
Configuring IP Services
4-14
Configuring a Boundary Router
A router connected to a non-OSPF network — called a
boundary
router
— injects
AS external advertisements (ASEs) into the AS. In Figure 4-2, for example,
routers R1 and R2 are boundary routers that use BGP and EGP to connect the
backbone to external ASs. R7 in Area A is also a boundary router, connecting the
area to an external RIP network. R14 in Area C connects the area to an external
AS via BGP.
A boundary router learns and propagates external routes. OSPF considers the
following routes to be external routes:
• A route to a destination outside the AS
• A static route
• A default route
• A route derived by RIP
• A directly connected network not running OSPF
A boundary router can be attached to the backbone or to any other area. In
addition to OSPF, a boundary router may also run some exterior gateway protocol
(on the interface that connects to another AS), such as BGP.
Boundary routers propagate external routes in the form of AS external (ASE)
advertisements. ASEs are flooded throughout every area in the AS (except stubs,
which by definition receive no information about external routes).
The features and functions of OSPF boundary routers are described in the
following sections:
• Specifying the Metric Type for an ASE Advertisement - page 4-15
• Using the Route Weight as the Type 2 Metric - page 4-15
• Using the External Route Tag in an ASE - page 4-16
For instructions on using Site Manager to configure a boundary router, see the AS
Boundary Router parameter on page 4-37.
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