
Configuring IP Services
4-8
• Each
border router
connects an area to the backbone. In Figure 4-2, R3, R8,
and R11 are border routers. Using the backbone, border routers ensure that
ASEs and summaries of routing information for all areas are distributed
throughout the AS.
Border routers can treat certain areas as
stubs
. Border routers forward no
information about external routes to stubs and can also be configured to forward
no information about internal OSPF routes. Figure 4-2 includes one stub area.
For an example of an OSPF AS that has been divided into areas, see Appendix C.
The network administrator creates areas by configuring OSPF routers as described
in the following sections:
• Configuring an Internal Area Router - page 4-8
• Configuring a Border Router - page 4-10
• Configuring a Boundary Router - page 4-14
Configuring an Internal Area Router
An
internal router
has OSPF interfaces to networks in one area only. Each internal
router
• Generates advertisements about the state of its local interfaces (updating its
routing table) and floods these internal advertisements to all adjacent routers.
• Receives link-state advertisements from other routers in the area and
summaries about routes in other areas. The router floods all received internal
advertisements to adjacent routers and uses this information to update its
routing table.
• Receives ASE advertisements, updates its routing table, and floods the ASEs
to adjacent routers in the area.
For instructions on using Site Manager to assign an ID to an area, see “Configuring
OSPF on the Router” on page 4-24.
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