There are three ways NBMA networks are typically deployed.
- Hub and Spoke: Pros= Ease of deployment, Easy to troubleshoot, Site Isolation; Cons = Redundancy Concerns if a non-redundant hub router fails
- Full Mesh: Pros = Redundancy, Direct Connectivity between all nodes on network eliminates the need to traverse a central hub; Cons = Expensive to implement and difficult to manage
- Partial Mesh: Pros = Combines the ease of deployment of a hub and spoke topology with the redundancy of a full mesh (or is deployed when specific circumstances require direct connectivity between specific sites), scales well.
Challenges in NBMA networks:
Split horizon is a problem for NBMA networks, since traffic cannot be transmitted out an interface in which it was previously received.
OSPF, which operates in a non-broadcast mode by default, requires special configuration when implemented on an NBMA network because each router does not automatically discover all others. DR election cannot occur, so it must be ensured that only the hub router can be the DR, as it is the only node with connections to all routers. Furthermore, neighbors must be statically configured.
Broadcast replication can be a problem when a single interface is provisioned for multiple P2P connections, as the broadcast packet must be replicated for every virtual circuit configured, thereby increasing the amount of bandwidth required.
Solutions in NBMA networks:
Point-to-Point sub-interfaces can resolve split-horizon and neighbor discovery challenges because each logical interface is unique between two routers and uses its own network addressing. This is recommended.
The other option is a point-to-multipoint sub-interface, which requires special configuration to mitigate issues with split-horizon and neighbor discovery. The WAN uses a single network for all routers, and requires manual configuration to map remote routers to a layer 2 address. The mapping can be very complex in large networks.
Stuck-in-Active: Journal of an IT-Network Administrator 