LAN Switching and WAN Networks

Topic 6 - OSPF

What we have done so far!

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Looked at the basic switching concepts and configuration from the Cisco IOS CLI

Looked at VLANs and seen how to configure basic VLANs on switches

Looked at VLAN trunking and seen how to setup trunks between switches and routers

Looked at switch port security features Looked at the Virtual Trunking Protocol

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Objectives

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Be able to describe OSPF characteristics Be able to define what an area is Be able to describe what the OSPF metric is a

nd how it is used Be able to describe how a router using OSPF is

uniquely identified Be able to describe how a router using OSPF f

orm adjacencies with out routers also using OSPF

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OSPF characteristics

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Developed in reference to the limitation of RIP in large enterprise networks

Based on open standards Runs on most routers Uses the SPF algorithm to provide a loop-free

topology Fast convergence with triggered and incremental

updates via LSA’s (it’s a link state protocol) Classless protocol allowing for VLSM and route

summarisation However, requires more memory, extra CPU

processing power, careful design, complex to troubleshoot and configure (multi-area designs)

OSPF areas

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OSPF uses Autonomous Systems and areas Areas basically control when and how much

routing information is shared across a network Area 0 is the backbone Areas 1-65,535 are “areas off the backbone”

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OSPF areas

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Route summarisation would mean that Area 1, 2 and 3 do not need to know all the subnets in each others area, only their own areas

Metric

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OSPF uses cost Cost is the inverse of the bandwidth of a link

The faster the speed of the link, the lower the cost Preferred path is the one with the lowest cost

OSPF supports load-balancing Six equal-cost paths to a single destination

On synchronous serial links the bandwidth defaults to 1544Kbps despite the clock rate settings Bandwidth needs configuring specifically so that

load-balancing works properly This is important is there are multiple synchronous

serial paths to a destination and the paths have different clock rates

Router identities

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Each router in an OSPF network requires a unique ID

The ID is included in OSPF messages The ID is based on:

The highest IP address on any loopback interfaces, OR

The highest IP address on it’s active interfaces If there are no active interfaces or loopback

interfaces then the OSPF process will not start Loopback interfaces are recommended

because they are always up

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Finding neighbours

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OSPF uses LSA’s to learn about neighbours OSPF generates hello LSA’s every 10 seconds If a router does not receive a hello LSA within 40 seconds

from a neighbour it declares that neighbour dead OSPF routers build an adjacency that makes them

neighbours To do this the following must match on both routers:

The area number and its type The hello and dead interval timers The OSPF password, if configured The area stub tag

OSPF routers go through three states to form an adjacency: Down state – no exchanges Init state – destination router receives a hello and adds it to it’s

neighbour list Two-way state – new and destination routers exchange hello

packets

Designated and backup routers

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OSPF also uses a client/server design when establishing adjacencies

Each segment will have a designated router (DR) and a backup designated router (BDR)

New routers form adjacencies with the DR and BDR A router talks to the DR using 224.0.0.6 DR and BDR talk to routers using 224.0.0.5

The router with the highest router ID (IP address) becomes the DR BDR is based upon the next highest router ID (IP

address) If the DR fails the BDR takes over and another

router becomes the BDR

Summary

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OSPF is an open routing protocol and very popular

It works on most routers It uses areas to control how much routing

information is passed around a network It uses cost (an inverse of bandwidth) as it’s

metric It uniquely identifies a router using either the

highest IP address on a loopback interface or the highest IP address on an active interface

Using a loopback address for identity purposes is recommended

Tutorial

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Click on the icon below to run the tutorial and work through it until completion

Tutorial

So, what do you know now?

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How does a router in an OSPF network become a designated router?

What happens if a designated router fails How often are OSPF LSA hello messages sent? What happens if a neighbour doesn't receive a

hello LSA packet from it’s neighbour after 40 seconds?

What must be configured to prevent errors with the load balancing feature of OSPF?

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How all this relates to the assignment

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You will need to know how to configure OSPF for the case study and the skills test

You need to be able to perform these configuration on physical Cisco kit

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Questions...

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...are there any?

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End!