3Com 4007, Implementation Manual, Page: 454 / 684

454 C
HAPTER
19: O
PEN
S
HORTEST
P
ATH
F
IRST
(OSPF) R
OUTING
Router Types OSPF routers serve several different, often overlapping, functions:
■
Internal routers — Internal routers connect only to networks that
belong to the same area. An internal router runs one copy of the OSPF
algorithm and maintains routing data only for its area.
In Figure 54, backbone area 0 and routers 1, 2, 3, and 4 are internal
routers. In area 1, routers 5 and 6 are internal routers.
■
Backbone routers
— Backbone routers have an interface to the
backbone area. Area border routers are always backbone routers
because you must configure them as being within the backbone area
or connected to it by a virtual link.
In Figure 54, routers 1, 2, 3, and 4, and area border routers 1, 2, 3,
and 4 are all backbone routers.
■
Area border routers (ABRs)
— Area border routers connect directly
to networks in two or more areas. An area border router runs a
separate copy of the OSPF algorithm and maintains separate routing
data for each area that is connected to it (including the backbone
area). Area border routers also send configuration summaries for their
attached areas to the backbone area, which then distributes this
information to other OSPF areas in the autonomous system.
In Figure 54, four area border routers link the areas in autonomous
system A.
■
Autonomous system boundary routers (ASBRs) — Autonomous
system boundary routers exchange their autonomous system topology
data with boundary routers in other autonomous systems. Every
router inside an autonomous system knows how to reach the
boundary routers for its autonomous system.
In Figure 54, two ASBRs control traffic between two autonomous
systems.
■
Designated routers (DRs) — Designated routers advertise network
link states for attached network segments. A link state advertisement
lists all routers that are connected to a segment.
The DR is considered adjacent to all routers in its area. As a result, the
DR exchanges routing data with all routers that are connected to its
network segment.
■
Backup designated routers (BDRs) — Backup designated routers
are given a lower priority value than the DR. They take over DR
functions if the DR fails.