Unicast Routing Protocols

Unicast routing protocols treat each IP network as an interface. The interface corresponding to the primary subnet is the active interface, and the interfaces corresponding to the secondary subnet are passive subnets.

For example, in the case of OSPF, the system treats each network as an interface, and hello messages are not sent out or received over the non-primary interface. In this way, the router link state advertisement (LSA) includes information to advertise that the primary network is a transit network and the secondary networks are stub networks, thereby preventing any traffic from being routed from a source in the secondary network.

Interface-based routing protocols (for example, OSPF) can be configured on per VLAN basis. A routing protocol cannot be configured on an individual primary or secondary interface. Configuring a protocol parameter on a VLAN automatically configures the parameter on all its associated primary and secondary interfaces. The same logic applies to configuring IP forwarding, for example, on a VLAN.

Routing protocols in the multinetted environment advertise the secondary subnets to their peers in their protocol exchange process. For example, for OSPF the secondary subnets are advertised as stub networks in router LSAs. RIP also advertises secondary subnets to its peers residing on the primary subnet.

This section describes the behavior of OSPF in an IPv4 multinetting environment:

This section describes the behavior of the Routing Information Protocol (RIP) in an IP multinetting environment:

There are no behavioral changes in the BGP in an IP multinetting environment.

This section describes a set of recommendations for using BGP with IP multinetting:

This section describes the behavior of IS-IS in an IPv4 multinetting environment: