Link Aggregation (LAG) is a mechanism used to aggregate physical interfaces or ports to create a logical entity called link bundle.

Traditionally LAG is a trunking technology that groups together multiple full-duplex IEEE 802.3 Ethernet interfaces to provide fault-tolerant high-speed links between switches, routers, and servers. LAG forms a single higher bandwidth routing or bridging endpoint and was designed primarily for host-to-switch connectivity. Following are the benefits:

• Logical aggregation of bandwidth

• Load balancing

• Fault tolerance

In NCS 4000 Series Routers, primary application of LAG is to provide connectivity to Access devices like NCS4200 Series and on the core side provide connectivity to Multi-service Edge (NCS 6000 Series) and Core Routers (like NCS 6000 Series).

A link bundle is a group of one or more ports that are aggregated or bundled together and act as a single link. This single link can be treated as a main interface or as a VLAN subinterface.

The advantages of link bundles are these:

• Multiple links can span several line cards to form a single interface. Thus, the failure of a single link does not cause a loss of connectivity.

• Bundled interfaces increase bandwidth availability, because traffic is forwarded over all available members of the bundle. Therefore, traffic can flow on the available links if one of the links within a bundle fails. Bandwidth can be added without interrupting packet flow.

NCS 4000 Series XR software supports following methods of forming bundles of Ethernet interfaces:

• IEEE 802.3ad—Standard technology that employs a Link Aggregation Control Protocol (LACP) to ensure that all the member links in a bundle are compatible. Links that are incompatible or have failed are automatically removed from a bundle.

• Static-LAG—Cisco proprietary technology that allows the user to configure links to join a bundle, but has no mechanisms to check whether the links in a bundle are compatible.

This list describes the properties and limitations of link bundles:

• 10 Gigabit, 40 Gigabit, and 100 Gigabit Ethernet interfaces can be bundled, with or without the use of LACP (Link Aggregation Control Protocol).

• Bundle membership can span across several line cards that are installed in the same chassis for NCS4000.

• The Cisco NCS 4000 Series Router supports a maximum of 128 Ethernet link bundles, 1000 Ethernet link bundles sub-interfaces. Each link bundle can have a maximum of 16 physical links.

• All the members in a link bundle shall be of same speed.

• Physical layer and link layer configuration are performed on individual member at physical interface layer.

• Configuration of network layer protocols and higher layer applications is performed on the bundle itself.

• A bundle can be administratively enabled or disabled.

• Each individual link within a bundle can be administratively enabled or disabled.

• Bundle member links are not supported on OTN terminated interfaces.

• Load balancing (the distribution of data between member links) is done with source and destination mac address.

  Note	Load balancing is done based on each stream, so each interface in the bundle may not carry equal traffic.

• QoS is supported and can be applied on the bundle interface and sub interfaces.

• LAG CFM is supported and can be applied on the bundle interface and subinterfaces.

• LAG is only supported for both L2 and L3 interfaces.

• Link layer protocols, such as LLDP and Link OAM , work independently on each link within a bundle.

• Upper layer protocols, such as routing updates and hellos, are sent over any member link of an interface bundle.

• Bundled interfaces are point to point.

• All links within a single bundle must be configured either to run 802.3ad (LACP) or Static-LAG (non-LACP). Mixed links within a single bundle are not supported.

• Only default LACP timer (30sec) is supported.

• To provision EVPL service with Bundle AC, user has to provision the bundle main interface along with the L2 bundle sub-interfaces. QOS or any other feature over the bundle main interface needs to be configured once the EVPL service is provisioned.

• When link-OAM is configured on the bundle interface, its recommended to configure one of the following command options:

  • RP/0/RP0:hostname(config)# ethernet oam profile <profile name> action wiring-conflict disable

  • RP/0/RP0:hostname(config)# ethernet oam profile <profile name> action wiring-conflict efd

  • RP/0/RP0:hostname(config)# ethernet oam profile <profile name> action wiring-conflict log

• While performing RPVM Switch Over or RP OIR or ISSU, the packet transmission stops for a duration of 3 to 20 seconds and causes CFM sessions with CCM interval 1 second and 10 seconds to flap (session goes down and recovers back).

The IEEE 802.3ad standard typically defines a method of forming Ethernet link bundles.

For each link configured as bundle member, this information is exchanged between the systems that host each end of the link bundle:

• A globally unique local system identifier

• An identifier (operational key) for the bundle of which the link is a member

• An identifier (port ID) for the link

• The current aggregation status of the link

This information is used to form the link aggregation group identifier (LAG ID). Links that share a common LAG ID can be aggregated. Individual links have unique LAG IDs.

The system identifier distinguishes one router from another, and its uniqueness is guaranteed through the use of a MAC address from the system. The bundle and link identifiers have significance only to the router assigning them, which must guarantee that no two links have the same identifier, and that no two bundles have the same identifier.

The information from the peer system is combined with the information from the local system to determine the compatibility of the links configured to be members of a bundle.

Bundle MAC addresses in the routers come from a set of reserved MAC addresses in the backplane. This MAC address stays with the bundle as long as the bundle interface exists. The bundle uses this MAC address until the user configures a different MAC address. The bundle MAC address is used by all member links when passing bundle traffic. Any unicast or multicast addresses set on the bundle are also set on all the member links.

Note	We recommend that you avoid modifying the MAC address, because changes in the MAC address can affect packet forwarding.

Before configuring LAG, be sure that these tasks and conditions are met:

• You must be in a user group associated with a task group that includes the proper task IDs. The command reference guides include the task IDs required for each command.

  If you suspect user group assignment is preventing you from using a command, contact your AAA administrator for assistance.

• You know the interface IP address (Layer 3 only).

• You know which links should be included in the bundle you are configuring.

• If you are configuring an Ethernet link bundle, you should have NCS4K-4H-OPW-QC2 line card installed in the router.

802.1Q VLAN subinterfaces can be configured on 802.3ad Ethernet link bundles. The maximum number of VLAN subinterfaces allowed per router is 1024 minus the number of main interface(s) configured. Example if one main bundle is configured then maximum 1023 VLAN subinterface bundles can be configured on the router.

Note	The memory requirement for bundle VLANs is slightly higher than standard physical interfaces.

To create a VLAN subinterface on a bundle, include the VLAN subinterface instance with the interface Bundle-Ether command:

interface Bundle-Ether instance.subinterface

After you create a VLAN on an Ethernet link bundle, all physical VLAN subinterface configuration is supported on that link bundle.

Aggregating interfaces on different line cards provides redundancy, allowing traffic to be quickly redirected to other member links when an interface or modular services card failure occurs.

LACP transmits frames containing the local port state and the local view of the partner system’s state. These frames are analyzed to ensure both systems are in agreement.