forwardandforward-then-verify.When using the
verify-then-forwardmode, the port is in a blocking state until the link configured for UDLD establishes bidirectional communication. When using theforward-then-verifymode, 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-forwardKeeps 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-verifyForwards 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. Thelink-keepaliveinterval 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:
Show commands
Syntax:
HP Switch(config)# show link-keepaliveSample 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.