* Question
What is a multi-rack link aggregation or a cross-stack Ethernet channel?
* Answer
A multi-rack link aggregation or cross-stack Ethernet channel refers to a network design technique that enables logical bundling of Ethernet links across multiple physical switches—typically in different racks or chassis—into a single logical channel. This approach enhances resiliency, load balancing, and redundancy in high-availability data center and enterprise network architectures.
1. Concept Overview
➤ Multi-Rack Link Aggregation:
This involves creating a Link Aggregation Group (LAG)—also called an EtherChannel—that spans multiple top-of-rack (ToR) switches. The aggregated link appears as a single interface to the connected device (like a server or another switch), even though it physically terminates on separate switches.
➤ Cross-Stack EtherChannel:
This is a similar concept, where switches in a stacked configuration (or virtual chassis) jointly participate in a single EtherChannel. It’s often vendor-specific:
Cisco: Called Multi-Chassis EtherChannel (MEC) or StackWise EtherChannel
Arista: Uses MLAG (Multi-Chassis Link Aggregation)
Juniper: Uses Virtual Chassis and LAGs
Dell/Force10: Uses VLT (Virtual Link Trunking)
2. Key Characteristics
Feature | Description |
Resiliency | If one switch or link fails, traffic continues through the other links/switches. |
Load Balancing | Traffic is distributed across all member links based on hashing algorithms. |
Single Logical Interface | To the connected server/switch, the group of physical links appears as one. |
Requires Vendor Support | Not all switches support cross-stack LAG; it depends on firmware, models, and vendor features. |
3. Use Case Example
Imagine a high-performance server with two NICs:
NIC 1 connects to Switch A (Rack 1)
NIC 2 connects to Switch B (Rack 2)
Using cross-stack link aggregation:
The server configures a bonded interface (LACP or static) across NIC 1 and NIC 2
Switch A and B are configured for MLAG or equivalent, and coordinate state and forwarding
Result: Higher throughput + failover protection + seamless connectivity
4. Standards & Protocols
IEEE 802.3ad / IEEE 802.1AX: Standard for LACP (Link Aggregation Control Protocol)
Proprietary Enhancements:
Cisco vPC (Virtual Port Channel)
Arista MLAG
Juniper MC-LAG
HPE IRF (Intelligent Resilient Fabric)
5. Benefits
High availability across physical infrastructure
No spanning-tree blocking (since links are active-active)
Seamless integration with NIC bonding on servers
Horizontal scalability in spine-leaf and ToR networks
Summary
A multi-rack link aggregation or cross-stack EtherChannel provides a way to combine multiple Ethernet links across different switches into a single logical interface, delivering high availability and balanced throughput. It is essential in modern redundant network topologies, especially in data center, cloud, and enterprise networks.
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