Motorola WiNG 5, System Reference Manual

Page: 132 / 802

5 - 72 WiNG 5 Access Point System Reference Guide
5.3.5.8 Dynamic Routing (OSPF)

Profile Network Configuration
Open Shortest Path First (OSPF) is a link-state interior gateway protocol (IGP). OSPF routes IP packets within a single
routing domain (autonomous system), like an enterprise LAN. OSPF gathers link state information from neighbor
routers and constructs a network topology. The topology determines the routing table presented to the Internet Layer
which makes routing decisions based solely on the destination IP address found in IP packets.
OSPF detects changes in the topology, like a link failure, and plots a new loop-free routing structure. It computes the
shortest path for each route using a shortest path first algorithm. Link state data is maintained on each router and is
periodically updated on all OSPF member routers.
OSPF uses a route table managed by the link cost (external metrics) defined for each routing interface. The cost could
be the distance of a router (round-trip time), link throughput or link availability.
An OSPF network can be subdivided into routing areas to simplify administration and optimize traffic utilization. Areas
are logical groupings of hosts and networks, including routers having interfaces connected to an included network.
Each area maintains a separate link state database whose information may be summarized towards the rest of the
network by the connecting router. Areas are identified by 32-bit IDs, expressed either in decimal, or octet-based
dot-decimal notation. Areas can defined as:
• stub area - A stub area is an area which does not receive route advertisements external to the autonomous system
(AS) and routing from within the area is based entirely on a default route.
• totally-stub - A totally stubby area does not allow summary routes and external routes. A default route is the only
way to route traffic outside of the area. When there’s only one route out of the area, fewer routing decisions are
needed, lowering system resource utilization.
• non-stub - A non-stub area imports autonomous system external routes and send them to other areas. However.
it still cannot receive external routes from other areas.
• nssa - NSSA is an extension of a stub that allows the injection of limited external routes into a stub area. If
selecting NSSA, no external routes, except a default route, enter the area.
• totally nssa - Totally nssa is an NSSA using 3 and 4 summary routes are not flooded into this type of area. It is
also possible to declare an area both totally stubby and not-so-stubby, which means that the area will receive only
the default route from area 0.0.0.0, but can also contain an autonomous system boundary router (ASBR) that
accepts external routing information and injects it into the local area, and from the local area into area 0.0.0.0.
A router running OSPF sends hello packets to discover neighbors and elect a designated router. The hello packet
includes link state information and list of neighbors. OSPF is savvy with layer 2 topologies. If on a
point-to-point link,
OSPF knows it is sufficient, and the link stays up . If on a broadcast link, the router waits for election before
determining if the link is functional.
To define a dynamic routing configuration:
1. Select the Configuration tab from the Web UI.
2. Select Devices .
3. Select System Profile from the options on left-hand side of the UI.
4. Expand the Network menu and select Dynamic Routing.