VPLS Path Selection Process for PE Routers
The VPLS path selection process is used to select the best path between a remote PE router and a local PE router in a VPLS network. This path selection process is applied to routes received from both single-homed and multi-homed PE routers.
In the VPLS documentation, the word router in terms such as PE router is used to refer to any device that provides routing functions.
When the VPLS path selection process is complete, a PE router is made the designated VPLS edge (VE) device. The designated VE device effectively acts as the endpoint for the VPLS pseudowire that is signaled from the remote PE router. Once a PE router is made the designated VE device, a pseudowire can be signaled between the remote PE router and the local PE router and then VPLS packets can begin to flow between the PE routers.
Routes from multihomed PE routers connected to the same customer site share the same site ID, but can have different route distinguishers and block offsets. You can alter the configurations of the route distinguishers and block offsets to make a router more likely or less likely to be selected as the designated VE device.
On each PE router in the VPLS network, the best path to the CE device is determined by completing the following VPLS path selection process on each route advertisement received:
If the advertisement has the down bit set to 0, the advertisement is discarded.
Select the path with a higher preference. The preference attribute is obtained from the site-preference configured using the
site-preferencestatement at the[edit routing-instances routing-instance-name protocols vpls site site-name]hierarchy level. When the site is down, the preference attribute is obtained from the local preference.If the preference values are the same, select the path with the lower router ID.
If the router IDs are the same, the routes are from the same PE router and the advertisement is considered to be an update. The router ID corresponds to the value of the originator ID for the BGP attribute (if present). Otherwise, the IP address for the remote BGP peer is used.
If the block offset values are the same, the advertisement is considered to be an update.
Once the VPLS path selection process has been completed and the designated VE device has been selected, a pseudowire is signaled between the remote PE router and the local PE router.
The VPLS path selection process works the same whether or not the route has been received from another PE router, a route reflector, or an autonomous system border router (ASBR).
When the remote PE router establishes or refreshes a pseudowire to the local PE router, it verifies that the prefix is in the range required for the site ID based on the block offset and label range advertised by the designated VE device. If the prefix is out of range, the pseudowire status is set to out of range.
The following cases outline the potential decisions that could be made when a PE router completes the VPLS path selection process for a Layer 2 advertisement in the VPLS network:
The PE router originated one of the advertisements and selected its own advertisement as the best path.
This PE router has been selected as the designated VE device. Selection as the designated VE device triggers the creation of pseudowires to and from the other PE routers in the VPLS network. If the remote customer site is multihomed, the designated VE device triggers the creation of pseudowires to and from only the designated VE device for the remote site.
The PE router originated one of the advertisements but did not select its own advertisement as the best path.
This PE router is a redundant PE router for a multihomed site, but it was not selected as the designated VE device. However, if this PE router has just transitioned from being the designated VE device (meaning it was receiving traffic from the remote PE routers addressed to the mulithomed customer site), the PE router tears down all the pseudowires that it had to and from the other PE routers in the VPLS network.
The PE router received the route advertisements and selected a best path. It did not originate any of these advertisements because it was not connected to the customer site.
If the best path to the customer site (the designated VE device) has not changed, nothing happens. If the best path has changed, this PE router brings up pseudowires to and from the newly designated VE device and tears down the pseudowires to and from the previously designated VE device.
If this PE router does not select a best path after running the VPLS path selection process, then the local PE router does not consider the remote site to exist.
When a VE device receives an advertisement for a Layer 2 NLRI that matches its own site ID but the site is not multihomed, the pseudowire between the VE device and the transmitting PE router transitions to a site collision state and is not considered to be up.