LAN Switching and WAN Networks Topic 6 - OSPF

What we have done so far! 18/09/2015Richard Hancock2  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 ***

Objectives 18/09/2015Richard Hancock3  Be able to describe OSPF characteristics Be able to describe OSPF characteristics  Be able to define what an area is 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 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 is uniquely identified  Be able to describe how a router using OSPF form adjacencies with out routers also using OSPF Be able to describe how a router using OSPF form adjacencies with out routers also using OSPF ***

OSPF characteristics 18/09/2015Richard Hancock4  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 18/09/2015Richard Hancock5  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” ***

OSPF areas 18/09/2015Richard Hancock6  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 18/09/2015Richard Hancock7  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 18/09/2015Richard Hancock8  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 ***

Finding neighbours 18/09/2015Richard Hancock9  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 18/09/2015Richard Hancock10  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  DR and BDR talk to routers using  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 18/09/2015Richard Hancock11  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 18/09/2015Richard Hancock12  Click on the icon below to run the tutorial and work through it until completion

So, what do you know now? 18/09/2015Richard Hancock13  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? ***

How all this relates to the assignment 18/09/2015Richard Hancock14  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 ***

Questions... 18/09/2015Richard Hancock15 ...are there any?

18/09/2015Richard Hancock16 End!