Enabling BGP Path Selection for Layer 2 VPNs and VPLS

Layer 2 VPNs and VPLS share the same path selection process for determining the optimal path to reach all of the destinations shared within a single routing instance. For Layer 2 VPN and VPLS topologies, the path selection process is straightforward if there is just a single path from each PE router to each CE device. However, the path selection process becomes more complex if the PE routers receive two or more valid paths to reach a specific CE device.

The following network scenarios provide examples of what might cause a PE router to receive more than one valid path to reach a specific CE device:

• Multihoming—One or more CE devices within a routing instance are multihomed to two or more PE routers. Each multihomed CE device has at least two valid paths.

• Route reflectors—There are multiple route reflectors deployed within the same network and they are supporting PE routers within the same routing instance. Due to time delays in large complex networks, the route reflectors can separately receive a different valid path to reach a CE device at different times. When they readvertise these valid paths, a PE router could receive two or more separate but apparently valid paths to the same CE device.

By default, Juniper Networks routers use just the designated forwarder path selection algorithm to select the best path to reach the Layer 2 VPN or VPLS destination (for more information, see VPLS Path Selection Process for PE Routers). However, you can also configure the routers in your network to use both the BGP path selection algorithm and the designated forwarder path selection algorithm as follows:

• On the Provider routers within the service providers network, the standard BGP path selection algorithm is used (for more information, see Understanding BGP Path Selection). Using the standard BGP path selection for Layer 2 VPN and VPLS routes allows a service provider to leverage the existing Layer 3 VPN network infrastructure to also support Layer 2 VPNs and VPLS. The BGP path selection algorithm also helps to ensure that the service provider’s network behaves predictably with regard to Layer 2 VPN and VPLS path selection. This is particularly important in networks employing route reflectors and multihoming.

  When a Provider router receives multiple paths for the same destination prefix (for example, a multihomed CE device), one path is selected based on the BGP path selection algorithm and placed in the bgp.l2vpn.0 routing table and the appropriate instance.l2vpn.0 routing table.

• When a PE router receives all of the available paths to each CE device, it runs the designated forwarder path selection algorithm to select the preferred path to reach each CE device, independently of the results of the earlier BGP path selection algorithm run on the Provider router. The VPLS designated forwarder algorithm uses the D-bit, preference, and PE router identifier to determine which of the valid paths to each CE device to use. The PE router might select a path to reach a CE device which is different from the path selected by the BGP-based Provider routers. In this scenario, the following is the expected behavior for traffic sent to the multihomed CE device:

  • If the path selected by the remote PE router is available, traffic will traverse the network to the multihomed CE device using the remote PE router’s preferred path (again, ignoring the path selected by the BGP-based Provider routers).

  • If the path selected by the remote PE router fails:

    1. The Provider routers switch the traffic destined for the multihomed CE device to the alternate path as soon as failure is detected.

    2. The Provider routers notify the remote PE routers of the path failure.

    3. The remote PE routers update their routing tables accordingly.

  For more information about the VPLS designated forwarder path selection algorithm, see VPLS Path Selection Process for PE Routers. This algorithm is also described in the Internet draft draft-kompella-l2vpn-vpls-multihoming-03.txt, Multi-homing in BGP-based Virtual Private LAN Service.

To enable the BGP path selection algorithm for Layer 2 VPN and VPLS routing instances, complete the following steps: