Exploring the Difference Between MLAG vs. vPC

As data center virtualization continues to advance, the demand for bandwidth and speed continues to climb. To meet this demand, network engineers are looking for more efficient ways to connect more and more network devices. One such solution is Link Aggregation Groups (LAGs), which bundle multiple Ethernet links into a single logical link to improve the efficiency of data transmission. There are two options for implementing LAGs: multi-chassis link aggregation (MLAG) and virtual port channel (vPC). While both enable link aggregation, they differ in many ways. Next, we'll dive into MLAG and vPC to understand how they work, the benefits of each, and the key differences between them.

 

What is MLAG?

 

MLAG (Multi-Chassis Link Aggregation) is a non-standard protocol used to implement Layer 2 multipathing to provide additional bandwidth and link redundancy. Simply put, MLAG allows multiple switches to work together and perform link aggregation like a single switch. Different vendors can customize the implementation of MLAG to suit their needs, making it a relatively open protocol with different implementations for each vendor. The MLAG protocol allows two or more enterprise switches to work as a single switch in a network when forming link bundles.

 

How do MLAG Work? 

 

In an MLAG network, the Link Aggregation Control Protocol (LACP, 802.3ad) is typically used to negotiate a north-south link between a host and an MLAG virtual switch or between two virtual MLAG switches. In contrast, member switches within the same virtual MLAG switch communicate using a proprietary east-west protocol. As an example, the following illustration shows an MLAG pair formed through multiple Gigabit switches, such as the S7600-48X8C. With these MLAG switches, redundant connections can be established between servers and enterprise network switches.

 

S7600-48X8C MLAG Solution

 

MLAGs can be formed into Link Aggregation Groups (LAGs) either by means of static link aggregation or by negotiation based on the LACP protocol. This not only improves bandwidth utilization but also provides redundant backups in the event of a failure, making the network more stable.

 

Advantages of MLAG

 

MLAG has many advantages in terms of providing system performance and resilience. First, with the LAG hashing algorithm, traffic could be balanced to different switches, thus eliminating performance bottlenecks due to a certain switch overload. To summarize, this practice effectively uses limited bandwidth, making the whole network more efficient. It can make the network continue to run smoothly during peak data traffic periods; organizations can also easily increase the bandwidth of their network and add LAG links (north-south and east-west) as required.

 

In terms of enhanced stability, MLAG switch utilizes a dual management and control plane architecture, making the system more fault-tolerant. When one switch fails, the other switch completes the task in the shortest time, avoiding network interruption and keeping the continuous and stable operation of the network. What's even better is that individual switches of the MLAG solution can be upgraded one by one without interrupting the work of other devices, so the network will gain a very high level of flexibility.

 

What is vPC?

 

vPC (Virtual Port Channel) is a unique technology in the Cisco Nexus Series switches that allows multiple physical connections to be presented through a single port channel (LAG). A port channel is a technology that aggregates multiple network interfaces together to achieve load balancing of traffic. In vPC, port channels are more than simple LAGs, allowing physical connections from two different Cisco switches to be presented to a third-party device through a unified logical channel. It is important to note that vPC is a Cisco proprietary protocol, so it can only be configured on Cisco Nexus switches, and switches from other vendors typically do not directly support this technology.

 

How does vPC work?

 

vPC enables a link to two Cisco MLAG switches to appear as a port channel on a third device. This third-party device can be any network device that supports the IEEE 802.3ad port-channel standard, such as Ethernet switches, servers, and so on. With vPC, users can create a Layer 2 port channel between two switches that spans two devices, thus simplifying the network topology and increasing network redundancy.

 

As an example, vPC can be used to create an inter-switch link between two MLAG switches while keeping the control planes of the two switches separate. When the vPC feature is enabled, the two vPC switches communicate over a peer-to-peer link, sending heartbeat messages to keep the connection alive. vPC domains consist of several key components: the vPC peer device, the vPC peer keep-alive link, the vPC peer link, and all the port channels connecting to the downstream device. Each device can operate in only one vPC domain, and each vPC domain must have a unique domain ID.

 

 

Advantages of vPC

 

One of the biggest advantages of vPC is its ability to allow a single device to connect to two upstream devices at the same time through a single port channel. This design not only improves bandwidth utilization but also eliminates potential redundant links by avoiding port issues that would be prevented by the Spanning Tree protocol in a traditional network. Since vPC provides a loop-free topology, loop problems in the network are effectively solved, ensuring smooth and uninterrupted data transmission. At the same time, vPC enables all uplink bandwidth to be fully utilized, avoiding wasted network bandwidth.

 

Furthermore, virtual port channel (vPC) has the capacity to converge expeditiously. When a link or device fails, the network can be quickly restored to ensure uninterrupted service. Under this structure, the high availability of the entire network is guaranteed, and neither link failure nor equipment failure will have a significant impact on the normal operation of the system.

 

The Main Difference Between MLAG vs. vPC

 

MLAG and vPC are both used to establish port aggregation between two switches, allow Layer 2 multipathing, and increase bandwidth and redundancy. But they do have very different implementation details and use cases.

 

Implementation Complexity

 

One of the biggest differences between the two is the complexity of the implementation. MLAG is a publicly available protocol that allows major vendors switch to customize the implementation to suit their needs, making it relatively simple to set up. In contrast, vPC is a proprietary protocol for the Cisco Nexus family of switches and can only be configured on Cisco devices, so it requires more customization and configuration steps during implementation, which makes it more complex to set up.

 

Compatibility Issues

 

vPC and MLAG also differ in terms of compatibility. vPC requires that the paired switches be the same model as the Cisco Nexus switch and that they run the same version of NX-OS. For example, it is not possible to use a Nexus 7000 series with a Nexus 5000 series switch for vPC pairing. In addition, while vPC supports uninterrupted operation during upgrades (ISSU), the paired switches must be identical in other cases. The MLAG is more compatible and can be deployed in a variety of different vendor switches with no strict limitations on product model or operating system.

 

Hierarchical Multipathing

 

In terms of multipathing support, vPC is more advanced. Not only does it support Layer 2 multipathing, but it also enables multipathing at Layer 3 to load-balance traffic and improve redundancy by means of parallel paths. Layer 3 multipathing can be further optimized through the Multi-Activity Gateway Protocol (MAGP) if required. MLAG, on the other hand, mainly supports Layer 2 multipathing, which provides some redundancy but is not as powerful as vPC for handling multipath traffic.

 

Application Scenarios

 

Application scenarios are a key difference between the two. vPC is primarily used in Cisco Nexus data center switches, so its application scenarios are relatively limited, and it is typically used in large-scale Cisco environments. MLAG, on the other hand, has a much broader scope of application and can be used not only in the traditional three-tier data center architecture but also in the two-tier spine-leaf architecture. MLAG-enabled switches can be flexibly configured at different layers to meet more diverse needs.

 

Conclusion

MLAG and vPC do provide good support for link aggregation and redundancy in different network environments. However, their choice and relevance are subject to network requirements and vendor compatibility. MLAG provides a cross-vendor, flexible methodology that can be deployed on a number of architectures and devices for wide applicability. vPC, on the other hand, is designed specifically for Cisco networks and offers more advanced and optimized multipathing options along with data flow and redundancy options. Understanding the differences between them helps network engineers make the best choice for the situation, thus improving data center performance and reliability.