| Broadcast: |
LAN interface-One Designated Router (DR) and one Backup Designated Router
(BDR) is elected per broadcast network. Multicast addresses 224.0.0.5
(ALLSPFRouters) and 224.0.0.6 (ALLDRouters) are used for communication between
the DR and their neighbors using the hello protocol. |
| Pt-to-Pt: |
Point to Point WAN interface such as Synchronous PPP and sometimes Frame
Relay. The hello protocol uses Multicast address 224.0.0.5 (ALLSPFRouters)
to acquire neighbor adjacency with the next hop router. |
| NBMA: |
Non Broadcast Multiple Access WAN interface such as X25 and Frame relay.
On there interfaces the IP unicast address is used to establish a neighbor
relationship. Extra configuration is required on NBMA interfaces to define
the NBMA interfaces and to establish the polling list for neighbors. |
| Designated Router: |
On Broadcast and NBMA networks a DR is required to establish a neighbor
adjacency with every other OSPF router on that Physical Network. The DR's
job is to create the same OSPF database in every router through the hello
protocol. A Backup DR will take over for the DR if it fails.The DR is elected
with the hello protocol using the router priority and or Router ID. |
| Router Priority: |
An assigned value to interface parameters. The router with the greater
priority will become DR.If equal the router with the greater Router ID will
become DR. |
| Router ID: |
Lowest number configured OSPF interface will become router ID. (ID is
dependant on order of entries in OSPF interface configuration. Entry 1 will
become router ID) |
| Routing Protocol Packets: |
There are 5 packet types to the OSPF routing protocol. |
| Hello Protocol: |
Protocol used by OSPF routers to acquire neighbor's and to synchronize
their topological database. |
| Adjacency: |
State between 2 routers on the same physical network or endpoints of
a virtual link that controls the distribution of routing protocol packets.
These packets are exchanged only between adjacencies. |
| Link State Advertisement (LSA): |
A OSPF router will periodically advertise the state of its Router links
(interfaces),Networks, Summaries, or AS external links. Link states are also
advertised when a link state changes. |
| Link State Flooding: |
On receiving a LSA indicating a change every router will flood this change
on all interfaces. This will ensure that every router in the Area will have
exactly the same topological database. From this database each router will
calculate its shortest path tree. |
| Shortest Path First Tree (SPF Tree): |
Every router in an area calculates Shortest Path to every other router
in its area using the shortest path first algorithm. (SPF) This SPF Tree
yields a routing table for the protocol. |
| Area: |
OSPF allows for a contiguous collection of networks and hosts to be grouped
into Areas. Each area runs a separate copy of the SPF algorithm and has its
own topological database. |
| Router Classification: |
A router can have 4 classifications: |
Internal
Router: |
A router with all of its directly connected interfaces or physical networks
belonging to the same area and containing no virtual connections to the backbone. |
| Area Border Router: (ABR) |
A router that attaches to the backbone and 1 other area. ABR's run separate
copies of the SPF algorithm for each area it attaches to. ABR's condense
the topological information of their attached areas and distribute it over
the backbone to the other areas. |
| Backbone Router: |
A router that has an interface into the backbone by a direct attachment
or a virtual link. |
| AS Boundary Router (ASBR): |
A router that exchanges information with routers from other Autonomous
Systems. ASBR's are also used to import routing information from non OSPF
attached interfaces. (i.e. RIP, DIRECT or STATIC Routes) |
| Backbone Area: |
Consists of networks and routers not contained in any area and ABR's.
The backbone is responsible for the distribution of routing information between
areas. This backbone area must be contiguous either physically or through
a virtual link. The number reserved for the backbone is 0.0.0.0. |
| Virtual Link: |
When a ABR cannot physically connect to the Backbone a virtual link will
logically connect the ABR to the backbone. The 2 endpoints of a virtual link
are always ABR's and must be configured in both ABR's. This virtual link
will use a transit area that is connected to the backbone. The transit area
cannot be a stub area. This link is treated as if it were a Pt-to-Pt link
belonging to the backbone. OSPF packets are sent to the neighbor IP address
at the other end of the Virtual Link. |
| Stub Area: |
An area where no external routes are imported into the area. A stub area
cannot contain ASBR's or cannot be transit area's for virtual links. Summary
advertisments external to the area are by default imported into the stub
area but my be squelched to further reduce area database size. In this case
the default route advertisment by the ABR's will handle all routes external
to the area. |
| OSPF Route: |
A route that is internal to the OSPF Autonomous System. |
| External Route: |
A route that is external to the OSPF Autonomous System and is imported
by an ASBR. |
| Intra-Area routing: |
Routing that takes place within an OSPF area. |
| Inter-Area routing: |
Routing that takes place between 2 OSPF areas. Inter-area routing path
is broken up into three pieces. |