How Smart Link Works - HP 4800G Series Configuration Manual

Receive control VLAN
The receive control VLAN is used for receiving and processing flush messages. When link switchover
occurs, the devices (such as Device A, Device B, and Device E in
messages in the receive control VLAN and refresh their MAC address forwarding entries and ARP/ND
entries.
Flush message
Flush messages are used by a smart link group to notify other devices to refresh their MAC address
forwarding entries and ARP/ND entries when link switchover occurs in the smart link group. Flush
messages are common multicast data packets, and will be dropped by a blocked receiving port.

How Smart Link Works

Link backup mechanism
As shown in Figure
Device C is the slave link. Normally, GE1/0/1 is in the forwarding state, while GE1/0/2 is in the standby
state. When the master link fails, GE1/0/2 takes over to forward traffic while GE1/0/1 is blocked and
placed in the standby state.
When a port switches to the forwarding state, the system outputs log information to notify the user
of the port state change.
To keep traffic forwarding stable, the master port that has been blocked due to link failure does not
take over immediately upon its recovery. Instead, link switchover will occur at next link switchover.
Topology change mechanism
As link switchover can outdate the MAC address forwarding entries and ARP/ND entries on all devices,
you need a forwarding entry update mechanism to ensure proper transmission. By far, the following two
update mechanisms are provided:
Uplink traffic-triggered MAC address learning, where update is triggered by uplink traffic. This
mechanism is applicable to environments with devices not supporting smart link, including devices
of other vendors'.
Flush update where a Smart Link-enabled device updates its information by transmitting flush
messages over the backup link to its upstream devices. This mechanism requires the upstream
devices to be capable of recognizing smart link flush messages to update its MAC address
forwarding entries and ARP/ND entries.
Role preemption mechanism
As shown in Figure
Device C is the slave link. Once the master link fails, GE1/0/1 is automatically blocked and placed in the
standby state, while GE1/0/2 takes over to forward traffic. During this period, if the smart link group is
1-1, the link on GE1/0/1 of Device C is the master link, and the link on GE1/0/2 of
1-1, the link on GE1/0/1 of Device C is the master link, and the link on GE1/0/2 of
Figure
1-4
1-1) receive and process flush