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Understanding Multichassis Link Aggregation Groups


Starting in Junos OS Release 17.4R1, MC-LAG is not supported on EX switches except for EX4600, EX4650-48Y, and EX9200 switches. Use the Virtual Chassis feature instead to provide equivalent functionality.

Layer 2 networks are increasing in scale mainly because of technologies such as virtualization. Protocol and control mechanisms that limit the disastrous effects of a topology loop in the network are necessary. The Spanning Tree Protocol (STP) is the primary solution to this problem because it provides a loop-free Layer 2 environment. STP has gone through a number of enhancements and extensions, and even though it scales to very large network environments, it still only provides one active path from one device to another, regardless of how many actual connections might exist in the network. Although STP is a robust and scalable solution to redundancy in a Layer 2 network, the single logical link creates two problems: At least half of the available system bandwidth is off-limits to data traffic, and network topology changes occur. The Rapid Spanning Tree Protocol (RSTP) reduces the overhead of the rediscovery process and allows a Layer 2 network to reconverge faster, but the delay is still high.

Link aggregation (IEEE 802.3ad) solves some of these problems by enabling users to use more than one link connection between switches. All physical connections are considered one logical connection. The problem with standard link aggregation is that the connections are point to point.

Multichassis link aggregation groups (MC-LAGs) enable a client device to form a logical LAG interface between two MC-LAG peers. An MC-LAG provides redundancy and load balancing between the two MC-LAG peers, multihoming support, and a loop-free Layer 2 network without running STP.

On one end of an MC-LAG, there is an MC-LAG client device, such as a server, that has one or more physical links in a link aggregation group (LAG). This client device uses the link as a LAG. On the other side of the MC-LAG, there can be a maximum of two MC-LAG peers. Each of the MC-LAG peers has one or more physical links connected to a single client device.

The MC-LAG peers use the Inter-Chassis Control Protocol (ICCP) to exchange control information and coordinate with each other to ensure that data traffic is forwarded properly.

The Link Aggregation Control Protocol (LACP) is a subcomponent of the IEEE 802.3ad standard. LACP is used to discover multiple links from a client device connected to an MC-LAG peer. LACP must be configured on both MC-LAG peers for an MC-LAG to work correctly.


You must specify a service identifier (service-id) at the global level; otherwise, multichassis link aggregation will not work.

Figure 1: Basic MC-LAG Topology
Basic MC-LAG Topology

The following sections provide information regarding the functional behavior of multichassis link aggregation, configuration guidelines, and best practices.

Benefits of MC-LAGs

  • Reduces operational expenses by providing active-active links within a Link Aggregation Group (LAG).

  • Provides faster layer 2 convergence upon link and device failures.

  • Adds node-level redundancy to the normal link-level redundancy that a LAG provides.

  • Improves network resiliency, which reduces network down time as well as expenses.