[Enterprise Switch] The behavior of MLAG and how to configure MLAG environment

The introduction of MLAG

• Multi-chassis Link Aggregation Group
• A non-standard protocol, implements link aggregation among multiple devices.
• The devices at both ends of the MLAG send MLAG negotiation packets through the peer-link.
• The main purpose of MLAG is to deliver system-level redundancy in the event one of the chassis fails.

Purpose

• A multi-chassis link aggregation group (MLAG) is a pair of links that terminate on two cooperating switches and appear as an ordinary link aggregation group (LAG). The cooperating switches are MLAG peer switches and communicate through an interface called a peer link. While the peer link’s primary purpose is exchanging MLAG control information between peer switches, but also carries data traffic from devices that are attached to only one MLAG peer and have no alternative path. An MLAG domain consists of the peer switches and the control links that connect these switches.

MLAG Domain Topology

MLAG Configuration

1. MLAG must be enabled globally using the mlag command.
2. The MLAG domain ID and peer link must be set using the mlag domain peer-link command.
3. The MLAG ID, associated MLAG domain ID and MLAG member must be configured using the mlag group member command. The associated MLAG domain may be nonexistent, which causes MLAG to be inactive locally.

Limitation: For a port to be configured as MLAG peer link or member.

1. STP cannot be enabled on a peer link or an MLAG member. An STP enabled port cannot be configured as a peer link or an MLAG member.
2. LACP status of the port must be disabled.
3. The port must not be any type of traffic segmentation port
4. Please don't connect to the cable before you configure the MLAG peer-link ports.
5. MLAG function is only supported by a static trunk with other devices.
6. The peer link port must bind to the appropriate VLAN for packet forwarding.
7. The up-link port and down-link port need to be added to MLAG member.

MLAG configure scenario:

ECS5520-1 & 2 Configuration and MLAG command Usage:

<Disable spanning-tree on port 1, 17>

Console(config)#interface ethernet 1/1,17

Console(config-if)#spanning-tree spanning-disabled
 

<Configure MLAG domain (peer link)>

Console(config)#mlag domain test peer-link ethernet 1/17

<Configure MLAG group>

Console(config)#mlag group 1 domain test member ethernet 1/1

ECS4120 configuration:

Console(config)#interface port-channel 1

Console(config-if)#interface ethernet 1/27,28

Console(config-if)#channel-group 1

ECS5520-1 MLAG Status:

5520-1#show mlag

 Global Status: Enabled

 Domain List: test

 MLAG List: 1

5520-1#show mlag domain test

 Peer Link: Eth 1/17

 MLAG List: 1

5520-1#show mlag group 1

 Domain ID: test

 Local Member: Eth 1/1

 Local State: Up

 Remote State: Up

5520-1#