Nortel Optical Multiservice Edge 6130 Planning Manual page 286

10-100 Appendix A: Data communications planning
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Topology considerations
An OSPF network has to be planned out in areas to take full advantage of the
protocol.
With OSPF packets destined for an area outside the current area are sent to
area 0. Thus it may be inferred that all areas must have a connection to area 0.
There may be more than one connection between an area and area 0 but there
must be no inter-area connections.
It can be concluded that OSPF networks are tree structures which lend
themselves to hierarchical addressing schemes using variable length subnet
masks.
OSPF places demands upon a routing processor. It is recommended that no
more than 50 OSPF routers be placed within an OSPF area.
The designated router on a LAN in a network running OSPF has a very high
processor utilization. It may be that some routers are unsuitable for this role
and so should be allocated a priority of 0. If it is not possible to have an area
connected directly to area 0, a virtual route may be used as in
(dashed line).
Optical Multiservice Edge 6130
Boundary router (or ASBR—Autonomous System Boundary Router) A
router which is the gateway between an OSPF network and another
network which uses a separate routing mechanism, including static routes.
Designated router Link state routing protocols have an inherent problem
when a number of routers exist on the same LAN. The solution is for a
router to be elected as a proxy for all the routers on the LAN, this is known
as a designated router (DR). The DR creates a dummy routing entity with
which all routers on the LAN form an adjacency. There are no other
adjacencies formed on the LAN. Thus, routers on a LAN will have only
one link on the LAN, rather than one for each of the other routers on the
LAN. This means that the number of links on a LAN with n routers is
reduced from n * (n - 1) to n * 2:
— A router with a priority of 0 never becomes DR.
— If a router with a higher priority connects to the network there is no
re-election.
— Changing router priorities does not cause a re-election.
— Changing router IDs does not cause a re-election.
OSPF multicasts. Packets sent out with 224.0.0.5 are sent to all OSPF
routers. Packets sent out with 224.0.0.6 are sent to all OSPF designated
routers.
NT6Q92MA Rel 1.0 Iss 1 Standard September 2006
Figure 10-28