1 CCNA 3 v3.1 Module 2

2 CCNA 3 Module 2 Single Area OSPF

333 Distance Vector Routing RIPv1, RIPv2, IGRP, EIGRP Interior gateway routing protocol Sends copies of routing tables to neighbors View network from neighbors perspective Updates at specified times (30 secs,90 secs) Slow convergence High bandwidth usage

444 Link State Protocols Examples OSPF (Open Shortest Path First) IS-IS(Intermediate System to Intermediate System Also known as Shortest Path First (SPF) Dijkstra algorithm calculates loop free topology Interior gateway protocol Sends routing updates to all routers within the autonomous systems All routers have a complete view of the network topology Routers maintain a complex database of the topology Link state database is identical on all routers Routers have full knowledge of distant routers and how they interconnect Require more memory and processing

555 Triggered updates Contain changes only Occur when topology change occurs Updates sent to all routers Triggered updates result in Fast convergence Efficient use of bandwidth OSPF configuration OSPF routing process must be enabled Networks must be identified

666 Link State Protocol Send triggered updates when a network change has occurred periodic updates known as link-state refreshes Each router keeps track of the state or condition of its directly connected neighbors by multicasting hello packets all the routers in its network using link-state advertisements (LSAs) Builds database about the network using hello information and LSAs Calculates shortest route to each network using shortest path first (SPF) algorithm Stores this route information in its routing table

777 Topological Database Every router advertises directly connected networks via Link State Advertisements Every router has it’s own view of the network it builds a ‘topological database’ Router A is aware of 2 paths to this provides redundancy should one of the routers fail

888 Advantages of Link State Protocols Use cost metrics to choose paths Cost metric reflects the capacity of the links Fast convergence because Triggers flooded updates No routing loops - router knows network topology Routing decision based on up to date of information LSAs are sequenced and aged Minimised Link-state database sizes because of Dijkstra calculations and faster convergence Supports Classless interdomain routing (CIDR) Variable-length subnet masking (VLSM)

999 Disadvantages of LSP More memory and processing power required In particular during initial setup Require strict hierarchical network design to reduce the size of the topology tables Administrator must have a good understanding of link-state routing Flooding of LSAs during the initial discovery process decrease network transport capability Uses the available bandwidth

10 Compare Distance Vector and Link State

11 OSPF Non-Proprietary protocol Preferred to RIP because it is scalable Open Standard - IETF RFC 2328 Link State routing protocol Interior Gateway Protocol for Autonomous systems Metric based on bandwidth Supports VLSM OSFP can use ‘areas’ for hierarchical design Multiple areas connect to a distribution area, area 0, also called the backbone

12 OSPF Terminology Link an interface on the router Link State Status of a link Relationship to neighbouring router Flooding sends information out all ports, except the port the information was received Link State Database – topological database List of information about all other routers Shows the internetwork topology

13 Area A collection of networks and routers Each router in the area has the same link-state information A router within an area is an internal router Area 0 - Backbone Cost Based on bandwidth Transmission speed Routing table Generated when SPF algorithm is run on link-state dbase Unique to each router

14 Adjacent database List of all directly connected routers Designated Router (DR) / Backup Designated Router (BDR) a router elected by all others to represent the network area All LSA sent to DR/BDR instead of to every single router Multicast address Reduces overhead of LSA updates Standard on multi-access networks DR is single point of failure – solution is BDR

15 OSPF Network Types Multi-access Number of routers is unknown Info sent to DR and BDR Point-to-Point 2 routers DR and BDR not elected NBMA Info sent to DR and BDR

16 OSPF Hello Protocol Hello protocol sends hello packets at Router starts up To initiate new adjacency Regular intervals ensure neighbor still functioning Every 10 seconds on broadcast multi-access and point-to- point networks Every 30 seconds to NBMA such as frame relay Hello packets have a L3 multicast address This address is all OSPF routers Hello packet has OSPF packet header with a type field of 1

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26 Configuring OSPF Only 1 area Number is 0 Multiple areas Number between 0 to 65,535 Must connect to area 0 Router(config)#router ospf process-id (1 to 65,535) Router(config-router)#network network-address wildcard-mask area-id Router(config)#router ospf 1 Router(config-router)#network area 0

27 Configure OSPF loopback address OSPF must have an active interface at all times Not always possible if ethernet / serial interface are down Create a loopback interface (logical interface) to overcome this OSPF uses the loopback interface address as the router ID (this overrides local IP address values) If there is more than one loopback interface highest loopback IP address is taken as router ID Loopback configuration Commands router(config)#interface loopback number Router(config-if)#ip address ip-address subnet-mask Subnet mask is always To turn off a loopback address Router(config)#no interface loopback number

28 Setting OSPF Priority Priorities can be set from 0 to prevents that router from being elected Highest OSPF priority will win the election for DR Configuring OSPF priority Router(config)#interface fa0/0 Router(config-if)#ip ospf priority 50 Router(config0if)#end

29 OSPF Cost Metric OSPF uses Cost metric for path selection Cost is based on bandwidth Default bandwidth is 1.544Mbps – cost is 64 Cost can be 1 (100Mbps) to Cost is 10 8 ÷ bandwidth Configuration Router(config)#interface serial 0/0 Router(config-if)#bandwidth 64 Router(config-if)#ip ospf cost number

30 OSPF Authentication To guarantee trust OSPF uses authenticatication Authentication key shared between routers password can be up to eight characters Router(config-if)#ip ospf authentication-key password After password is configured, enable authentication Router(config-router)#area area-number authentication This is sent in plain text – easy to decode To send encrypted authentication Router(config-if)#ip ospf message-digest-key key-id md5 encryption-type key MD5 keyword is the message-digest hashing algorithm to use Encryption type is the type of encryption where 0 means none and 7 means proprietary. key-id 1 through 255 key is an alphanumeric password up to sixteen characters Neighbor routers must use the same key identifier with the same key value Router(config-router)#area area-id authentication message-digest

31 OSPF Default Route Used to reach networks outside the domain gateway of last resort Command Router(config)#ip route [interface | next-hop address ] referred to as the quad-zero route

32 What Causes a failure to establish a neighbor relationship Hellos are not sent from both neighbors. Hello and dead interval timers are not the same. Interfaces are on different network types. Authentication passwords or keys are different. Other possibilities All interfaces haven’t correct addresses or subnet mask network area statements hasn’t correct wildcard masks network area statements interfaces not in correct area

33 Verifying OSPF Show ip protocol Displays, times, filters, metrics, networks Show ip route Displays routes known and how they were learned Show ip ospf interfaces Verifies interfaces configured in an area Show ip ospf Displays how many times SPF has been executed Show ip ospf neighbor details Displays a list of neighbors, priorities and states Show ip ospf database Displays the contents of the topological database