Bud Node Support with the Looping Back Interface (LBI)

This topic introduces the Looping Back Interface (LBI) and its role in supporting bud node scenarios.

The Looping Back Interface (LBI) enhances flexibility in label handling and routing decisions within multicast virtual private network (MVPN) environments. The LBI is automatically created during system boot-up and doesn’t require manual configuration. The LBI can be associated to any label and is not tied to a Flexible PIC Concentrator (FPC). This provides an advantage over mechanisms that terminate core traffic on a VRF like the Label-Switched Interface (LSI) or the Virtual Tunnel (VT) interface. This feature optimizes multicast forwarding by reducing core network traffic and improving efficiency. The LBI ensures compatibility with Multicast VPN (MVPN) and is optimized for use with Label Distribution Protocol (LDP) tunnels.

The LBI integrates seamlessly with existing Label Switch Path (LSP) setups, utilizing dynamic label allocation to direct traffic efficiently. Any dynamic incoming label can be associated to the nexthop which includes the LBI and a swap-to label in the LSI range to terminate multicast traffic on the appropriate VRF. On detection of the LBI interface, the newly labelled route will be installed in the mpls.0 table with an updated nexthop by the tunnel module.

For egress-only scenarios, the nexthop used will still point to the LSI interface with the corresponding routing-instance. The dynamic label assigned will contain the same next hop as the LSI. The label in LSI label range will still be available for use for unicast but will not be used to multicast.

In a bud node scenario where PE routers are both transit and egress for MVPN flows, a copy of the traffic is sent to the local routing-instance and another copy label swapped to other egress PE routers. This way, the LBI allows traffic to be looped back for a second look up to ensure that multicast traffic reaches the correct local routing instance.

When the P2MP mLDP tunnel detects a bud node, the nexthop is updated to include the LBI and the additional branch to the next egress PE. Next, a label swap operation ensures that the swap-to label is the vrf-table-label for the corresponding routing-instance.

This mechanism effectively reduces the number of duplicate packet copies within the core network.

Multicast forwarding routes will be programmed with the LSI as the upstream interface, even for scenarios where the VT interface would be previously used.