Router roles

FIGURE 6 . 2
Router roles
Starting at the core of the given network and working our way outward, consider RouterA.
Notice that RouterA is part of Area 0. As we learned in the previous chapter, Area 0 is referred
to as the
backbone area
. Therefore, we can make the following definition:
Backbone router
A backbone router is any router that exists (wholly or in part) in OSPF Area 0.
Another distinction we can make about RouterA is that it is contained completely within a
single area, in this case Area 0. Because all of RouterA’s interfaces are internal to a single area,
we can make the following definition:
Internal router
An internal router is any router that has all of its interfaces as members of the
same area.
Remember that a router can play more than one role. In our example, RouterA
is both a backbone router and an internal router.
Now consider RouterB. Notice that RouterB meets the requirement to be classified as a backbone
router (in other words, RouterB has one or more interfaces that are part of Area 0). However,
unlike RouterA, RouterB is partially in Area 0 and partially in Area 10. There is yet
another term used to define routers that have interfaces in more than one area:
Area border router (ABR)
An area border router is any router that is connected to multiple
OSPF areas. Cisco recommends that an ABR belong to only one OSPF process and that no
router belong to more than three areas.
Recall that the topology of an OSPF area is contained in a link-state database. Therefore,
if a router is connected to multiple areas, it will contain multiple link-state databases.
EIGRP
Autonomous
System
Area 10 Area 0
Internal Router
Area Border Router
Autonomous System Boundary Router
Backbone Router
Internal Router
Backbone Router
RouterC RouterB RouterA
Categories of Multi-Area Components
179
This should be a design consideration when sizing a router that will function as an area
border router.
Notice also that RouterB is connected to an EIGRP network. Whether an OSPF network is
connected to an EIGRP network, a BGP network, an OSPF network with a different Process ID,
or a network running any other such external routing process, this external network may be
referred to as an
autonomous system
. The scenario of an OSPF router sitting at the boundary
of an external routing process leads us to a fourth category of OSPF router:
Autonomous system boundary router (ASBR)
An autonomous system boundary router is
any OSPF router that is also connected to an external routing process and exchanges routing
information between the OSPF routing process and the external routing process.
The ability of an ASBR to exchange routing information between its OSPF routing process and
the external routing process to which the router is connected is not an automatic process. Such
routes are exchanged through a process called
route redistribution
, which is the focus of Chapter 10,
“Route Optimization.”