17: Configuring OSPF (Open Shortest Path First)
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© Virtual Access 2018
GW2020 Series User Manual
Issue: 2.1
Page 139 of 423
17.1.1
OSPF areas
Figure 79: OSPF areas
OSPF has a number of features that allow it to scale well for larger networks. One of
these features is OSPF areas. OSPF areas break up the topology so that routers in one
area know less topology information about the subnets in the other area, and they do
not know anything about the routers in the other area at all. With smaller topology
databases, routers consume less memory and take less processing time to run SPF.
The Area Border Router (ABR) is the border between two areas. The ABR does not
advertise full topology information about the part of the network in area 0 to routers in
area 1. Instead the ABR advertises summary information about the subnets in area 0.
Area 1 will just see a number of subnets reachable via area 0.
17.1.2
OSPF neighbours
OSPF forms neighbour relationships, called adjacencies, with other routers in the same
Area by exchanging ‘Hello’ packets to multicast address 224.0.0.5. Only after an
adjacency is formed can routers share routing information.
Each OSPF router is identified by a unique router ID. The router ID can be determined in
one of three ways:
•
The router ID can be manually specified.
•
If not manually specified, the highest IP address configured on any Loopback
interface on the router will become the router ID.
•
If no loopback interface exists, the highest IP address configured on any physical
interface will become the router ID.
By default, Hello packets are sent out OSPF-enabled interfaces every 10 seconds for
broadcast and point-to-point interfaces, and 30 seconds for non-broadcast and point-to-
multipoint interfaces.
OSPF also has a ‘Dead Interval’, which indicates how long a router will wait without
hearing any hellos before announcing a neighbour as ‘down’. The default setting for the
Dead Interval is 40 seconds for broadcast and point-to-point interfaces, and 120 seconds
for non-broadcast and point-to-multipoint interfaces. By default, the Dead Interval timer
is four times the Hello interval.
