MLAG vs. vPC: What's the Difference?
Updated at Feb 18th 20251 min read
As data center virtualization continues to evolve, the demand for higher bandwidth and faster speeds is increasing. To meet this need, network engineers are exploring ways to interconnect more networking devices efficiently. One such method is Link Aggregation Group (LAG), which combines multiple Ethernet links into a single logical link to enhance data transmission efficiency. Both Multi-Chassis Link Aggregation (MLAG) and Virtual Port Channel (vPC) can be used to implement LAG, but they differ in several aspects. This article provides an in-depth comparison of MLAG and vPC, covering their working principles, benefits, and key differences.
What Is MLAG?
MLAG (Multi-chassis Link Aggregation) is a non-standard protocol that enables Layer 2 multipathing from the host to gain either additional bandwidth or link resiliency. It’s a public protocol so that every vendor can support MLAG by using their own custom-rolled implementation of it. MLAG can make two or more enterprise network switches act like a single switch when forming link bundles.
The Working Principle of MLAG
In MLAG links, Link Aggregation Control Protocol (LACP, 802.3ad) is typically used to negotiate north and south between the host and an MLAG virtual switch or between two virtual MLAG switches. East and west proprietary protocols are used between MLAG enterprise network switches that are members of the same virtual MLAG switch. Figure 1 shows the example of using multiple gigabit MLAG switches - FS S5850-48T4Q to form an MLAG Pair. With four leaf switches in an MLAG pair, you can build redundant connections from the server to the MLAG enterprise network switches. The LAG groups can be formed using static link aggregation or LACP-based negotiation methods.
Figure 1: Using FS S5850-48T4Q Switches to Form an MLAG Pair
Benefits of MLAG
Traffic is evenly distributed to each of the switches through the use of LAG hashing.
Simply bundle more links into the LAG to increase bandwidth for North & South as well as East & West
Offers stability with dual management and control planes
Able to upgrade one switch at a time without affecting other devices
Expands port capacity freely and simply adds another switch East or West by creating another MLAG to another switch
Interested in how to configure it, you can watch the following video, taking the FS S5800-8TF12S hyper-converged infrastructure switch as an example.
What Is vPC?
vPC (Virtual Port Channel) is a Cisco Nexus technology where a Port Channel is a regular LAG. A Port Channel is a technology that provides a way to aggregate (bond) multiple interfaces together. Traffic is then load-balanced across each of the connections. It is a Cisco Nexus-specific protocol that is hard to configure on any other type of switch.
The Working Principle of vPC
vPC allows links physically connected with two different Cisco MLAG switches to appear as a single Port Channel to a third device. The third device can be an ethernet switch, a server, or any other networking device supporting IEEE 802.3ad Port Channels. vPC also allows the creation of Layer 2 Port Channels that span two switches.
As illustrated below, vPC is utilized to create inter-switch links spanning two MLAG switches while keeping two control planes of these MLAG switches separate. After you enable the vPC function, you create a peer keep-alive link, which sends heartbeat messages between the two vPC peer devices. The vPC domain includes vPC peer devices, the vPC peer keep-alive link, the vPC peer link, and all the Port Channels in the vPC domain connected to the downstream device. You can have only one vPC domain ID on each device.
Figure 2: Form a vPC Pair by Creating Inter-switch Links
Benefits of vPC
Allows a single device to use a Port Channel across two upstream devices
Eliminates Spanning Tree Protocol blocked ports
Provides a loop-free topology
Uses all available uplink bandwidth
Provides fast convergence if either the link or a device fails
Provides link-level resiliency
Helps ensure high availability
MLAG vs. vPC: 4 Key Differences
Both MLAG and vPC can create a port group between two switches and enable Layer 2 multipathing. In MLAG or vPC domain, each MLAG enterprise network switch is managed and configured independently and is able to forward/route traffic without passing to a master switch. Despite their similarities, they still differ in some ways.
Implementation Complexity
Obviously, the biggest difference between them is the difficulty of implementation. MLAG is a public protocol that is supported by almost every vendor using their own custom rolled implementation, while vPC is a Cisco Nexus-specific protocol, not all the vendors have this technology. Thus, MLAG setup is a bit easier than vPC.
Compatibility issues
Another issue is compatibility. For vPC pairing, the same type of Cisco Nexus switches must be used. For example, it is not possible to configure vPC on a pair of switches, including a Nexus 7000 series and a Nexus 5000 series switch. And the vPC peers must run the same NX-OS version except during the non-disruptive upgrade, that is, the In-Service Software Upgrade (ISSU).
Layer Multipathing
Besides, the vPC peer link must consist of at least two 10G Ethernet ports in dedicated mode. vPC is more advanced than MLAG. It supports both Layer 2 and Layer 3 multipathing, which allows you to create redundancy by enabling multiple parallel paths between nodes and load-balancing traffic where alternative paths exist. If you want to enable Layer 3 multipathing, you could also use the Multi-Active Gateway Protocol (MAGP).
Application Scenarios
Normally, vPC can only be used on Cisco Nexus data center switches, while MLAG can be applied to a wide range of scenarios. Whether in a traditional 3-tier data center architecture or a 2-tier spine-leaf architecture, switches that support MLAG can form an MLAG pair at different layers. For a deeper dive into how MLAG enhances data center network design, check out this guide on MLAG implementation.
Item | MLAG | vPC |
Simplifies Network Design | Yes | Yes |
Eliminates Spanning Tree Protocol (STP) | Yes | Yes |
Multipathing | Layer 2 | Layer 2 & Layer 3 |
Difficulty of Implementation | Easier | Relatively Difficult |
Switch Type for Pairing | No requirement | Strict Requirements |
Using Scenarios | Common seen in distribution or data center switches | Usually in Cisco Nexus data center switches |
Conclusion
Both MLAG and vPC offer powerful solutions for link aggregation and redundancy, but their suitability depends on network requirements and vendor compatibility. MLAG provides a vendor-agnostic, flexible approach, whereas vPC is tailored for Cisco environments with enhanced multipathing capabilities. Understanding their differences can help network engineers choose the right solution to optimize data center performance and reliability.
FS PicOS® data center switches support MLAG, providing system-level redundancy and improving network reliability to help you steadily advance in an evolving digital environment.
