Skip to main content

OSPF network types

OSPF network types
2.2.5 This page will introduce the three types of OSPF networks.
A neighbor relationship is required for OSPF routers to share routing information. A router will try to become adjacent, or neighbor, to at least one other router on each IP network to which it is connected. OSPF routers determine which routers to become adjacent to based on the type of network they are connected to. Some routers may try to become adjacent to all neighbor routers. Other routers may try to become adjacent to only one or two neighbor routers. Once an adjacency is formed between neighbors, link-state information is exchanged.
OSPF interfaces automatically recognize three types of networks:
  • Broadcast multi-access, such as Ethernet
  • Point-to-point networks
  • Nonbroadcast multi-access (NBMA), such as Frame Relay
A fourth type, point-to-multipoint, can be manually configured on an interface by an administrator. 
In a multi-access network, it is not known in advance how many routers will be connected. In point-to-point networks, only two routers can be connected.
In a broadcast multi-access network segment, many routers may be connected. If every router had to establish full adjacency with every other router and exchange link-state information with every neighbor, there would be too much overhead. If there are 5 routers, 10 adjacency relationships would be needed and 10 link-states sent. If there are 10 routers then 45 adjacencies would be needed. In general, for n routers, n*(n-1)/2 adjacencies would need to be formed.
The solution to this overhead is to hold an election for a designated router (DR). This router becomes adjacent to all other routers in the broadcast segment. All other routers on the segment send their link-state information to the DR. The DR in turn acts as the spokesperson for the segment. The DR sends link-state information to all other routers on the segment using the multicast address of 224.0.0.5 for all OSPF routers.
Despite the gain in efficiency that electing a DR provides, there is a disadvantage. The DR represents a single point of failure. A second router is elected as a backup designated router (BDR) to take over the duties of the DR if it should fail. To ensure that both the DR and the BDR see the link-states all routers send on the segment, the multicast address for all designated routers, 224.0.0.6, is used.
On point-to-point networks only two nodes exist and no DR or BDR is elected. Both routers become fully adjacent with each other.
The Interactive Media Activity will help students recognize the three types of OSPF networks.
The next page will describe the OSPF Hello protocol.

Comments

Post a Comment

Popular posts from this blog

OSI layers / Peer-to-peer communications / TCP/IP model

OSI layers 2.3.4 This page discusses the seven layers of the OSI model. The OSI reference model is a framework that is used to understand how information travels throughout a network. The OSI reference model explains how packets travel through the various layers to another device on a network, even if the sender and destination have different types of network media. In the OSI reference model, there are seven numbered layers, each of which illustrates a particular network function. - Dividing the network into seven layers provides the following advantages: • It breaks network communication into smaller, more manageable parts. • It standardizes network components to allow multiple vendor development and support. • It allows different types of network hardware and software to communicate with each other. • It prevents changes in one layer from affecting other layers. • It divides network communication into smaller parts to make learning it easier to understand. In the foll...
Post a Comment
Read more

Advantages and disadvantages of link-state routing

Advantages and disadvantages of link-state routing 2.1.5  This page lists the advantages and disadvantages of link-state routing protocols. The following are advantages of link-state routing protocols:  Link-state protocols use cost metrics to choose paths through the network. The cost metric reflects the capacity of the links on those paths. Link-state protocols use triggered updates and LSA floods to immediately report changes in the network topology to all routers in the network. This leads to fast convergence times. Each router has a complete and synchronized picture of the network. Therefore, it is very difficult for routing loops to occur. Routers use the latest information to make the best routing decisions. The link-state database sizes can be minimized with careful network design. This leads to smaller Dijkstra calculations and faster convergence. Every router, at the very least, maps the topology of it...
Post a Comment
Read more

Ports for services

Ports for services 10.2.2  Services running on hosts must have a port number assigned to them so communication can occur. A remote host attempting to connect to a service expects that service to use specific transport layer protocols and ports. Some ports, which are defined in RFC 1700, are known as the well-known ports. These ports are reserved in both TCP and UDP.  These well-known ports define applications that run above the transport layer protocols. For example, a server that runs FTP will use ports 20 and 21 to forward TCP connections from clients to its FTP application. This allows the server to determine which service a client requests. TCP and UDP use port numbers to determine the correct service to which requests are forwarded. The next page will discuss ports in greater detail.
Post a Comment
Read more