Nortel Optical Multiservice Edge 6130 Planning Manual page 284

10-98 Appendix A: Data communications planning
The topology of each OSPF area is invisible to entities outside the area. This
area partitioning system speeds up routing, because all packets with
destinations within an area are contained within that area; packets destined for
another area are sent to the backbone area for redirection.
The rules for area use within OSPF networks contrast with the way areas are
implemented in OSI in the following ways:
•
•
Interfacing OSPF networks with non-OSPF networks
The DCN design does not support running other IP routing protocols within
the OSPF network. However, there are applications where non-OSPF is the
only protocol available and these networks must be able to send to and receive
packets from the OSPF network.
There are two ways in which this can be done:
1 static routes (see
2 redistribution.
Redistribution
Redistribution gives visibility of both networks using dynamic routing. Routes
from one area to subnets in the other are costed within limitations. There is no
easy way for non-OSPF to interpret OSPF metrics and vice versa.
•
•
A router (boundary router) must always be used as the interface between the
two networks. There may be more than one router performing this role.
Implementing OSPF in a network
This section covers issues that the implementer of an OSPF network needs to
be aware of.
Optical Multiservice Edge 6130
There is no requirement for a backbone area within OSI.
The border between OSI areas is between routers (that is, a OSI router can
only reside in one area), whereas the border between OSPF areas runs
through a router (that is, an OSPF router may be in more than one area).
Figure
The boundary router advertises all OSPF routes into the non-OSPF
network with a cost of one hop.
Redistributing non-OSPF routes into the OSPF network is more complex.
To a router within the OSPF network, networks in the non-OSPF network
within one hop of the boundary router have the same cost as the cost of
reaching the boundary router. To a router within the OSPF network,
networks in the non-OSPF network beyond one hop of the boundary router
have a cost equal to the number of hops required in the non-OSPF network.
The cost to reach the boundary router within the OSPF area is considered
to be zero. How this is implemented varies between router manufacturers.
NT6Q92MA Rel 1.0 Iss 1 Standard September 2006
10-30)