If you have two or more redundant Lumen® MPLS circuits, you can configure your router for failover. Configuring your router for failover ensures that if one of your Lumen circuits goes down, traffic can be routed to the other circuit(s)—keeping your system functioning properly. Doing this requires a one-time, configuration change, which you can do yourself.
If you have redundant circuits with Lumen and another provider, then you'll need to use a different method—like AS Path Prepending—to configure your primary/backup systems.
You can use BGP communities to configure your router for failover. BGP communities are used to signal a desired behavior. Just like rerouting traffic on highways, BGP can be used to reroute traffic in a network.
BGP has many attributes that can be used for path selection (e.g., weight, local preference, AS_PATH), and BGP evaluates these attributes in a specific order. For instance, weight is evaluated before local preference, which is evaluated before AS_PATH, and so on. When BGP evaluates two paths with the same prefix, it chooses the path with the largest local preference. (The weight attribute could impact this, as weight is evaluated before local preference. Weight is an attribute that is only used on Cisco routers. In the Lumen MPLS network, the weight attribute isn’t used.)
By sending a community via the external BGP session (eBGP), the CE router is signaling to the PE router to modify the local preference (LP). LP isn’t advertised between two eBGP peers. LP is a non-transitive attribute, so won’t be advertised outside of an autonomous system (AS).
In the Lumen network, route maps are built on every provider edge (PE) to modify LP when certain BGP communities are sent by the customer edge (CE). The local preference value is used by BGP to determine the best path. (The largest local preference wins the tiebreaker.)
BGP only advertises its best path to its peers, so when a BGP router has two paths for a prefix, only the path with the largest LP is advertised inside the MPLS network. In the Lumen network the following communities can be used:
- 209:90 – changes LP to 90
- 209:80 – changes LP to 80
- 209:70 – changes LP to 70
When no communities are sent with a prefix, the LP will default to 100. If the DallasCE is the only CE advertising a route, then traffic can only route through the Dallas circuit.