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Support of Inner VLAN List and Inner VLAN Range for Qualified BUM Pruning on a Dual-Tagged Interface for a VPLS Routing Instance Overview

Junos OS provides the qualified-bum-pruning-mode statement, which supports constraining of broadcast, multicast, and unknown (BUM) traffic in a VPLS instance to a specific subscriber VLAN stack. This cuts down unnecessary consumption of bandwidth and thereby, improves network performance.

All subscriber VLANs that need to be backhauled to their respective retail-ISPs are also created in the same VPLS instance as the retail-ISP. Each retail-ISP is allocated a VPLS instance. Any traffic on subscriber VLANs is hauled over the VPLS tunnel to the retail-ISP. Similarly, any traffic from a retail-ISP on a VPLS tunnel is forwarded to all the subscriber VLANs in that VPLS instance. For multicast traffic (which includes broadcast, unknown DMAC, and layer 2 and layer 3 multicast), the standard Layer-2 VPLS instance-forwarding creates a BUM packet, which comes from a retail-ISP over a VPLS tunnel, to be flooded to all subscriber VLANs in the VPLS instance. This causes the individual subscribers who might have subscribed to specific multicast traffic channels to receive all the traffic instead of the multicast traffic that the subscriber has signed up for.

To overcome this, BUM traffic needs to be forwarded only to subscribers who are the intended recipients, by mapping the VLAN-tags present in the BUM packet to these subscribers. A subscriber is assigned a stacked VLAN-tag and BUM packets are sent only to a subscriber whose stacked VLAN-tags match the VLAN-tags present in the BUM packet. This ensures that subscribers receive only the BUM traffic that is specifically intended for them, which prevents the normal flooding of BUM traffic. This is called BUM-pruning. Hitherto, Junos OS supported only single VLAN tagged and dual VLAN tagged subscriber interfaces. This implies that for each subscriber, a different interface must be configured. Such a BUM-pruning solution does not scale well. To address this issue, Junos OS now supports configuring VLAN ranges on the subscriber interfaces. This enables better management of subscriber services.

BUM-pruning on Junos OS allows subscriber interfaces to be configured with inner-vlan-lists. Each inner VLAN list includes all the subscriber VLANs that must be grouped on a particular subscriber interface mapped to a VPLS instance. You can configure BUM-pruning on each VPLS instance allocated to a retail-ISP by using the set routing-instances routing-instance-name qualified-bum-pruning-mode command.

For normal VPLS flooding, BUM traffic is received over the VPLS tunnel and flooded to all the subscriber interfaces mapped to the vpls-instance. These interfaces might be or might not be mapped to the same subscriber VLAN as the packet received over the VPLS tunnel. By enabling the qualified BUM pruning mode in a VPLS instance, VPLS flooding is restricted to a combination of service-provider VLAN and subscriber VLAN. The qualified-bum-pruning statement implements BUM-pruning on the Packet Forwarding Engine in the egress list of each subscriber interface mapped to the VPLS instance. On the Packet Forwarding Engine, BUM-pruning is implemented as a vlan-check nexthop installed in the egress list of the subscriber interface. The vlan-check nexthop checks whether the BUM packet exiting the ifl has the same combination of service-provider VLAN and subscriber VLAN as that configured on the interface. If the VLAN check matches, the packet is forwarded or else it is discarded. This ensures that only the subscriber that is the intended recipient of the BUM traffic receives the packet. This feature is supported for both single-tagged and dual-tagged subscriber interfaces as well as for subscriber interfaces configured with vlan-map operations. If vlan-map operations are configured on the interfaces, then the normalized VLAN on the interface is considered for the vlan-check nexthop. This feature is supported for both default and logical systems.

Currently, dual-tagged subscriber interfaces can support a single pair of service-provider VLAN and subscriber VLAN. With the support for BUM-pruning of VPLS traffic on dual-tagged interfaces, you can configure a single service provider VLAN(S) on the subscriber interface and map it to multiple customer VLANs using a single inner VLAN list or inner VLAN range. The inner VLAN list on a subscriber interface can have multiple elements. Each element of the inner VLAN list can be as follows:

  • A single VLAN tag

  • A range of VLANs

The BUM traffic flow through dual-tagged interfaces is supported on both aggregated and non-aggregated subscriber interfaces. When BUM traffic exits a subscriber interface configured with inner VLAN list or inner VLAN range, the service provider VLAN (S) and the subscriber VLAN (C) in the packet are checked against all combinations of (S,C) of the S and C VLANs possible on the interface. If the packet matches any of the combination, it is forwarded on the subscriber interface. If the packet does not match any combination, it is discarded. If the subscriber has vlan-map configured, then the S and C VLANs to be checked are modified based on the VLAN normalization on the interface.