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Routers and Routing Basics - Routing and Routing Protocols

Routed vs. Routing Protocols

• Routed protocols can be routed using their Layer 3 addressing scheme.

– IP: Internet Protocol

– Novell IPX: Internetwork Packet eXchange

– AppleTalk

• Routing protocols are how routers communicate with each other, to determine paths and share routing tables.

– RIP: Routing Information Protocol

– IGRP: Interior Gateway Routing Protocol

– OSPF: Open Shortest Path First

Introduction to Routing

• Routing is about making path determination decisions based upon logical addresses.

• Routing table entries in routers are crucial for effective path determination. Routes can be learnt by routers, or set manually.

• Static routing

– Administrator manually enters all routes.

– Suitable for small networks and backup routes.

– Less traffic as no routing protocols enabled, and no updates.

– Becomes a problem if a link goes down.

• Dynamic routing

– Routers learn the topology from neighbours.

– Quick to adapt to changing topologies.

– Works well if a link goes down.

– Has greater overheads than static routing: more routing updates means more network traffic.

Configuring Static Routes

• Configure static route.

– Router(config)# ip route

• Include the optional administrative distance.

– Indicates route reliability.

– Lower values indicate greater reliability.

– If a static route is being configured as a backup for dynamic routes, use a greater admin distance.

• A router only installs a static route in its routing table if the destination is reachable.

– Router# show ip route


Configuring Default Routes

• A default route is a special static route to direct traffic whose destination doesn’t match any dynamic or static routes in the routing table.

– Use the ip route command in global config mode, then specify both the destination network and subnet mask as 0.0.0.0.

Basics of Routing Protocols

• A routing protocol learns all available routes and selects the best routes for use in the routing table.

• The algorithm used by the dynamic routing protocol determines how quickly the routing table accurately reflects the internetwork topology.

– When all routers share the same view of the internetwork topology, then the routers have converged.

– Algorithms mostly classified as distance vector or link-state.

Distance Vector Protocols

• Measures distance to other routers by number of hops.

• Periodically sends entire routing table to directly connected routers.

– This is how all routers accumulate distance vectors to other routers in the network.

– Routing updates are sent at regular intervals.

– Problem is a router only sees the topology of the entire internetwork from its own, and its neighbours’, perspectives. Distant routers usually remain unknown.

• Examples include RIP, IGRP, BGP.

Link-State Protocols

• Routers send Link-State Advertisements (LSAs) to directly connected routers upon topology change. An example is the OSPF routing protocol.

• A router adds information it receives in an LSA to its topological database, then passes on any new information in a new LSA.

– Topological database is updated with most recent information, helping create an accurate common view of the internetwork.

• The router uses the topological database with the Shortest Path First (SPF) algorithm to sort all available paths from shortest to longest into a shortest Path First tree.

– Final routing table is the result of applying SPF to the topological database.

Routing: Inside and Out

• Interior Gateway Protocols

– Designed to find the most rapid and reliable path through an Autonomous System (AS).

• Metrics used for path determination are of paramount importance to IGPs.

– An AS divides internetworks into smaller, more manageable networks under a common administration.

• Exterior Gateway Protocols

– Designed to link independent ASesthat use IGPs and are under different administrations, such as BGP routing protocol.

– Requires a list of networks to advertise as directly reachable.

Routing Protocols

• Routing Information Protocol (RIP)

– Distance vector interior routing protocol, specified in RFC 1058.

– Only metric used is hop count, max of 15.

– Routing updates broadcast every 30 secs.

• Interior Gateway Routing Protocol (IGRP)

– Distance vector interior routing protocol, developed by Cisco.

– Composite metric built from bandwidth, load, delay, reliability.

– Uses Autonomous Systems, routing updates broadcast every 90 secs.

• Open Shortest Path First (OSPF)

– Link-state interior routing protocol, specified in RFC 2328.

– Builds a topological database using the SPF algorithm.

– Uses process numbers, routing updates triggered by topology changes.

• Enhanced Interior Gateway Routing Protocol (EIGRP)

– Hybrid distance vector/link-state interior routing protocol by Cisco.

– Uses Diffused Update Algorithm (DUAL) to calculate the shortest path.

– Uses Autonomous Systems, routing updates triggered by topology changes.

• Border Gateway Protocol (BGP)

– Distance vector exterior routing protocol.

– Used to route Internet traffic between autonomous systems.

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