O S P F

OSPF stands for Open Shortest Path First It is an open standard LSRP

OSPF Neighbors

A neighbor in OSPF is a Router that shares the same network link or the same network segment

OSPF Hello Protocol

A router running OSPF discovers its neighbors by sending and receiving a simple protocol called the hello protocol

A router configured for OSPF sends out a small hello packet periodically (10 seconds is the default on broadcast multi-access media)

OSPF Hello Protocol

It has a source address of the router and a multicast destination address set to AllSPFRouters (224.0.0.5)

All routers running OSPF (or the SPF algorithm) listen to the protocol and send their own hello packets periodically

OSPF Topologies

OSPF identifies five distinct network types or topologiesBroadcast Multi-accessPoint-to-PointPoint-to-MultipointNBMA

Broadcast Multi-access

This is any LAN network such as Ethernet, Token Ring, or FDDI

In this environment, OSPF sends out multicast traffic

A designated router and backup designated router will be elected

Point-to-Point

This technology is used where there is only one other router directly connected to the transmitting or receiving router

A typical example of this is a serial line

OSPF has no need of a designated or backup designated router in this scenario.

OSPF messaging sent using the multicast address for AllSPFRouters, 224.0.0.5

Point-to-Multipoint

This is a single interface that connects to multiple destinations

The underlying network treats the network as a series of point-to-point circuits

OSPF traffic is sent as multicast.

There is no DR or BDR election

N B M A

This physically resembles a point-to-point line, but in fact, many destinations are possible

WAN clouds, including X.25 and Frame Relay, are examples of this technology

NBMA uses a fully meshed or partially meshed network

N B M A

OSPF sees it as a broadcast network, and it will be represented by one IP subnet

This technology requires manual configuration of the neighbors and the DR and BDR selection

The Hello Packet

Although the routers running OSPF transmit a small packet called the hello packet to establish neighbor relations, it serves other functions

Adjacent OSPF Neighbors

After neighbors have been established by means of the Hello protocol, they exchange routing information

When their topology databases are the same or synchronized, the neighbors are fully adjacent

The Hello protocol continues to transmit by default every 10 seconds on broadcast multi-access media and by default every 30 seconds on point-to-point links

The transmitting router and its networks reside in the topology database for as long as the other routers receive the Hello protocol

The Designated Router The designated router is a router on broadcast multi-access media that is responsible for maintaining the topology table for the segment

If routers are connected to a broadcast segment, one router (DR) on the segment is assigned the duty of maintaining adjacencies with all the routers on the segment

It is elected by the use of the Hello protocol The election is determined by either the highest IP

address or this command (if it is defined):ip ospf priority number

All other routers need only peer with the designated router, which informs them of any changes on the segment.

Backup Designated Router Redundancy has been built into the network

with the backup designated router (BDR) All routers actually have an adjacency not only

with the designated router, but also with the backup designated router, which in turn has an adjacency with the designated router

If the designated router fails, the backup designated router immediately becomes the new designated router

DR / BDR Election The network administrator can manually elect the

designated and backup designated routers, or they can be dynamically selected using the Hello protocol

After the designated and backup designated routers have been elected, all routers on the broadcast medium will communicate directly with the designated routers

They will use the multicast address to all designated routers

The backup router will listen but will not respond

Manual Election To manually determine which router will be the

designated router, it is necessary to set the priority of the router

A router interface can have a priority of 0 to 255 The value of 0 means that the router cannot be a

designated router or backup designated router the higher the priority, the more favorable the chances

are of winning the election If there is more than one router on the segment with

the same priority level, the election process picks the router with the highest router ID

Dynamic Election The selection is made on the basis of the

highest router ID or IP address present on the network segment

Dynamic ElectionThe following is the process used to elect the designated and backup designated routers:• All the neighbors who have a priority greater than 0

are listed.

• The neighbor with the highest priority is elected as the BDR.

• If there is no DR, the BDR is promoted as DR.

• From the remaining routers, the router with the highest priority is elected as the BDR.

• If there is a tie, the highest router IDs are used.

Building the RT After a neighbor is discovered in OSPF, an

adjacency is formed Routing tables are built in two different ways.

When a router joins the Network When a change in the Network takes place

When a Routerjoins the Network When a new Router is added to a Network, it

builds it routing table by listening to the existing Routers with complete routing tables

Every router within an area will have the same database and will know of every network within the area

The routing table built from this database is unique to the router because the decisions depend on the individual router’s position within the area, relative to the remote destination network

Building RT ona new OSPF Router Five packet types are used to build the RT for the first

time:

1. Hello ProtocolUsed to find neighbors and to determine DR/BDR

2. Database DescriptorUsed to send summary info to neighbors for topological database synchronization

3. Link-state RequestRequest for a more detailed info, sent when router receives database descriptor with new information

4. Link-state UpdateWorks as LSA (Link state Advertisement) packet issued in response to the above made request

5. Link-state AcknowledgmentAcknowledges the LSA

Building RT ona new OSPF Router

OSPF STATES The different stages or states that the router

goes through with OSPF are:

1. The DOWN state

2. The INIT state

3. The TWO-WAY state

4. The EXSTART state

5. The EXCHANGE state

6. The LOADING state

7. The FULL state

1. DOWN state The new router is in a

down state The 2500 router

transmits its own hello packets to introduce itself to the segment and to find any other OSPF-configured routers

This is sent out as a hello to the multicast address 224.0.0.5

2. INIT state

The new router waits for a reply. Typically this is four times the length of the hello timer. The router is in the init state

Within the wait time, the new router hears a hello from another router and learns the DR and the BDR, if there is no DR or BDR stated in the incoming hello, an election takes place

2. INIT state

Upon hearing the Hello protocol from the 2500, a router on the segment adds the router ID of the 2500 and replies as a multicast (224.0.0.5) with its own ID and a list of any other neighbors

3. TWO-WAY state

The new router sees its own router ID in the list of neighbors, and a neighbor relationship is established

The new router changes its status to the two-way state

4. EXSTART state

One of the routers will take seniority, becoming the master router. This is the exstart state

The two neighbors determine a master/slave relationship based on highest IP interface address

This designation is not significant; it just determines who starts the communication

5. EXCHANGE state

Both routers will send out database description packets, changing the state to the exchange state

The 7200 sends out a series of database description packets containing the networks held in the topology database

These networks are referred to as links

6. LOADING state

If the receiving router, the 2500, requires more information, it will request that particular link in more detail using the link-state request packet (LSR)

The LSR will prompt the master router to send the link-state update packet (LSU)

This is the same as a link-state advertisement (LSA) used to flood the network with routing information

While the 2500 is awaiting the LSUs from its neighbor, it is in the loading state

7. FULL state

When these LSRs are received and the databases are updated and synchronized, the neighbors are fully adjacent

The Topology Database The topology database is the router’s view of the

network within the area It includes every OSPF router within the area and all

the connected networks The topology database is updated by the LSAs Each router within the area has exactly the same

topology database All routers must have the same vision of the network;

otherwise, confusion, routing loops, and loss of connectivity will result

The synchronization of the topology maps is ensured by the intricate use of sequence numbers in the LSA headers

TT RT From the topology map, a routing database is

constructed This database will be unique to each router,

which creates a routing database by running the shortest path first (SPF) algorithm called the Dijkstra algorithm

Each router uses this algorithm to determine the best path to each network

If there are equal metrics for a remote network, OSPF includes all the paths and load balances the route data traffic among them.

TT RT Occasionally a link may flap or go up and

down. This is more usual on a serial line If this happens, it could cause many LSAs to

be generated in updating the network To prevent this from happening, OSPF

introduced timers These timers forced OSPF to wait before

recalculating SPF These timers are configurable

Choosing best paths As with any routing protocol, OSPF examines

all the available paths to every network that it knows about

It selects the shortest, most direct path to that destination

The METRIC As with all routing protocols, this decision will be based

on the metric used by the routing protocol RIP uses hop count, which shows how many routers

must be passed through to get to the destination OSPF chooses the metric of cost Cisco’s implementation of a dynamic and default cost

uses a predefined value based on the bandwidth of the router interface

The cost is applied to the outgoing interface The routing process will select the lowest accumulated

cost of the interfaces to the remote network

OSPF Configuration in a Single Arearouter ospf process-numbernetwork network-number wildcard-mask area area-number The network command in OSPF plays a similar role to

that of the network command in RIP or IGRP. The difference is the level of granularity afforded to the administrator. In RIP and IGRP, the network command is defined at the class level. In OSPF, it is possible to identify the specific address of an interface.

After the network command has been entered, OSPF identifies which interfaces are participating in OSPF by comparing the interface IP address with the address given in the network command, filtered through the wildcard mask

The wildcard mask states how much of the address to pay attention to

The COST Command This command manually overrides the default cost that

the router assigns to the interface The default cost is calculated based on the bandwidth

parameter assigned to the outgoing interface The cost command syntax is as follows:

ip ospf cost cost

The range of values configurable for the cost of a link is 1 to 65535

In general, the path cost in Cisco routers is calculated using the formula 108 / Bandwidth

The COST Command

Show commands for OSPFshow ip ospf— Shows the OSPF process and its details—for

example, how many times the router has recalculated its routing table.

show ip ospf database— Shows the contents of the topological database.show ip ospf interface— Gives information on how OSPF has been

configured on each interfaceshow ip ospf neighbor— Displays all the information about the relationship

that the router has with its neighbors—for example, the status of communication and whether it is initializing

or transferring DDP packets.show ip protocols— Enables you to view the IP configuration on the router.show ip route— Shows detailed information on the networks that the

router is aware of and the preferred paths to those networks. Also gives the next logical hop as the next step in the path

OSPF Router Types The following list identifies the different OSPF

routers: Internal Router Backbone Router Area Border Router (ABR) Autonomous System Boundary Router (ASBR)

OSPF Router Types