Basic OSPF Topology Configuration
Fixed Switch Configuration Guide 22-3
OSPF Router Types
OSPF router type is an attribute of an OSPF process. A Fixed Switch device uses one OSPF router
process that can be any number between 1 and 65535. OSPF defines four router types:
• Area border router (ABR)
An ABR is a router that connects one or more areas to the backbone area, and is a member of
every area to which it is connected. An ABR keeps a separate copy of the link-state database
for each area to which it is connected.
• Autonomous system boundary router (ASBR)
An ASBR is a router that is connected to more than one routing protocol and exchanges
routing information with routers running other protocols. An ASBR distributes routes
received from external autonomous systems through its own autonomous system.
• Internal router (IR)
An internal router has all its interfaces in a single area.
• Backbone router (BR)
Backbone routers are located in the backbone area and can be ABRs or IRs.
Each router has an identifier that uniquely identifies that router in the AS. The router ID is written
in dotted decimal format. If the router ID is not explicitly configured, the highest configured
loopback IP address, if one exists, is used, or the highest routing VLAN IP address is used.
Designated Router
A designated router (really an interface on a physical router device) is elected by all OSPF routers
on a particular network segment, based on highest priority. Each network that has at least two
attached routers has a designated router.
The purpose of the designated router (DR) is to reduce network traffic by being the source for
routing updates. Other routers on the network send their updates to the DR (and the backup
designated router or BDR). The DR keeps a complete topology map of the network and sends the
updates it receives from the other routers out to the other routers on the network via multicast.
This reduces the amount of network traffic because the routers do not have to update each other
and can get all their updates from the DR only.
The backup designated router (BDR), as the name indicates, is the backup to the DR and becomes
the DR if the current DR fails. The BDR is also elected by the other routers on the network based
on OSPF priority.
A designated router is different from an OSPF router type, which is an attribute of an OSPF
process. A DR is an interface on a physical router, and the election of a DR is based on the priority
assigned to the OSPF process, and if necessary, the highest router ID.
Authentication
OSPF v2 supports optional authentication between routers. When configured, only trusted routers
can participate in the AS’s routing. The fixed switch platforms support either simple or MD5
authentication schemes. Separate authentication schemes can be configured for each IP subnet.
Basic OSPF Topology Configuration
Figure 22-1 on page 22-4 provides an overview of a basic OSPF topology. This topology displays
two areas: a backbone area which must exist in any OSPF topology and a directly connected area
To Next Page
Loading...
Need help?
General