LAN Switching and WAN Networks

1. LAN Switching and WAN Networks Topic 6 - OSPF

2. What we have done so far! • 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 *** Richard Hancock

3. Objectives • Be able to describe OSPF characteristics • Be able to define what an area is • Be able to describe what the OSPF metric is and 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 form adjacencies with out routers also using OSPF *** Richard Hancock

4. OSPF characteristics • 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) Richard Hancock

5. OSPF areas • 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” *** Richard Hancock

6. OSPF areas • 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 Richard Hancock

7. Metric • 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 Richard Hancock

8. Router identities • 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 *** Richard Hancock

9. Finding neighbours • 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 adjacencythat 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 Richard Hancock

10. Designated and backup routers • 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 Richard Hancock

11. Summary • 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 Richard Hancock

12. Tutorial • Click on the icon below to run the tutorial and work through it until completion Richard Hancock

13. So, what do you know now? • 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? *** Richard Hancock

14. How all this relates to the assignment • 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 *** Richard Hancock

15. Questions... • ...are there any? Richard Hancock

16. End! Richard Hancock