Understanding Support for Targeted Distribution of Logical Interface Sets of Static VLANs over Aggregated Ethernet Logical Interfaces

A logical interface set represents logical bundling of subscribers. Targeted distribution enables the selection of links for subscribers like VLAN demultiplexing (demux), PPPoE, or static VLANs that are managed using logical interfaces stacked over a logical interface set. When aggregated Ethernet is configured as the underlying interface for a logical interface set, the traffic from the logical interface set either flows according to a hash-based distribution or, if link-protection is configured on the aggregated Ethernet bundle, then it flows through one aggregated Ethernet link. Subscribers are managed by the underlying logical interfaces or interface sets and the traffic passes through the underlying aggregated Ethernet by choosing one of the member links from the bundle. Link selection is done per subscriber basis.

Manual targeted distribution for VLAN demux, PPPoE, and static VLANs over aggregated Ethernet logical interfaces requires explicit configuration of primary and backup member links for the VLANs. To overcome this overhead, Junos OS allows automatic targeted distribution for aggregated logical interfaces to automatically select the primary and the backup links and also to ensure that the load is balanced across all the member links. Figure 1 shows configuration of static VLANs managed by logical interfaces ae0.0 and ae0.1 configured over logical interface set IFLset over aggregated Ethernet interfaces.

Figure 1: Targeted Distribution of Logical Interface sSet over Aggregated Ethernet Interfaces

By default, Junos OS allows automatic targeted distribution of static VLANs. For manual targeting, you configure targeting type as manual.

For a logical interface set whose underlying interface is an aggregated Ethernet interface, VLAN demux or PPPoE, the targeted distribution is configured and the DCD assigns one of the links of the aggregated Ethernet bundle as the primary link and another as the backup link for each of the logical interface set in order to evenly distribute the subscribers. The primary member link selection is based on the minimum primary load and the backup member link selection is based on the minimum overall load on the member links.

By default, the weight of the VLAN is 1 and the weight of the logical interface set is the accumulation of the underlying logical interfaces along with the default value of the logical interface set that is 1. Junos OS allows you to assign weight for all the targeted subscribers like VLAN demux, PPPoe, and static VLANs. The value of the weight assigned is based on the business, or class of service, or bandwidth requirement. The value of the weight ranges from 1 through 1000. The member links are then assigned based on the value of the weight. Targeted distribution is configured at the logical interface level or at logical interface set level or both. When logical interface set participates in targeting there is no special link selected for underlying logical interfaces. Link selected for logical interface set is propagated to underlying VLANs.

Consider upper as the logical interface set and lower as the underlying logical interfaces. When targeted distribution is configured, then the following applies:

• If the value of lower is set ,then value of upper has to be set. Upper can be set irrespective of whether lower is set or not.

• If the VLANs have configured weights and the logical interface set has no configured weight ,then the weight of the logical interface set is the accumulation of the VLAN weights.

• If the logical interface sets have configured weights, then the weights are used to distribute the logical interface sets irrespective of the weight of any member VLANs.

Rebalancing happens when the subscribers get added or removed from the bundle. The bundle is considered to be balanced as long as the difference of load on any two links in that bundle is not greater than the value of the highest weighted logical interface set or logical interface.

In CoS, there are two modes of operation with respect to aggregated Ethernet - scale and replicate. Scale mode allows distribution of load across the links while replicate mode load on all the links. The default scale mode is useful when all the links are active. When there are only two links active at a given point then replicate mode is more useful. Hence, Junos OS allows replicate mode for each member link on the bundle for static VLANs irrespective of whether it is auto or manual targeting.