Enabling Bgp Path Selection For Layer 2 Vpns And Vpls - Juniper EX9200 Features Manual

VPLS Feature Guide for EX9200 Switches

Enabling BGP Path Selection for Layer 2 VPNs and VPLS

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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.
NOTE: In the VPLS documentation, the word router in terms such as PE router
is used to refer to any device that provides routing functions.
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 each Layer 2 VPN or VPLS routing instance
destination (for more information, see
page 57). 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.
"VPLS Path Selection Process for PE Routers" on
Copyright © 2016, Juniper Networks, Inc.