242 Chapter 7: OSPF, EIGRP, and IGRP
Table 7-4
OSPF Characteristics
Term
Definition
OSPF
Open Shortest Path First protocol. a nonproprietary link-state routing protocol for
TCP/IP. Developed in response to a need to overcome limitations of RIP, a distance-
vector protocol.
Link-state
protocol
A protocol that describes the state of the existing interface and link. A link-state
database includes information about the interface's IP address, the type of network
on which it resides, and information about neighboring routers.
Shortest Path
First algorithm
An algorithm used by OSPF to determine the shortest path to each destination. This
algorithm places each router at the root of a tree and calculates the shortest path to
each destination based on the cost metric.
OSPF cost
A metric used by OSPF to determine overhead required to send a packet across an
interface. The cost of an interface is inversely proportional to the interface's
bandwidth.
cost = 10
8
/ BW
Area
A group of OSPF routers that contain the same link-state database.
Authentication
Security procedures that require passwords to be defined before a router can
participate in area activity.
Backbone
Area 0 is the hub of all OSPF activity. All OSPF information must pass
through Area 0.
Virtual link
A link used when an area cannot be physically connected to Area 0.
Neighbors
OSPF routers that share a common segment and common handshake information.
Table 7-5
Six OSPF Packet Types
Name
Type
Comments
Hello
1
Database description
2
Link-state request
3
Used to request full path information associated
with a received database description packet.
Link-state update
4
Contains one or more link-state advertisements.
Link-state acknowledgment
5
Used to acknowledge the receipt of path
information contained in link-state updates.
AS external link-state acknowledgment
7
Used by the ASBR to distribute external routing
information throughout the NSSA.
87200333.book Page 242 Wednesday, August 22, 2001 2:37 PM