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Introduction to Smart Link

Latest reply: Dec 26, 2019 19:02:51 264 1 3 0 0

Hello, everyone. 

I will introduce Smart Link in this article.

Network Topology and Technical Background



Figure 1 Smart Link networking

Smart Link is a solution customized for dual-uplink networking. What is dual-uplink networking? As shown in Figure 1, Switch D is connected to SwitchB and SwitchC through two uplinks. This is dual-uplink networking. Smart Link has the following advantages:

1. When both uplinks are running normally, one link is in a forwarding state (active) and the other is in a blocking state (inactive), preventing packet looping on the network.

2. Smart Link is easy to configure and use.

3. If the active link fails, traffic is switched to the standby link in milliseconds to ensure continued data forwarding and minimal packet loss.

4. Smart Link does not require protocol packets to be exchanged, greatly improving the convergence speed and reliability of the.

5. Smart Link is a Huawei proprietary feature.


Smart Link group

A Smart Link group consists of two interfaces: a master interface and a slave interface. Normally, the master interface is an inactive state, and the slave interface is blocked and in an inactive state.

As shown in Figure 1, Interface1 and Interface2 on SwitchD form a Smart Link group.

Master and Slave Interfaces

In the Smart Link group shown in Figure 1, Interface1 is the master interface, and Interface2 is the slave interface. When the two interfaces are both up, the master interface is active and the slave interface is inactive. If the link on the master interface fails, the slave interface becomes active and transitions to the forwarding state.

Flush Packet

When a link switchover occurs in a Smart Link group, existing forwarding entries no longer apply to the new topology. Therefore, all the MAC address entries and ARP entries on the network need to be updated. In this situation, the Smart Link group sends Flush packets to notify other devices of updates in MAC address entries and ARP entries. In Figure 1, when a link switchover occurs, SwitchD sends a multicast Flush packet to notify SwitchA, SwitchB, and SwitchC that they need to update their MAC address entries and ARP entries.

Control VLAN

Control VLAN for sending Flush packets

A Smart Link group broadcasts Flush packets in this VLAN. In Figure 1, if Switch D is enabled to send Flush packets, it broadcasts Flush packets in the control VLAN when a link switchover occurs in the Smart Link group.

Control VLAN for receiving Flush packets

The upstream devices use the control VLAN to accept and process Flush packets. In Figure 1, upstream devices SwitchA, Switch B, and SwitchC are able to identify Flush packets and are enabled to process Flush packets. When a link switchover occurs on SwitchD, the upstream devices process the received Flush packets and update their MAC address entries and ARP entries.

Working Principle


Figure 2 Topology

As shown in Figure 2, SW3 connects to SW1 and SW2 through two links. Configure a Smart Link group on SW3, add port 1 and port 2 to the Smart Link group, and set port 1 as the master port and port 2 as the slave port.

Both Links Are Working Properly


Figure 3 Both Links Are Working Properly

When both uplinks are working normally, the master port (port1) is inactive state and its link is the primary link; the slave port (port2) is in an inactive state and its link is the backup link. In this case, port 2 of SW3 is blocked and does not forward data. Therefore, service traffic is transmitted along the path indicated by the red line. No loop or broadcast storm occurs on the network.

Primary Link Fails


Figure 4 Primary Link Fails

When the primary link fails, the master port (port1) switches to the inactive state and the slave port (port2) switches to the active state. The entire switchover process is completed at a very fast speed. Because no protocol packets need to be exchanged, the convergence time is at the millisecond level.

Existing MAC address entries and ARP entries on the switches no longer apply to the network. A mechanism is required to update MAC address entries and ARP entries. Two mechanisms are available:

1. Flush packet-triggered

2. Traffic-triggered automatic update.

Method 1: Flush packet-triggered


Figure 5 Flush packet-triggered 1

This mode is applicable to the scenario where the upstream devices (such as SW1 and SW2) support the Smart Link function and can identify Flush packets. To implement a fast link switchover, you need to enable SW3 to send Flush packets and enable the dual-uplink interface of the upstream device to receive Flush packets.

After a link switchover occurs on SW3, SW3 sends Flush packets through the new primary link. The VLAN Bitmap field of the flush packet is filled with the protection VLAN ID of the smart link group to which port1 is in the forwarding state before the link switching belongs. The Control VLAN ID field is filled with the transmit control VLAN ID configured for the smart link group.


Figure 6 Flush packet-triggered 2

When an upstream switch receives the Flush packet, it checks whether the control VLAN carried in the packet is included in the list of control VLANs for receiving Flush packets configured on the interface. If the VLAN is not in the control VLAN list, the switch directly forwards the Flush packet without processing it. If the VLAN is in the control VLAN list, the device extracts the VLAN bitmap data from the Flush packet and updates the MAC address and ARP entries learned in the VLAN.


Figure 7 Flush packet-triggered 3

To ensure that Flush packets can be correctly transmitted in the control VLAN, ensure that all interfaces in the dual-homed networking allow the control VLAN to be sent. You are advised to send Flush packets with reserved tags. If you want to send Flush packets without tags, ensure that the default VLAN ID of the peer port is the same as the VLAN ID of the sending control VLAN. Otherwise, Flush packets are not sent in the sending control VLAN.

Method 2: Traffic-triggered automatic update


Figure 8 Traffic-triggered automatic update 1

This method is applicable to the scenario where the switch is connected to a non-Huawei device that does not support the Smart Link function. The upstream traffic needs to be triggered. In the preceding figure, SW3 supports Smart Link, but SW1 and SW2 do not.


Figure 9 Traffic-triggered automatic update 2

If SW3 does not receive upstream traffic (maybe from the downstream terminal) from SW3 to trigger MAC and ARP entry updates on SW1 and SW2, traffic may be interrupted when SW1 and SW2 receive data from SW3. SW2 is used as an example. When SW2 receives a data packet destined for SW3, SW2 still forwards the packet through the interface connected to SW1. However, the packet cannot reach SW3, causing traffic loss until the MAC-level ARP entry of SW2 is aged out. Therefore, this mode is much less efficient than the mode using flush packets.

Link recovery processing principle


Figure 10 Link recovery processing principle 1

As shown in the preceding figure, port 1 of SW3 is the master port of the Smart Link group, and port 2 is the slave port. By default, port 1 is in the active state. However, port 1 or its directly connected link is faulty. As a result, a switchover is performed, and the slave port (port 2) is in the active state. So what happens when the master port (port1) recovers? A Smart Link group supports two modes: role preemption mode and non-role preemption mode.

Role preemption mode:


Figure 11 Link recovery processing principle 2

As shown in the preceding figure, if the Smart Link group is configured to work in role preemption mode, when the primary link recovers, the master port preempts to be the active interface and the slave port enters the inactive state.

Non-role preemption mode:


Figure 12 Link recovery processing principle 3

As shown in the preceding figure, if the Smart Link group is configured to work in non-role preemption mode, the master interface (port2) remains in the active state and the slave interface (port1) remains in the inactive state after the master link recovers. In this way, traffic is stable. By default, the non-role preemption mode is used.

You can run the smart-link manual switch command to manually perform a link switchover.

This is my understanding of Smart Link. I will introduce how to configure Smart Link later. Welcome to discuss!

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Created Dec 26, 2019 19:02:51

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