HP (Hewlett-Packard) HP-UX 11i v2 Manuale Utente

Configuring gated

Multi-access networks (networks that can be accessed through two or
more neighbor routers) must have one of the routers identified as a
designated router.

Designated routers initiate OSPF protocol functions on behalf of the
network. In Figure 3-3, you can access network 1 through the neighbor
routers A, B, or C; one of these routers is elected to become the
designated router for network 1.

The set of routers that exchange OSPF protocol packets between areas in
an autonomous system is called the backbone. In Figure 3-3, routers C,
D, E, F, G, and I form an AS backbone that allows protocol packets to
travel between the three areas.

OSPF routers exchange various types of link state advertisements to
build their topological databases. Most link state advertisements are
flooded (sent to every router) throughout the attached area. An exception
is the link state advertisement sent out by AS boundary routers that
describe routes to destinations outside the AS; these advertisements are
flooded throughout the AS. Table 3-1 shows the various types of link
state advertisements used by the OSPF protocol.

AS boundary routers exchange routing information with routers in
other autonomous systems. An AS boundary router can be an area
border router or an internal router. It can also be a backbone router, but

Router link

Router’s links to
area

Internal and
area border
routers

Area

Network
link

List of routers
attached to network

Designated
router

Area

Summary
link

Routes to
destinations outside
area but within AS

Area border
router

Area

AS
external
link

Routes to
destinations outside
AS

AS boundary
router

AS