CCNA Course

OSPF

OSPF Features

OSPF is an open standards routing protocol

This works by using the Dijkstra algorithm

OSPF provides the following features:

Minimizes routing update traffic

Allows scalability (e.g. RIP is limited to 15 hops)

Has unlimited hop count

Supports VLSM/CIDR

Allows multi-vendor deployment (open standard)

OSPF PACKETS

There are different types of OSPF Packets:

Hello

Data Base Description (DBD)

Link State Request (LSR)

Link State Update (LSU)

Link State Ack

Neighbor Discovery

When router starts, it (R1) send Hello packet – uses 224.0.0.5

Hello packets are received by all neighbors

R2 receive hello and check all parameters in hello

R2 write RID (1.1.1.1) in its neighbor table

Neighbor Discovery

R2 send hello to R1 with RID (1.1.1.1) in its neighbor list

OSPF neighbor state 2-way means that the router is available to

exchange its LSDB

Hello packets and Link State Advertisements (LSAs) build and maintain the topological database

Hello Packet Information

• Router ID: 32-bit

• Hello and dead intervals: must be the same on neighboring routers (must match)

• Neighbors: list of adjacent routers

• Area ID: Contains the full LSA entries. Multiple LSA entries can fit in one OSPF update packet (must match)

• Router Priority: 8-bit

• DR and BDR IP Addresses

• Authentication Password: (must match)

• Stub Area Flag: (must match)

Exchange LSA

Each router tell the other a list of LSAs in their databases using LSDB —not all the details of the LSAs, just a list (Exstart)

Each router check which LSAs it already has, and then ask other router for only the LSAs that are not known yet using LSR

LSU sent to reply the request from adjacent router (loading)

LSAs are used to update and maintain the topology database.

OSPF Tables

There are three type of tables

Neighbor

Topology

Routing

Neighbor Table

Contain information about the neighbors

Neighbor is a router which shares a link on

same network

Another relationship is adjacency

Not necessarily all neighbors

LSA updates are only when adjacency is

established

Topology Table

Contain information about all network and path to

reach any network

All LSA’s are entered in to topology table

When topology changes LSA’s are generated and

send new LSA’s

On topology table an algorithm is run to create a

shortest path, this algorithm is known as SPF or

dijkstra algorithm

Routing Table

Also knows as forwarding database

Generated when an algorithm is run on

the topology database

Routing table for each router is unique

OSPF Terms: Link State

Advantages and disadvantages Note that OSPF is a more sophisticated routing protocol

Converges rapidly and accurately

Can use a metric calculation that effectively selects the "best" route(s) primarily based on bandwidth, although an OSPF cost can be administratively assigned

Use of OSPF requires

More powerful routing hardware

More detailed knowledge by the administrator, especially when large multi-area networks are used

Neighbors Neighbours are two or more routers that

have an interface on a common network

• E.g. two routers connected on a serial link

• E.g. several routers connected on a

common Ethernet or Frame relay network

Communication takes place between

among neighbours

Not all neighbors will form adjacencies

Database and Neighbors Maintenance

Routers perform three tasks to main its LSDB and

neighbor relationship:

1. Maintain neighbor state by sending Hello messages

based on the Hello Interval, and listening for Hellos

before the Dead Interval expires

2. Flood any changed LSAs to each neighbor

3. Reflood unchanged LSAs as their lifetime expires

(default 30 minutes )

OSPF Area

Autonomous System Border Router (ASBR)

Areas Reasons and Features A larger topology database requires more memory on

each router.

Processing the larger topology database with the SPF

algorithm requires more processing power

A single interface status change, anywhere in the

internetwork forces every router to run SPF again.

A router can be a member of more than one area (ABR)

All routers in the same area have same topology

database

When multiple areas exist, there must always be an area

0 (the backbone) to which other areas connect

Path Calculation Changes to the topological database of a router trigger a recalculation to re-establish the best route(s) to known networks

Uses the SPF (shortest path first) algorithm developed by a computer scientist named Dijkstra

This is done by each individual router using its detailed "knowledge" of the whole network

Leads to rapid and accurate convergence

Based on detailed knowledge of every link in the area and the OSPF "cost" of each

builds an OSPF tree with itself at the route

Network Types

OSPF Election

DR and BDR selection required

All neighbor routers form full adjacencies with the DR

and BDR only

Packets to the DR use 224.0.0.6

Packets from DR to all other routers use 224.0.0.5

Router ID The Router ID (RID) is an IP address used to identify

the router

Cisco chooses the Router ID by using the highest IP

address of all configured loopback interfaces

If no loopback interfaces are configured with

addresses, OSPF will choose the highest IP address

of all active physical interfaces.

You can manually assign the router ID.

The RID interface MUST always be up, therefore

loopbacks are preferred

LSA Types

LSA type 1 (Router LSA):

Describe RID, interfaces, IP address/mask, current interface state (status)

LSA type 2 (Network LSA):

Describe DR and BDR IP addresses, subnet ID, mask

LSA type 3 (Summary LSA): Describe subnet ID, mask, RID of ABR that advertises the LSA

Example of LSA Types 1 and 2

Multi-Area OSPF

LSDB for each area

LSA type 3 floods summary for subnets in other

areas and RID of ABR