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Appendix A: Data communications planning 10-99
Planning Guide NT6Q92MA Rel 1.0 Iss 1 Standard September 2006
Terms
Some terms associated with OSPF are:
• Costs Routes have a cost associated with them. The higher the cost the less
favourable the route. OSPF has a number of metrics which are converted
with algorithm into a cost.
• Policy filters This parameter only applies when an OSPF network uses
external routes. An announce filter acts on the outward advertisements
form the OSPF area and the accept filter acts on inward advertisements. As
the LSPs are modified by the filter and the resultant used to produce a
routing table, it follows that policy filters need to be applied to all routers
in the OSPF network and not just to the boundary router.
• Link stat e is the status of a link between two routers.
• Cost of a link is computed from bandwidth, real cost, availability,
reliability and other link metrics.
• OSPF area is a collection of connected routers which exchange link state
updates.
• Adjacencies database lists all a router’s neighbours.
• Link State Database is a list of link states from all other routers in the
OSPF area. All routers have identical link state databases.
• OSPF routing table is produced from the OSPF link state database.
• Routing table (forwarding table) . The best routes are chosen from all
protocol routing tables. Note that each router has a different routing table.
• Backbone area Area to which all other OSPF areas are connected, either
directly or via a virtual link. It is referred to as area 0.0.0.0 or area 0.
• Standard area Area which is not the backbone area but which receives all
link state updates from external networks.
• Stub areas These are areas which can have more than one interface, but by
definition do not carry transit data and do not receive link state updates
from external networks. All routers in a stub area must be set to be stub
routers. How this is implemented varies between router manufacturers.
• Totally stubby areas Stub areas which do not receive summary LSAs.
• NSSA (Not So Stubby Areas) Stub areas which receive certain link state
updates from external networks.
• Router ID This is the number by which each router is known to OSPF. On
a Bay router the default is the IP address of the first configured interface.
On Cisco the default is the highest configured IP address. On both routers
it should be manually configured to be the same as the circuitless
IP/loopback
address.
• Border router A router which is in the backbone area and one or more
other OSPF areas.