Transcript OSPF
LAN Switching and WAN Networks Topic 6 - OSPF 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 *** 2 Richard Hancock 13/04/2015 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 *** 3 Richard Hancock 13/04/2015 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) 4 Richard Hancock 13/04/2015 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” *** 5 Richard Hancock 13/04/2015 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 6 Richard Hancock 13/04/2015 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 7 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 13/04/2015 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 *** 8 Richard Hancock 13/04/2015 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 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: 9 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 13/04/2015 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 The router with the highest router ID (IP address) becomes the DR A router talks to the DR using 224.0.0.6 DR and BDR talk to routers using 224.0.0.5 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 10 Richard Hancock 13/04/2015 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 11 Richard Hancock 13/04/2015 Tutorial Click on the icon below to run the tutorial and work through it until completion 12 Richard Hancock 13/04/2015 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? *** 13 Richard Hancock 13/04/2015 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 *** 14 Richard Hancock 13/04/2015 Questions... ...are there any? 15 Richard Hancock 13/04/2015 End! 16 Richard Hancock 13/04/2015