Unicast IP Routing
Unicast IP Routing |
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To further reduce the amount of routing information flooded into areas, OSPF allows the use of stub areas. A stub area can contain a single entry and exit point (a single ABR), or multiple ABRs when any of the ABRs can be used to reach external route destinations. For stub areas with multiple ABRs, the external routes are advertised by an ASBR that is outside of the area. AS external routes are not flooded into and throughout a stub area. Routing to all AS external networks in a stub area is done through a default route (destination 0.0.0.0 with the network mask of 0.0.0.0). Thus, a single entry in the routing tables of the routers in a stub area is used to route to all AS external locations.
To create the default route, the ABR of a stub area advertises a default route into the stub area. The default route is flooded to all the routers within the stub area, but not outside the stub area. The default route is used by the routers in a stub area for any destination IP address that is not reachable within the AS.
For example, Area 0.0.0.3 in Figure 3.22 is configured as a stub area because all external traffic must travel through its single ABR, router R3. R3 advertises a default route for distribution inside Area 0.0.0.3 instead of flooding the AS external networks into the area.
Figure 3.22 OSPF Stub Areas
All routers in a stub area must be configured so that they do not import or flood AS external routes within the stub area. Therefore, all area configurations for all router interfaces within a stub area must be configured for a stub area. Whether or not a router interface is in a stub area is indicated in a special option bit called the E-bit in the OSPF Hello packet. When the E-bit is set to 1, the router is allowed to accept and flood AS external routes. When the E-bit is set to 0, the router is not allowed to accept and flood AS external routes. Routers receiving Hello packets on interfaces verify that the E-bit of the received Hello packet matches their configuration before establishing an adjacency.
Stub areas have the following constraints:
Stub areas as defined in the OSPF RFC collapse all external routes into a single default route. Therefore, within a stub area, a router's routing table contains intra-area routes, inter-area routes, and a default route. The Windows 2000 Router also supports the collapsing of all non-intra-area routes into a single default route. This is known as a totally stubby area. A router's routing table within a totally stubby area contains intra-area routes and a default route. The default route summarizes all inter-area routes and all external routes.
To configure a stub area on a Windows 2000 Router, when configuring general properties for an area, select the Stub area check box and select the Import summary advertisements check box. To configure a totally stubby area on a Windows 2000 Router, select the Stub area check box but do not select the Import summary advertisements check box.
