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MC-LAG and STP Configuration on Enterprise SONiC Switch

1. Background

MC-LAG inherently provides loop avoidance mechanisms. In normal operation, MC-LAG-based networks do not generate Layer 2 loops. However, in multi-tier MC-LAG deployments, loops may still occur due to incorrect network design, incomplete initial configuration, or device reboot with empty configuration. In such cases, STP can be deployed as a safeguard to prevent loop formation.

2. Topology Requirements

Switches adopt cross-device link aggregation (MC-LAG) to ensure high service reliability at the device level. Under normal conditions, traffic is load-balanced across all active links. With STP enabled, the network is still able to successfully break potential loops, while ensuring that data forwarding is not impacted.

networking topology about mc-lag and stp

3. Configuration Steps

3.1 Create the aggregate interface, and add the physical interfaces connected to each switch into the corresponding Link Aggregation Group (LAG).

Switch A

sonic(config)# interface link-aggregation 1
sonic(config)# interface ethernet 20
sonic(config-if-20)# link-aggregation-group 1
sonic(config-if-20)# exit
sonic(config)# interface ethernet 35
sonic(config-if-35)# link-aggregation-group 1
sonic(config-if-35)# exit

Switch B

sonic(config)# interface link-aggregation 1
sonic(config)# interface link-aggregation 2
sonic(config)# interface link-aggregation 3
sonic(config)# interface ethernet 14
sonic(config-if-14)# link-aggregation-group 1
sonic(config-if-14)# exit
sonic(config)# interface ethernet 11
sonic(config-if-11)# link-aggregation-group 2
sonic(config-if-11)# exit
sonic(config)# interface ethernet 12
sonic(config-if-12)# link-aggregation-group 3
sonic(config-if-12)# exit

Switch C

sonic(config)# interface link-aggregation 1
sonic(config)# interface link-aggregation 2
sonic(config)# interface link-aggregation 3
sonic(config)# interface ethernet 35
sonic(config-if-35)# link-aggregation-group 1
sonic(config-if-35)# exit
sonic(config)# interface ethernet 21
sonic(config-if-21)# link-aggregation-group 2
sonic(config-if-21)# exit
sonic(config)# interface ethernet 49
sonic(config-if-49)# link-aggregation-group 3
sonic(config-if-49)# exit

Switch D

sonic(config)# interface link-aggregation 3
sonic(config)# interface ethernet 12
sonic(config-if-12)# link-aggregation-group 3
sonic(config-if-12)# exit
sonic(config)# interface ethernet 49
sonic(config-if-49)# link-aggregation-group 3
sonic(config-if-49)# exit

3.2 Create the VLAN interface, and assign both the physical interfaces and the Link Aggregation Group (LAG) to the corresponding VLAN.

Switch A

sonic(config)# vlan 10
sonic(config)# interface link-aggregation 1
sonic(config-lagif-1)# switchport trunk vlan 10
sonic(config-lagif-1)# exit
sonic(config)# interface ethernet 37
sonic(config-if-37)# switchport trunk vlan 10
sonic(config-if-37)# exit
sonic(config)# interface ethernet 10
sonic(config-if-10)# switchport trunk vlan 10
sonic(config-if-10)# exit

Switch B

sonic(config)# vlan 10
sonic(config)# vlan 400
sonic(config)# interface link-aggregation 1
sonic(config-lagif-1)# switchport trunk vlan 10
sonic(config-lagif-1)# exit
sonic(config)# interface link-aggregation 2
sonic(config-lagif-2)# switchport trunk vlan 400
sonic(config-lagif-2)# switchport trunk vlan 10
sonic(config-lagif-2)# stp bpdu-filter enable
sonic(config-lagif-2)# exit
sonic(config)# interface link-aggregation 3
sonic(config-lagif-3)# switchport trunk vlan 10
sonic(config-lagif-3)# exit

Switch C

sonic(config)# vlan 10
sonic(config)# vlan 400
sonic(config)# interface link-aggregation 1
sonic(config-lagif-1)# switchport trunk vlan 10
sonic(config-lagif-1)# exit
sonic(config)# interface link-aggregation 2
sonic(config-lagif-2)# switchport trunk vlan 400
sonic(config-lagif-2)# switchport trunk vlan 10
sonic(config-lagif-2)# stp bpdu-filter enable
sonic(config-lagif-2)# exit
sonic(config)# interface link-aggregation 3
sonic(config-lagif-3)# switchport trunk vlan 10
sonic(config-lagif-3)# exit

Switch D

sonic(config)# vlan 10
sonic(config)# interface link-aggregation 3
sonic(config-lagif-3)# switchport trunk vlan 10
sonic(config-lagif-3)# exit
sonic(config)# interface ethernet 38
sonic(config-if-38)# switchport trunk vlan 10
sonic(config-if-38)# exit
sonic(config)# interface ethernet 3
sonic(config-if-3)# switchport trunk vlan 10
sonic(config-if-3)# exit

3.3 Configure MC-LAG and set a consistent Bridge MAC address across both peer devices.

Switch B

sonic(config)# bridge mac 00:00:04:12:05:01
sonic(config)# interface vlan 400
sonic(config-vlanif-400)# ip address 50.1.1.2/24
sonic(config-vlanif-400)# exit
sonic(config)# mclag domain 1
sonic(mclag-domain)# peer-address 50.1.1.1
sonic(mclag-domain)# local-address 50.1.1.2
sonic(mclag-domain)# peer-link link-aggregation 2
sonic(mclag-domain)# member lag 1
sonic(mclag-domain)# member lag 3
sonic(mclag-domain)# commit
sonic(mclag-domain)# exit

Switch C

sonic(config)# bridge mac 00:00:04:12:05:01
sonic(config)# interface vlan 400
sonic(config-vlanif-400)# ip address 50.1.1.1/24
sonic(config-vlanif-400)# exit
sonic(config)# mclag domain 1
sonic(mclag-domain)# peer-address 50.1.1.2
sonic(mclag-domain)# local-address 50.1.1.1
sonic(mclag-domain)# peer-link link-aggregation 2
sonic(mclag-domain)# member lag 1
sonic(mclag-domain)# member lag 3
sonic(mclag-domain)# commit
sonic(mclag-domain)# exit

3.4 Enable MSTP on the switch, and verify the STP loop-prevention status across the entire device.

Switch A

sonic(config)# stp enable mstp
sonic(config)# stp instance 1
sonic(config-stp-1)# exit
sonic(config)# stp bind vlan 10 1
sonic(config)# stp name test

Switch B

sonic(config)# stp enable mstp
sonic(config)# stp instance 1
sonic(config-stp-1)# exit
sonic(config)# stp bind vlan 10 1
sonic(config)# stp name test

Switch C

sonic(config)# stp enable mstp
sonic(config)# stp instance 1
sonic(config-stp-1)# exit
sonic(config)# stp bind vlan 10 1
sonic(config)# stp name test

Switch D

sonic(config)# stp enable mstp
sonic(config)# stp instance 1
sonic(config-stp-1)# exit
sonic(config)# stp bind vlan 10 1
sonic(config)# stp name test

4. Verification

1. Check MC-LAG status

sonic# show mclag state
The MCLAG’s keepalive is: OK
MCLAG info sync is: completed
Domain id: 1
MCLAG session Channel: Primary channel
VRF Name: default
consistency Check Action: idle
Local Ip: 50.1.1.1
Peer Ip: 50.1.1.2
Peer Link Interface: PortChannel0002
Keepalive time: 1
sesssion Timeout : 15
Peer Link Mac: 00:11:24:1a:1b:61
Admin Role: None
Role: Standby
MCLAG Interface: PortChannel0001, PortChannel0003
Loglevel: DEBUG

2. Check the STP status on Switch D

sonic# show stp status
role Flags: Root - Root, Desg - Designated, Altn - Alternate, Back - Backup, Mstr - Master, Disa - Disabled
state Flags: disc - Discard/Blocking/Listening,  lear - Learning, forw - Forwarding
Spanning-tree Mode: mstp

  mst_instance  port             port_role    stp_state
--------------  ---------------  -----------  -----------
             1  Ethernet3        Desg         forw
             1  PortChannel0003  Root         forw
             1  Ethernet38       Altn         disc

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