CCNA 3 Week 2 Link State Protocols OSPF

Copyright © 2005 University of Bolton Distance Vector vs Link State Distance Vector –Copies Routing Table to neighbours –Updates frequently –Distance metrics e.g. hop count –Views network from prospective of neighbours –Slow to converge –Loops a problem –Easy to configure –Consumes bandwidth Link State –Uses shortest path –Updates event triggered –Sends link-state packets to all –Common view of network –Fast convergence –Less vulnerable to loops –Harder to Configure –Requires more memory and CPU power

Copyright © 2005 University of Bolton Link State Features Router learn network info from ALL routers in a net Routers multicast frequent hello packets containing details of directly connected networks Link-State Advertisments keep track of other routers Information used by router to calculate Shortest Path to each network

Copyright © 2005 University of Bolton Example Topology

Copyright © 2005 University of Bolton Maintaining Routing Information When a failure occurs, LSAs are multicast –Routers forward multicasts on all ports except receiver Each Link-State router updates its topology database with new network state and recalculates A link = an interface Link-State info includes: IP Addr & subnet, net type, connected routers Best routes calculated from topology database and placed in routing table

Copyright © 2005 University of Bolton Network View

Copyright © 2005 University of Bolton Advantages & Disadvantages Advantages –Fast convergence: changes reported at once –Robust against loops –Routers all know topology –Link-state packets sequenced and aged –Careful design minimises link-state table size Disadvantages –Significant RAM and CPU –Network design follows strict rules –Network admin must be knowledgable (you guys!) –Initial flooding impedes performance

Copyright © 2005 University of Bolton Comparison After initial flood, routing table only changes if net conditions change –The partial update only contains information about links that have changed. –Reduces network load, faster propagation of changes Link-state protocols support CIDR and VLSM. In fact, link-state protocols generally outperform distance vector protocols on any size network. –Not always implemented due to hardware requirements and admin expertise

OSPF Open Shortest Path First

Copyright © 2005 University of Bolton Introduction Open non-proprietary standard protocol for TCP/IP Widely used in industry Preferable to RIP v1 or v2 because metrics are better than simple hop count Uses a hierarchical approach, with different areas defined within a network

Copyright © 2005 University of Bolton OSPF Terminology Link: an interface on a routerLink-state: The status of the link between two routersTopological Database: The status of all routers on the network Area: each router within an area has the same link-state information Cost: assigned to each link, based on bandwidthRouting Table: generated when routing algorithm runs on topological database Adjacency Database: List of routers communicating directlyDesignated Router (DR): Elected to represent all routers on the network

Copyright © 2005 University of Bolton OSPF vs Distance Vector Speed of convergence Support for Variable Length Subnet Mask (VLSM) Network size Path selection Grouping of members

Copyright © 2005 University of Bolton SPF Algorithm

Copyright © 2005 University of Bolton SPF Algorithm

Copyright © 2005 University of Bolton OSPF Network Types OSPF interfaces automatically recognize 3 types of networks: –Broadcast multi-access, such as Ethernet –Point-to-point networks –Nonbroadcast multi-access (NBMA), e.g. Frame Relay Not desirable for every router to be adjacent to every other in a broadcast network – overhead Pick Designated Router (and Backup)

Copyright © 2005 University of Bolton Use of Designated Router Routers send their updates to the designated router (Multicast to ) Designated Router updates all routers on network using As Designated Router is single point of failure, Backup is also established

Copyright © 2005 University of Bolton Hello Packets Small layer 3 packets Sent every 10 seconds to multicast Used to ensure neighbours still running Protocol used in election of DR

Copyright © 2005 University of Bolton Protocol Steps Use hello packets to announce selves Develop adjacency relationships with neighbours Use LSA and LSU packets to learn about topology Calculate routing tables from topology If changes occur, recalculate

Copyright © 2005 University of Bolton Configure R1(config)# router ospf R1(config-router)# network area

Copyright © 2005 University of Bolton Need to add loopback address No active interfaces means router drops out of OSPF Logical loopback keeps process available If present, loopback used as router ID R1(config)# interface loopback 1 R1(config-if)# ip address

Copyright © 2005 University of Bolton Link costs OSPF uses 10 8 /bandwidth as link cost Set bandwidth of interface if needed: R1(config-if)# bandwidth 56 May be necessary if mixed network uses other calculations R1(config-if)# ip ospf cost

Copyright © 2005 University of Bolton Authentication Can require common password for updates May be encrypted using MD5 See lab for details

Copyright © 2005 University of Bolton Timers OSPF routers must use the same timer values Router(config-if)#ip ospf hello-intervalseconds Router(config-if)#ip ospf dead-intervalseconds

Copyright © 2005 University of Bolton Default Route On the router connected to default connection: Router(config)#ip route [interface | next-hop address ] Must then instruct router to propagate default route Router(config-router)#default-information originate