Chapter 8: Single-Area OSPF

backup designated router

(BDR) A router that becomes the designated router if the current designated router fails. The BDR is the OSPF router with the second-highest priority at the time of the last DR election.

Router ID

A field in an OSPF Hello packet that is a 32-bit value expressed in dotted decimal notation (an IPv4 address ) used to uniquely identify the originating router.

data structures

A group of data elements that are stored together under one name. The adjacency database, link-state database, and forwarding database are all examples of data structures.

OSPF area

A logical set of network segments (CLNS-, DECnet-, or OSPF-based) and their attached devices. Areas are usually connected to other areas through routers , making up a single autonomous system.

database description

A packet used in OSPF that contains link-state advertisement (LSA) headers only and describes the contents of the entire link-state database. Routers exchange DBDs during the exchange phase of adjacency creation. A DBD is an OSPF Type 2 packet.

link-state router

A router that uses a link- state routing protocol.

link-state routing protocol

A routing protocol in which routers exchange information with one another about the reachability of other networks and the cost or metric to reach the other networks. These routers use Dijkstra's algorithm to calculate shortest paths to a destination, and normally update other routers with whom they are connected only when their own routing tables change.

wildcard mask

A string of 32 binary digits used by the router to determine which bits of the address to examine for a match.

link-state database

A table used in OSPF that is a representation of the topology of the autonomous system. It is the method by which routers "see" the state of the links in the autonomous system.

link-state acknowledgment

Acknowledges receipt of link-state advertisement (LSA) packets. Link-state acknowledgment packets are Type 5 OSPF packets. (LSAck)

cost

An arbitrary value, typically based on hop count, media bandwidth, or other measures, that is assigned by a network administrator and used to compare various paths through an internetwork environment. Routing protocols use cost values to calculate the most favorable path to a particular destination: the lower the cost, the better the path.

passive interface

An interface that does not take part in the advertisement of routing information. This interface command enables the suppression of routing updates over some interfaces while it allows updates to be exchanged normally over other interfaces. A neighbor adjacency cannot be formed over this interface. This is because link-state packets cannot be sent or acknowledged.

link-state advertisement

Broadcast packet used by link-state protocols that contains information about neighbors and path costs. LSAs are used by the receiving routers to maintain their routing tables. (LSA)

link-state packet

Broadcast packet used by link-state protocols that contains information about neighbors and path costs. LSAs are used by the receiving routers to maintain their routing tables. (LSP)

Multiarea OSPF

In an OSPF Network Design, one large autonomous system (AS) is divided into smaller areas , to support hierarchical routing. All areas must connect to the backbone area (area 0).

Single-Area OSPF

In an OSPF Network design, all routers are in one area called the backbone area (area 0).

exchange state

In this state, OSPF routers exchange database descriptor (DBD) packets. Database descriptors contain link-state advertisement (LSA) headers only and describe the contents of the entire link-state database.

Full State

In this state, routers are fully adjacent with each other. All the router and network LSAs are exchanged and the routers' databases are fully synchronized.

Loading State

In this state, the actual exchange of link state information occurs. Based on the information provided by the DBDs, routers send link-state request packets. The neighbor then provides the requested link -state information in link-state update packets. During the adjacency, if a router receives an outdated or missing LSA, it requests that LSA by sending a link-state request packet. All link-state update packets are acknowledged.

Exstart State

In this state, the routers and their DR and BDR establish a master-slave relationship and choose the initial sequence number for adjacency formation. The router with the higher router ID becomes the master and starts the exchange.

link-state request

Link-state request packets are Type 3 OSPF packets. The link-state request packet is used to request the pieces of the neighbor's database that are more up to date. (LSR)

link-state update

Link-state update packets are Type 4 OSPF packets. A link-state update packet carries a collection of link-state advertisements one hop farther from its origin. (LSU)

designated router

OSPF router that generates LSAs for a multiaccess network and has other special responsibilities in running OSPF. Each multiaccess OSPF network that has at least two attached routers has a designated router that is elected by the OSPF Hello protocol. The DR router enables a reduction in the number of adjacencies required on a multiaccess network, which in turn reduces the amount of routing protocol traffic and the size of the topological database.

OSPF

Open Shortest Path First. Link-state, hierarchical IGP routing algorithm proposed as a successor to RIP in the Internet community. Features include least-cost routing, multipath routing, and load balancing. It was derived from an early version of the IS-IS Protocol.

link state

Refers to the status of a link, including the interface IP address /subnet mask, type of network, cost of the link, and any neighbor routers on that link.

Area Border Router

Routers interconnecting the areas in a multi-area OSPF network. (ABR)

DROthers

Routers that are not a DR or BDR. They are the other routers in the OSPF network.

Hello interval

Specifies the frequency, in seconds, at which a router sends Hello packets.

reference bandwidth

The number, measured in Mb/ s, which is used by OSPF routers to calculate cost. The default reference bandwidth is 100 Mb/ s. Changing the this does not actually affect the bandwidth capacity on the link; rather, it simply affects the calculation used to determine the metric.

dead interval

The time in seconds that a router waits to hear from a neighbor before declaring the neighboring router out of service.

Hello packet

These are Type 1 OSPF packets. They are used to establish and maintain adjacency with other OSPF routers.

shortest path first algorithm

This algorithm uses accumulated costs along each path, from source to destination, to determine the total cost of a route. Routing algorithm that iterates on the length of path to determine a shortest-path spanning tree. Commonly used in link-state routing algorithms. Sometimes called Dijkstra's algorithm. (SPF)

Down State

This is the first OSPF neighbor state. It means that no information (hellos) has been received from this neighbor, but hello packets can still be sent to the neighbor in this state.

2-way state

This state designates that bidirectional communication has been established between two routers. Bidirectional means that each router has seen the other's hello packet.

Init State

This state specifies that the router has received a hello packet from its neighbor, but the receiving router's ID was not included in the hello packet.

Router Priority

Used in a DR/ BDR election . The default priority for all OSPF routers is 1 but can be manually altered from 0 to 255. The higher the value, the more likely the router becomes the DR on the link.

OSPFv2

Version 2 of the OSPF routing protocol. It is used to support IPv4 unicast address families.

OSPFv3

Version 3 of the OSPF routing protocol. It is used to support both IPv4 and IPv6 unicast address families.