H3C Technologies H3C S7500E Series Switches - Manual (Page 113)

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Transmit control VLAN
The transmit control VLAN is used for transmitting flush messages. When link switchover occurs, the
devices (such as Device C and Device D in Figure 8-1 ) broadcast flush messages within the transmit
control VLAN.
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 Figure 8-1 ) receive and process
flush 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 8-1 , the link on GE2/0/1 of Device C is the master link, and the link on GE2/0/2 of
Device C is the slave link. Normally, GE2/0/1 is in the forwarding state, while GE2/0/2 is in the standby
state. When the master link fails, GE2/0/2 takes over to forward traffic while GE2/0/1 is blocked and
placed in the standby state.
z When a port switches to the forwarding state, the system outputs log information to notify the user
of the port state change.
z 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:
z 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’.
z 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.