Tài liệu học CCNA kỳ 3 ccna3 mod2 SingleAreaOSPF

1 Module 2 - Single Area OSPF CCNA 1 version 3.1 Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com Contents • Link-state Routing Protocol • Single Area OSPF Concepts • Single Area OSPF Configuration 2 Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com Introduction • The two main classes of interior gateway routing protocols (IGP) are distance vector and link-state. • Link-state routing algorithms, also known as shortest path first (SPF) algorithms, maintain a complex database of topology information. A link-state routing algorithm maintains full knowledge of distant routers and how they interconnect. • Link-state routing protocols have some advantages over distance vector: efficient use of bandwidth and faster convergence, maintains full knowledge of distant routers, suitable for a large network • The basics of OSPF are covered in this module (single area) Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com Overview of link- state routing Let’s compare two types of routing protocol 3 Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com Link-state routing protocol features •Link-state routing protocols collect routing information from all other routers in the network or within a defined area of the network. •Once all of the information is collected, each router, independently of the other routers, calculates its best paths to all destinations in the network. •Each router maintains its own view of the network so it is less likely to propagate incorrect information provided by any of its neighboring routers •Each router keeps track of the state or condition of its directly connected neighbors by multicasting hello packets. Each router also keeps track of all the routers in its network or area of the network by using link-state advertisements (LSAs). The hello packets contain information about the networks that are attached to the router. Some functions of Link State •Respond quickly to network changes •Send triggered updates only when a network change has occurred •Send periodic updates known as link-state refreshes •Use a hello mechanism to determine the reachability of neighbors Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com How routing information is maintained Link-state routing uses the following features (cover detail in next pages): 1) Link-state advertisements (LSAs) 2) A topological database 3) The shortest path first (SPF) algorithm 4) The resulting SPF tree 5) A routing table of paths and ports to each network to determine the best paths for packets 4 Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com How routing information is maintained Link state = link + state Link = interface of a router State = description of an interface and the relationship to its neighboring routers (IP addr. , subnet mask, type of network to which it is connected, routers connected to that network) Link state 1 Link state 2 …………… Link state n Link-state data base / topological database Dijkstra shortest path first (SPF) algorithm Router A LSAs LSAs Router A Select the shortest path from root (router A) to one destination, use this list of paths to create routing table Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com How routing information is maintained when a failure is occur s Network 1 is down LSAs L S A s LSAs LSAs L S A s 1. When a failure occurs in the network, such as a neighbor becomes unreachable 2. Link-state protocols flood LSAs using a special multicast address throughout an area. 3. Each link-state router takes a copy of the LSA, updates its link-state/topological database. 4. The link-state router will then forward the LSA to all neighboring devices. 5. LSAs cause every router within the area to recalculate routes. Because LSAs need to be flooded throughout an area Note: all routers within that area need to recalculate their routing tables ==> the number of link-state routers that can be in an area should be limited Area x 5 •Link-state routing algorithms have the following characteristics: 1.They are known collectively as shortest path first (SPF) protocols. 2.They maintain a complex database of the network topology. 3.They are based on the Dijkstra algorithm. •Maintain full knowledge of the network routers as well as how they interconnect (achieved through the exchange of LSAs) •Topological database is built from all received LSAs •LSA exchange is triggered by an event in the network instead of periodic updates (fast convergence) Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com Compare with RIPv1/IGRP, Link-state protocols: 1.Use cost metrics to choose paths through the network (The cost metric reflects the capacity of the links) 2.Use triggered, flooded updates which can generally leads to fast convergence times. 3.Each router has a complete and synchronized picture of the network. Therefore, it is very difficult for routing loops to occur 4.Routers always use the latest set of information on which to base their routing decisions because LSAs are sequenced and aged 5.Need careful network design to have small database 6.Every router is capable of knowing full network knowledge, so easier for troubleshooting 7.Support CIDR and VLSM Distance vector vs. Link state 6 Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com Disadvantages of link-state routing protocols • They require more memory and processing power • They require strict hierarchical network design, so that a network can be broken into smaller areas to reduce the size of the topology tables. • They require an administrator with a good understanding of link-state routing. • They flood the network with LSAs during the initial discovery process, which can significantly decrease the capability of the network to transport data. This flooding process can noticeably degrade the network performance depending on the available bandwidth and the number of routers exchanging information Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com •Link-state routers advertise (LSA) the states of their links to all other routers in the area (DV protocols learn routes and then send these routes to directly connected) •Link-state routers can form special relationships with their neighbors and other link-state routers. •The initial flood of LSAs provides routers with the information that they need to build a link-state database. •Routing updates occur only when the network changes. •If there is no changes, the routing updates occur after a specific interval. •If the network changes, a partial update is sent immediately. •The partial update only contains information about links that have changed, not a complete routing table. Distance vector vs. Link state When a change occurs, link-state routers are all notified simultaneously by the partial update. (Distance vector routers wait for neighbors to note the change, implement the change, and then pass it to the neighboring routers) 7 Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com •OSPF is based on open standards. It is described in several standards of IETF (The most recent description is RFC 2328) •OSPF can be used and configured as a single area for small networks •OSPF routing scales to large networks if hierarchical network design principles are used Single Area OSPF Concepts: Overview Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com Link-state routers need to identify neighboring routers and then communicate with the identified neighbors Single Area OSPF Concepts: OSPF terminology 8 Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com •Information is gathered from OSPF neighbors about the status, or links, of each OSPF router. •This information is flooded to all its neighbors (Flooding is a process that sends information out all ports, with the exception of the port on which the information was received) •An OSPF router advertises its own link states and passes on received link states. Single Area OSPF Concepts: Link-State Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com •A collection of networks and routers that has the same area ID. •Each router within an area has the same link-state information •A router within an area is called internal router Single Area OSPF Concepts: OSPF area terminology 9 Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com •The routers process the information about link-states and build a link-state database. •Every router in the OSPF area will have the same link-state database. •Every router has the same information about the state of the links and the neighbors of every other router. Single Area OSPF Concepts: Topology database Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com •Cost is a value which is assigned to a link •The cost is based on the bandwidth of the link Single Area OSPF Concepts: Link cost 10 Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com •Forwarding database is routing table •Forwarding database is generated after SFP algorithm is run on its own link- state database •Routing table (forwarding database) on each router is unique •Routing table is collection of best paths to destinations (if there are many paths to one destination, only the best path is put to routing table) Single Area OSPF Concepts: Forwarding database Họcviệnmạng Cisco Bách Khoa - Website: www.bkacad.com •Adjacencies database is a listing of all the neighbors to which a router has established a bi-directional communication Single Area OSPF Concepts: Adjacencies database [...]... the default time is 30 s Học viện mạng Cisco Bách Khoa - Website: www.bkacad.com OSPF Hello protocol Common OSPF header data Authentication type Contains the authentication type All OSPF protocol exchanges are authenticated The authentication type is configurable on per-area basis Checksum - Checksum for the entire packet contents Authentication - Contains authentication information Học viện mạng Cisco... used to manually select the DR and BDR Học viện mạng Cisco Bách Khoa - Website: www.bkacad.com 17 Steps in the operation of OSPF OSPF routers progress through the following five distinct steps of operation: 1 Establish router adjacencies 2 Elect a DR and BDR, if necessary 3 Discover routes 4 Select the appropriate routes to use 5 Maintain routing information Học viện mạng Cisco Bách Khoa - Website:... own multicast address, 224.0.0.6 Non DR and non BDR routers send their LSUs to 224.0.0.6, or all DR and BDR routers Học viện mạng Cisco Bách Khoa - Website: www.bkacad.com 20 Steps in the operation of OSPF 2 3 1 On multiaccess network, member router (DRother) only update link-state to DR Học viện mạng Cisco Bách Khoa - Website: www.bkacad.com Steps in the operation of OSPF When the DR receives, it acknowledges... timers are not the same Interfaces are on different network types Authentication passwords or keys are different Học viện mạng Cisco Bách Khoa - Website: www.bkacad.com 28 Verifying the OSPF configuration Học viện mạng Cisco Bách Khoa - Website: www.bkacad.com Verifying the OSPF configuration Học viện mạng Cisco Bách Khoa - Website: www.bkacad.com 29 Summary • • • • • • • • • • • • • • • • • • The features... header, in bytes •Router ID - Identifies the source of the packet (a 32 -bit number The highest IP address on the router is used as the ID If a loopback address is configured, this will be used, even if it is not the highest address) •Area ID - Identifies the area to which the packet belongs All OSPF packets are associated with a single area Học viện mạng Cisco Bách Khoa - Website: www.bkacad.com OSPF Hello... information exchange •On a network a non-DR/BDR router only exchange routing information with DR/BDR Học viện mạng Cisco Bách Khoa - Website: www.bkacad.com DR/BDR on point-to-point network On point-to-point networks only two nodes exist and no DR or BDR is elected Both routers become fully adjacent with each other Học viện mạng Cisco Bách Khoa - Website: www.bkacad.com 14 OSPF network types OSPF interfaces... only if an adjacency is formed between neighbors Học viện mạng Cisco Bách Khoa - Website: www.bkacad.com OSPF network types Point-to-multipoint, can be configured on an interface by an administrator •In a multi-access network, the number of routers that will be connected in advance is unknown •In point-to-point networks, only two routers can be connected Học viện mạng Cisco Bách Khoa - Website: www.bkacad.com... routers in that network Học viện mạng Cisco Bách Khoa - Website: www.bkacad.com Steps in the operation of OSPF •Upon receiving an LSU that includes new information, an OSPF router updates its link-state database It then runs the SPF algorithm using the new information to recalculate the routing table •After the SPF hold timer expires, the router switches over to the new routing table Học viện mạng Cisco... subnet mask The wildcard mask represents the links or host addresses that can be present in this segment Area IDs can be written as a whole number or dotted decimal notation Học viện mạng Cisco Bách Khoa - Website: www.bkacad.com 23 Configuring OSPF loopback address and router priority •When the OSPF process starts, the Cisco IOS uses the highest local active IP address as its OSPF router ID If there... OSPF priority values Học viện mạng Cisco Bách Khoa - Website: www.bkacad.com 24 Configuring OSPF loopback address and router priority •Modify the OSPF priority by entering global interface configuration ip ospf priority command on an interface that is participating in OSPF •The command show ip ospf interface will display the interface priority value as well as other key information Học viện mạng Cisco