© 2002, Cisco Systems, Inc. All rights reserved.

2 Frank Mann CCAI- CCNA Module 6: Routing and Routing Protocols

6.1 Introduction to Static Routing Students completing this module should be able to: Explain the significance of static routing Configure static and default routes Verify and troubleshoot static and default routes Identify the classes of routing protocols Identify distance vector routing protocols Identify link-state routing protocols Describe the basic characteristics of common routing protocols Identify interior gateway protocols Identify exterior gateway protocols Enable Routing Information Protocol (RIP) on a router

6.1.1 Introducing routing Routing is the process that a router uses to forward packets toward the destination network. A router makes decisions based upon the destination IP address of a packet. When routers use dynamic routing, this information is learned from other routers. When static routing is used, a network administrator configures information about remote networks manually.

6.1.2 Static route operation Static route operations can be divided into these three parts: Network administrator configures the route Router installs the route in the routing table Packets are routed using the static route The administrator could enter either of two commands to accomplish this objective. Specify the outgoing interface. Specify the next-hop IP address of the adjacent router.

6.1.2 Static route operation

Use a local interface as the gateway

Using the next hop

6.1.4 Configuring default route forwarding Default routes are used to route packets with destinations that do not match any of the other routes in the routing table. Routers are typically configured with a default route for Internet-bound traffic, since it is often impractical and unnecessary to maintain routes to all networks in the Internet.

6.1.4 Configuring default route forwarding Identify another place that needs a default route configured!

6.1.4 Configuring default route forwarding A default route on both Sterling and Waycross will provide routing for all packets that are destined for non-directly connected networks

6.1.5 Verifying static route configuration After static routes are configured it is important to verify that they are present in the routing table and that routing is working as expected. The command show running-config is used to view the active configuration in RAM to verify that the static route was entered correctly. The show ip route command is used to make sure that the static route is present in the routing table.

6.1.6 Troubleshooting static route configuration

6.1.6 Troubleshooting static route using Ping and Traceroute

15 Frank Mann CCAI- CCNA 6.2 Dynamic Routing Overview

6.2.1 Introduction to routing protocols A routing protocol is the communication used between routers. A routing protocol allows one router to share information with other routers regarding the networks it knows about as well as its proximity to other routers. The information a router gets from another router, using a routing protocol, is used to build and maintain a routing table. Examples of routing protocols are: Routing Information Protocol (RIP) Interior Gateway Routing Protocol (IGRP) Enhanced Interior Gateway Routing Protocol (EIGRP) Open Shortest Path First (OSPF)

6.2.1 Routed versus Routing protocols

6.2.2 Autonomous systems An autonomous system (AS) is a collection of networks under a common administration sharing a common routing strategy. To the outside world, an AS is viewed as a single entity.

6.2.2 American Registry of Internet Numbers The American Registry of Internet Numbers (ARIN), a service provider, or an administrator assigns an identifying number to each AS. This autonomous system number is a 16 bit number. Routing protocols, such as Cisco’s IGRP, require assignment of a unique, autonomous system number

6.2.3 Purpose of a routing protocol and autonomous systems When all routers in an internetwork are operating with the same knowledge, the internetwork is said to have converged. Fast convergence is desirable because it reduces the period of time in which routers would continue to make incorrect routing decisions

6.2.4 Identifying the classes of routing protocols Most routing algorithms can be classified into one of two categories: distance vector link-state The distance vector routing approach determines the direction (vector) and distance to any link in the internetwork. The link-state approach, also called shortest path first (SPF), recreates the exact topology of the entire internetwork

6.2.4 Identifying the classes of routing protocols

6.2.5 Distance vector routing protocol features Distance vector routing algorithms pass periodic copies of a routing table from router to router. These regular updates between routers communicate topology changes. Distance vector based routing algorithms are also known as Bellman-Ford algorithms.

Periodic updates to Neighboring routers

Routing updates occur when topology changes Distance vector algorithms call for each router to send its entire routing table to each of its adjacent neighbors.

Routing metrics The routing tables include information about the total path cost as defined by its metric and the logical address of the first router on the path to each network contained in the table.

6.2.6 Link-state routing protocol features The second basic algorithm used for routing is the link-state algorithm. Link-state algorithms are also known as Dijkstras algorithm or as SPF (shortest path first) algorithms. Link-state routing algorithms maintain a complex database of topology information. The distance vector algorithm has nonspecific information about distant networks and no knowledge of distant routers. A link-state routing algorithm maintains full knowledge of distant routers and how they interconnect.

Link State Concepts

Link State Routing Process Link-state routing uses: Link-state advertisements (LSAs) – A link-state advertisement (LSA) is a small packet of routing information that is sent between routers. Topological database – A topological database is a collection of information gathered from LSAs. SPF algorithm – The shortest path first (SPF) algorithm is a calculation performed on the database resulting in the SPF tree. Routing tables – A list of the known paths and interfaces.

Link State Routing Process

Link State Update process

6.3 Routing Protocols Overview A router determines the path of a packet from one data link to another, using two basic functions: A path determination function A switching function

6.3 Routing Protocols Overview A router determines the path of a packet from one data link to another, using two basic functions: A path determination function A switching function

Path determination Path determination occurs at the network layer based on the IP Address of the destination. The path determination function enables a router to evaluate the paths to a destination and to establish the preferred handling of a packet. The router uses the routing table to determine the best path and proceeds to forward the packet using the switching function

6.3.1 Path determination

6.3.2 Routing configuration Enabling an IP routing protocol on a router involves the setting of both global and routing parameters. Global tasks include selecting a routing protocol, such as RIP, IGRP, EIGRP or OSPF. The major task in the routing configuration mode is to indicate IP network numbers. Dynamic routing uses broadcasts and multicasts to communicate with other routers.

Routing Configuration

6.3.3 Routing protocols

Routing protocols At the Internet layer of the TCP/IP suite of protocols, a router can use an IP routing protocol to accomplish routing through the implementation of a specific routing algorithm. Examples of IP routing protocols include: RIP – A distance vector interior routing protocol IGRP – Cisco's distance vector interior routing protocol OSPF – A link-state interior routing protocol EIGRP – Cisco’s advanced distance vector interior routing protocol BGP – A distance vector exterior routing protocol

Routing Information Protocol Routing Information Protocol (RIP) was originally specified in RFC Its key characteristics include the following: It is a distance vector routing protocol. Hop count is used as the metric for path selection. If the hop count is greater than 15, the packet is discarded. Routing updates are broadcast every 30 seconds, by default.

Interior Gateway Routing Protocol Interior Gateway Routing Protocol (IGRP) is a proprietary protocol developed by Cisco. Some of the IGRP key design characteristics emphasize the following: It is a distance vector routing protocol. Bandwidth, load, delay and reliability are used to create a composite metric. Routing updates are broadcast every 90 seconds, by default.

Open Shortest Path First Open Shortest Path First (OSPF) is a nonproprietary link-state routing protocol. The key characteristics of OSPF are as follows: It is a link-state routing protocol. Open standard routing protocol described in RFC Uses the SPF algorithm to calculate the lowest cost to a destination. Routing updates are flooded as topology changes occur.

EIGRP EIGRP is a Cisco proprietary enhanced distance vector routing protocol. The key characteristics of EIGRP are as follows: It is an enhanced distance vector routing protocol. Uses load balancing. Uses a combination of distance vector and link-state features. Uses Diffused Update Algorithm (DUAL) to calculate the shortest path. Routing updates are broadcast every 90 seconds or as triggered by topology changes.

Border Gateway Protocol Border Gateway Protocol (BGP) is an exterior routing protocol. The key characteristics of BGP are as follows: It is a distance vector exterior routing protocol. Used between ISPs or ISPs and clients. Used to route Internet traffic between autonomous systems

Interior vs. Exterior

Administrative Distance: Ranking Routes

6.3.4 Autonomous systems and IGP versus EGP An exterior routing protocol must isolate autonomous systems. Remember, autonomous systems are managed by different administrations. Networks must have a protocol to communicate between these different systems. Autonomous systems have an identifying number, which is assigned to it by the American Registry of Internet Numbers (ARIN) or a provider. This autonomous system number is a 16-bit number. Routing protocols such as Cisco’s IGRP and EIGRP require that a unique, autonomous system number be assigned

6.3.5 Distance vector summary Distance vector algorithms (also known as Bellman-Ford algorithms) call for each router to send all or some portion of its routing table only to its neighbors. Distance vector algorithms perform routing decisions based upon information provided by neighboring routers. Distance vector protocols use fewer system resources but can suffer from slow convergence and may use metrics that do not scale well to larger systems. Distance vector protocols are based on finding the distance (number of hops) and vector (direction) to any link on the internetwork. The algorithms involve passing copies of a complete routing table from router to router on a periodic basis.

6.3.6 Link-state Link-state algorithms (also known as shortest path first algorithms) flood routing information to all routers in the internetwork that creates a map of the entire network. Each router sends packets to all its neighbors. These packets contain descriptions of the network or networks to which the router is linked. The routers assemble all the information into a complete view of the internetwork topology to calculate the shortest path to all known sites on the network. It then generates routing tables showing the best path for any destination on the network. Once converged, link state protocols use small update packets, which contain only changes rather than copies of the entire routing table. Update packets are passed across the network in event-triggered updates, so convergence is fast.

Labs Module 6: Routing and Routing Protocols Lab: Configuring Static Routes e-Lab: 6.1.2a Static Route Operation 6.1.2b Static Routes Configuring Static Routes Configuring Default Route Forwarding Verifying Static Route configuration Static Routes Routing Configuration