Understanding MLAG: A Deep Dive into Multi-Chassis Link Aggregation

In today’s networks, high availability and redundancy are not just nice-to-have features – they’re essential requirements. Multi-Chassis Link Aggregation (MLAG) technology stands out as a powerful solution for achieving these goals. Let’s dive deep into what MLAG is, how it works, and why it’s becoming increasingly important in modern network architectures.

What is MLAG and Why Does it Matter?

Imagine you’re building a data center or even a campus network and need to ensure that no single switch failure can bring down your critical connections. This is where MLAG comes into play. At its core, MLAG combines switches using a peer link so they appear as one logical switch to L2 protocols such as STP, LACP and IGMP. This delivers system-level redundancy and network-level resiliency.

The magic happens through a clever trick: the switches communicate with each other to negotiate a common MLAG System ID (MSI), effectively fooling LACP (Link Aggregation Control Protocol) into treating them as one device. This simple yet powerful concept opens up new possibilities for network redundancy and load balancing.

Benefits

MLAG SSO allows an MLAG pair to sustain the failure of one MLAG peer switch with minimal impact on the traffic forwarding plane and layer 2 protocols. Essentially sub-second failover recovery ensures minimal disruption

MLAG ISSU allows an MLAG pair to be upgraded with minimal impact on the traffic forwarding plan and layer 2 protocols.

MLAG configuration is standards based and simple to set up.

LACP and IGMP work transparently across peers

Btw: VARP (Virtual ARP) provides an alternative to traditional redundancy protocols

The Building Blocks of MLAG

The Peer Link: More Than Just a Connection

The foundation of MLAG is the peer link, which might seem like just another connection between switches, but it’s much more sophisticated than that. Using standard front-facing ports and operating over a TCP/IP connection, the peer link handles two distinct types of traffic:

1. Coordination traffic: This includes MLAG advertisements and keepalives, ensuring both switches stay in sync
2. Data traffic: Only comes into play for single-attached hosts or during failure scenarios

The Election Process: Democracy in Action

Like a well-organized democracy, MLAG peers automatically negotiate their roles:

• Each switch puts forward its MLAG System ID (calculated from its bridge MAC address plus a locally administered bit)
• The switch with the lowest MSI becomes the primary
• Both switches maintain this negotiated identity until MLAG is explicitly deconfigured
• Importantly, there’s no preemption – once roles are established, they stick

Spanning Tree and MLAG: A Careful Ballet

MLAG’s handling of Spanning Tree Protocol (STP) is particularly elegant. The primary MLAG peer takes the lead role:

• It runs the active STP agent
• Processes all BPDUs
• Calculates forwarding ports for both peers

Meanwhile, the secondary peer plays a supporting role:

• Keeps its STP agent inactive
• Only handles BPDUs in specific scenarios
• Creates the illusion of a single STP bridge to the rest of the network

Designing for High Availability

Best Practices That Matter

When implementing MLAG, following these best practices can make the difference between a robust setup and a fragile one:

1. Create redundant peer links across different linecards
2. Maintain identical configurations across both switches
3. Keep spanning tree enabled as a safety net
4. Ensure software version compatibility
5. Configure consistent reload delays

Note: The system provides detailed compatibility information during upgrades

Handling Failure Scenarios

MLAG includes sophisticated mechanisms for handling failures:

• Split-brain prevention through shared MAC addressing
• Error-disabled interfaces on returning switches
• Configurable reload delays (default: 300 seconds)

Troubleshooting and Monitoring

For network administrators, MLAG provides comprehensive troubleshooting commands:

show mlag                    ! Basic MLAG status
show mlag detail            ! Detailed MLAG information
show mlag interfaces        ! MLAG interface status
show mlag config-sanity     ! Configuration consistency check
show spanning-tree          ! STP status and configuration

Remember

To ensure a successful MLAG implementation, focus on these key areas:

1. Configuration Consistency: Keep VLAN configurations, port channel settings, and STP parameters identical across peers
2. Spanning Tree Strategy: Maintain STP as a backup while considering modern alternatives like BGP with ECMP
3. Version Management: Stay on top of software versions and follow proper upgrade procedures
4. Failure Recovery Planning: Understand the non-preemptive behavior and plan accordingly