Juniper EX9200 Features Manual page 80

VPLS Feature Guide for EX9200 Switches
62
Figure 5 on page 61 illustrates how a CE device could be multihomed to two PE routers.
Device CE1 is multihomed to Routers PE1 and PE2. Device CE2 has two potential paths
to reach Device CE1, but only one path is active at any one time. If Router PE1 were the
designated VPLS edge (VE) device (also called a designated forwarder), BGP would
signal a pseudowire from Router PE3 to Router PE1. If a failure occurred over this path,
Router PE2 would be made the designated VE device, and BGP would re-signal the
pseudowire from Router PE3 to Router PE2.
Multihomed PE routers advertise network layer reachability information (NLRI) for the
multihomed site to the other PE routers in the VPLS network. The NLRI includes the site
ID for the multihomed PE routers. For all of the PE routers multihomed to the same CE
device, you need to configure the same site ID. The remote VPLS PE routers use the site
ID to determine where to forward traffic addressed to the customer site. To avoid route
collisions, the site ID shared by the multihomed PE routers must be different than the
site IDs configured on the remote PE routers in the VPLS network.
Although you configure the same site ID for each of the PE routers multihomed to the
same CE device, you can configure unique values for other parameters, such as the route
distinguisher. These values help to determine which multihomed PE router is selected
as the designated VE device to be used to reach the customer site.
BEST PRACTICE:
distinguishers for each multihomed PE router. Configuring unique route
distinguishers helps with faster convergence when the connection to a primary
multihomed PE router goes down. If you configure unique route distinguishers,
the other PE routers in the VPLS network must maintain additional state for
the multihomed PE routers.
Remote PE routers in the VPLS network need to determine which of the multihomed PE
routers should forward traffic to reach the CE device. To make this determination, remote
PE routers use the VPLS path-selection process to select one of the multihomed PE
routers based on its NLRI advertisement. Because remote PE routers pick only one of the
NLRI advertisements, it establishes a pseudowire to only one of the multihomed PE
routers, the PE router that originated the winning advertisement. This prevents multiple
paths from being created between sites in the network, preventing the formation of
Layer 2 loops. If the selected PE router fails, all PE routers in the network automatically
switch to the backup PE router and establish new pseudowires to it.
BEST PRACTICE:
devices and the multihomed PE routers, we recommend that you employ the
Spanning Tree Protocol (STP) on your CE devices. Layer 2 loops can form
due to incorrect configuration. Temporary Layer 2 loops can also form during
convergence after a change in the network topology.
The PE routers run the BGP path selection procedure on locally originated and received
Layer 2 route advertisements to establish that the routes are suitable for advertisement
to other peers, such as BGP route reflectors. If a PE router in a VPLS network is also a
We recommend that you configure unique route
To prevent the formation of Layer 2 loops between the CE
Copyright © 2016, Juniper Networks, Inc.