Chapter 3: Introduction to Dynamic Routing Protocols
This document is a sample chapter from the Network Academy'scourse booklet, "CCNA Exploration Course Booklet: Routing Protocols and Concepts, Version 4.0." The booklet is one of several supplemental materials used to complement/support the online course of the same name. The course teaches students how a router learns about remote networks and how it determines best path to those networks. The course content also covers both static and dynamic routing protocols.
The chapter covers the following subjects:
Introduction and Advantages
Classifying Dynamic Routing Protocols
- Administrative Distances
- Routing Protocols and Subnetting Activities
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Presentation on theme: "Chapter 3 Dynamic Routing Protocols CIS 82 Routing Protocols and Concepts Rick Graziani Cabrillo College Last Updated: 3/2/2008."— Presentation transcript:
1 Chapter 3 Dynamic Routing Protocols CIS 82 Routing Protocols and Concepts Rick Graziani Cabrillo College firstname.lastname@example.org Last Updated: 3/2/2008
2 2 Note This presentation will be updated prior to March. 25, 2008 The audio of the lecture for this presentation will be available on my web site after March. 25, 2008 My web site is www.cabrillo.edu/~rgraziani. For access to these PowerPoint presentations and other materials, please email me at email@example.com.
3 3 For further information This presentation is an overview of what is covered in the curriculum/book. For further explanation and details, please read the chapter/curriculum. Book: Routing Protocols and Concepts By Rick Graziani and Allan Johnson ISBN: 1-58713-206-0 ISBN-13: 978-58713- 206-3
4 4 Topics Introduction to Dynamic Routing Protocols Perspective and Background Network Discovery and Routing Table Maintenance Dynamic Routing Protocol Advantages Classifying Dynamic Routing Protocols IGP and EGP Distance Vector and Link- State Classful and Classless Convergence Metrics Purpose of the Metric Metrics and Routing Protocols Load Balancing Administrative Distance Purpose of Administrative Distance Dynamic Routing Protocols and Administrative Distance Static Routes and Administrative Distance Directly Connected Networks and Administrative Distance
5 Introduction to Dynamic Routing Protocols Perspective and Background Network Discovery and Routing Table Maintenance Dynamic Routing Protocol Advantages
6 6 Perspective and Background Dynamic routing protocols have evolved over several years As networks have evolved and become more complex, new routing protocols have emerged. Most institutions have migrated to new protocols, others are still in use. The first version of RIP was released in 1982, but some of the basic algorithms within the protocol were used on the ARPANET as early as 1969.
7 7 Perspective and Background Interior Routing Protocols or Interior Gateway Protocols (IGP) Distance Vector RIPv1 – Simple, Classful, limited metrics (hop count) RIPv2 – Simple, Classless, limited metrics (hop count) Cisco Proprietary IGRP – Simple, Classful, better metric (BW, delay, reliab., load) EIGRP – Simple, Classless, same metric, DUAL (backup routes) Link State OSPF – Perceived complex, classless, Cisco metric BW, IETF IS-IS - Perceived complex, classless, metric “default”, ISO Classful (does not support CIDR and VLSM) Classless (supports CIDR and VSLM)
8 8 Perspective and Background Exterior Routing Protocols or Exterior Gateway Protocols (EGP) Border Gateway Protocol (BGP) is now used between Internet service providers (ISP) as well as between ISPs and their larger private clients to exchange routing information. Path Vector routing protocol, metric – attributes (policies) Replaced EGP
9 9 Role of Dynamic Routing Protocol Dynamic Routing Protocols: Exchange of routing information between routers Dynamically learn information about remote networks Determines the best path to each network Adds routes to routing tables Automatically learn about new networks Automatically finds alternate paths if needed (link failure in current path)
10 10 Role of Dynamic Routing Protocol Compared to Static Routes: Advantages of Dynamic Routing Protocols: Less administrative overhead (change modifications) Disadvantage of Dynamic Routing Protocols More CPU and memory requirements This is not that big an issue in most networks and with modern routers. Configuration is less error-prone Scales better with larger networks “Less secure” if routing updates are sent unencrypted. Most networks use both dynamic and static routes
11 11 Purpose of Dynamic Routing Protocols A routing protocol is a set of processes, algorithms, and messages that are used to exchange routing information and populate the routing table with the routing protocol’s choice of best paths. Purpose: Discovering remote networks Maintaining up-to-date routing information Choosing the best path to destination networks Having the ability to find a new best path if the current path is no longer available Components of a routing protocol (depending upon the routing protocol): Data structures: Tables or databases for their operations, kept in RAM. Algorithm: An algorithm is a finite list of steps used in accomplishing a task. Routing protocols use algorithms for processing routing information and for best-path determination. Routing protocol messages: Discover neighboring routers Exchange routing information Learn and maintain accurate information about the network
12 12 Dynamic Routing Protocol Operation The operations of a dynamic routing protocol vary depending on the type of routing protocol, but in general: 1. The router sends and receives routing messages on its interfaces. 2. The router shares routing messages and routing information with other routers that are using the same routing protocol. 3. Routers exchange routing information to learn about remote networks. 4. When a router detects a topology change, the routing protocol can advertise this change to other routers.
13 13 Static Routing Usage, Advantages, and Disadvantages Primary uses: Smaller networks that are not expected to grow significantly. Routing to and from stub networks Default route
14 Classifying Dynamic Routing Protocols IGP and EGP Distance Vector and Link-State Classful and Classless Convergence
15 15 Classifying Routing Protocols Routing Protocols can be classified by: IGP or EGP Distance vector or link-state Classful or classless
16 16 IGP and EGP An autonomous system (AS)—otherwise known as a routing domain—is a collection of routers under a common administration. Company’s internal network An ISP’s network. Because the Internet is based on the autonomous system concept, two types of routing protocols are required: Interior routing protocols Exterior routing protocols
17 17 IGP and EGP Interior gateway protocols (IGP): Used for intra-autonomous system routing Routing inside an autonomous system Exterior gateway protocols (EGP): Used for inter-autonomous system routing Routing between autonomous systems
18 18 Distance Vector and Link-State Routing Protocols Interior gateway protocols (IGP) can be classified as two types: Distance vector routing protocols Link-state routing protocols
19 19 Distance Vector Routing Protocol Operation Distance vector Routes are advertised as vectors of distance and direction. Distance is defined in terms of a metric Such as hop count, Direction is simply the: nexthop router or exit interface. Typically use the Bellman-Ford algorithm for the best-path route determination
20 20 Distance Vector Routing Protocol Operation Routing protocol Does not know the topology of an internetwork. Only knows the routing information received from its neighbors. Like signposts along the path to the final destination.
21 21 Distance Vector Routing Protocol Operation Distance vector protocols work best in situations where: The network is simple and flat and does not require a hierarchical design. The administrators do not have enough knowledge to configure and troubleshoot link-state protocols. Specific types of networks, such as hub-and-spoke networks, are being implemented. Worst-case convergence times in a network are not a concern. More in Chapter 4.
22 22 Link-State Protocol Operation Link-state routing protocol can create a “complete view,” or topology, of the network. Like having a complete map of the network topology Link-state protocols are associated with Shortest Path First (SPF) calculations. A link-state router uses the link- state information to: Create a topology map Select the best path to all destination networks in the topology.
23 23 Link-State Protocol Operation Link-state protocols work best in situations where The network design is hierarchical, usually occurring in large networks. The administrators have a good knowledge of the implemented link-state routing protocol. Fast convergence of the network is crucial. More in later chapters.
24 24 Classful and Classless Routing Protocols All routing protocols can also be classified as either Classful routing protocols Classless routing protocols IPv6 routing protocols are classless
25 25 Classful Routing Protocols Classful routing protocols do not send subnet mask information in routing updates. The first routing protocols, such as RIP When network addresses were allocated based on classes. Class A, B, or C. Routing protocol did not need to include the subnet mask in the routing update. Network mask determined based on value of first octet of the network address.
26 26 Classful Routing Protocols Classful routing protocols do not include the subnet mask Therefore do not support VLSM and CIDR. All subnets within the same “major classful network address” must have the same mask. Other limitations to classful routing protocols, including: Inability to support discontiguous networks (later) More later! 172.16.0.0/16 Major Classful Network All /24 subnets
27 27 Classless routing Protocols Classless routing protocols include the subnet mask with the network address in routing updates. Today’s networks are no longer allocated based on classes Subnet mask cannot be determined by the value of the first octet. Classless routing protocols are required in most networks today because of their support for: VLSM CIDR Discontiguous networks. 172.16.128.0/30172.16.132.0/30 172.16.136.0/30 172.16.0.0/16 Major Classful Network /27 and /30 subnets
28 28 An important characteristic of a routing protocol: How quickly it converges when there is a change in the topology. Convergence is when the routing tables of all routers are at a state of consistency. The network has converged when all routers have complete and accurate information about the network. Convergence time is the time it takes routers to: share information calculate best paths update their routing tables. A network is not completely operable until the network has converged; therefore, most networks require short convergence times. R2’s Routing Table R1’s Routing Table R3’s Routing Table Dynamic Routing Protocols and Convergence
29 29 Dynamic Routing Protocols and Convergence Generally, convergence time: Slow: RIP and IGRP Faster: EIGRP, OSPF, and IS-IS R2’s Routing Table R1’s Routing Table R3’s Routing Table
30 Metrics Purpose of the Metric Metrics and Routing Protocols Load Balancing
31 31 Purpose of a Metric Metrics are a way to measure or compare. Determine which route is the best path. Assign costs to reach remote networks. Routing protocol learns multiple routes to the same destination. Metric is used to determine which path is most preferable ?
32 32 Purpose of a Metric Routing protocol metrics: RIP: Hop count IGRP and EIGRP: Bandwidth, delay, reliability and load OSPF (Cisco’s version): Bandwidth IS-IS: Four values (Cisco uses “default”) – Covered in CCNP BGP: Attributes – Covered in CCNP More later
33 33 Metric Parameters R1 to reach the 172.16.1.0/24 network. RIP: Fewest number of hops via R2. OSPF: Path with the highest cumulative bandwidth through R3. This results in faster packet delivery. 56 Kbps
34 34 Metric Field in the Routing Table The routing table displays the metric for each dynamic and static route. Static routes always have a metric of 0. Routing protocols install route in routing table with the lowest metric.
35 35 All routers running RIP R2 has a route to the 192.168.8.0/24 network that is 2 hops away. The 2 in the command output is where the routing metric is displayed. 120 is the Administrative Distance (later) R2# show ip route Gateway of last resort is not set R 192.168.1.0/24 [120/1] via 192.168.2.1, 00:00:24, Serial0/0/0 C 192.168.2.0/24 is directly connected, Serial0/0/0 C 192.168.3.0/24 is directly connected, FastEthernet0/0 C 192.168.4.0/24 is directly connected, Serial0/0/1 R 192.168.5.0/24 [120/1] via 192.168.4.1, 00:00:26, Serial0/0/1 R 192.168.6.0/24 [120/1] via 192.168.2.1, 00:00:24, Serial0/0/0 [120/1] via 192.168.4.1, 00:00:26, Serial0/0/1 R 192.168.7.0/24 [120/1] via 192.168.4.1, 00:00:26, Serial0/0/1 R 192.168.8.0/24 [120/2] via 192.168.4.1, 00:00:26, Serial0/0/1
36 36 What happens when two or more routes to the same destination have identical metric values? The router load balances between these equal-cost paths. The packets are forwarded using all equal-cost paths. Load Balancing
37 37 All the routing protocols discussed in this course are capable of automatically load balancing traffic for up to four equal-cost routes by default. EIGRP is also capable of load balancing across unequal-cost paths. This feature of EIGRP is discussed in the CCNP courses. Load Balancing R2# show ip route R 192.168.6.0/24 [120/1] via 192.168.2.1, 00:00:24, Serial0/0/0 [120/1] via 192.168.4.1, 00:00:26, Serial0/0/1
38 Administrative Distance Purpose of Administrative Distance Dynamic Routing Protocols and Administrative Distance Static Routes and Administrative Distance Directly Connected Networks and Administrative Distance
39 39 Purpose of Administrative Distance There can be times when a router learns a route to a remote network from more than one routing source. Can’t compare hop count and bandwidth (apples and oranges) Administrative distance (AD) is: Used to determine which routing source takes precedence. Used to determine which routing source to use when there are multiple routing sources for the same destination network address. Lower the AD the more preferred the routing source.
40 40 Purpose of Administrative Distance Cisco uses Administrative distance (AD) to define the preference of a routing source. Routing sources: Directly connected networks Static routes Specific routing protocols It is possible to modify the administrative distance for static routes and dynamic routing protocols. (in CCNP) Note The term trustworthiness is commonly used when defining administrative distance. The lower the administrative distance value, the more trustworthy the route.
41 41 Purpose of Administrative Distance AD has value from 0 to 255. The lower the value, the more preferred the route source. AD of 0 is the most preferred. Only a directly connected network has an administrative distance of 0, which cannot be changed. No better route to a network than being directly connected to that network. AD of 255 means the router will not believe the source of that route Route will not be installed in the routing table.
42 42 Multiple Routing Sources R2# show ip route Gateway of last resort is not set D 192.168.1.0/24 [90/2172416] via 192.168.2.1, 00:00:24, Serial0/0 C 192.168.2.0/24 is directly connected, Serial0/0/0 C 192.168.3.0/24 is directly connected, FastEthernet0/0 C 192.168.4.0/24 is directly connected, Serial0/0/1 R 192.168.5.0/24 [120/1] via 192.168.4.1, 00:00:08, Serial0/0/1 D 192.168.6.0/24 [90/2172416] via 192.168.2.1, 00:00:24, Serial0/0/0 R 192.168.7.0/24 [120/1] via 192.168.4.1, 00:00:08, Serial0/0/1 R 192.168.8.0/24 [120/2] via 192.168.4.1, 00:00:08, Serial0/0/1 Although not common, more than one dynamic routing protocol can be deployed in the same network. R2 running both EIGRP and RIP
43 43 Multiple Routing Sources R2# show ip route D 192.168.6.0/24 [90/2172416] via 192.168.2.1, 00:00:24, Serial0/0/0 R2 running both EIGRP and RIP R2 has learned of the 192.168.6.0/24 route from both: R1 through EIGRP updates R3 through RIP updates. RIP: AD = 120, EIGRP: AD = 90 (lower, more preferred AD) R2 adds the route learned using EIGRP to the routing table and forwards all packets for the 192.168.6.0/24 network to Router R1.
44 44 Verifying AD: show ip route R2# show ip route D 192.168.6.0/24 [90/2172416] via 192.168.2.1, 00:00:24, Serial0/0/0
45 45 Verifying AD: show ip protocols R2# show ip protocols Routing Protocol is “eigrp 100 “ Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Default networks flagged in outgoing updates Default networks accepted from incoming updates EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0 EIGRP maximum hopcount 100 EIGRP maximum metric variance 1 Redistributing: eigrp 100 Automatic network summarization is in effect Automatic address summarization: Maximum path: 4 Routing for Networks: 192.168.2.0 192.168.3.0 192.168.4.0 Routing Information Sources: Gateway Distance Last Update 192.168.2.1 90 2366569 Distance: internal 90 external 170
46 46 show ip protocols (continued) Routing Protocol is “rip” Sending updates every 30 seconds, next due in 12 seconds Invalid after 180 seconds, hold down 180, flushed after 240 Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Redistributing: rip Default version control: send version 1, receive any version Interface Send Recv Triggered RIP Key-chain Serial0/0/1 1 2 1 FastEthernet0/0 1 2 1 Automatic network summarization is in effect Maximum path: 4 Routing for Networks: 192.168.3.0 192.168.4.0 Passive Interface(s): Routing Information Sources: Gateway Distance Last Update 192.168.4.1 120 Distance: (default is 120) More on show ip protocols later
47 47 Static Routes and Administrative Distance Static routes Default AD = 1 After directly connected networks (AD = 0), static routes are the most preferred route source.
48 48 Static Routes and Administrative Distance R2# show ip route 172.16.0.0/24 is subnetted, 3 subnets C 172.16.1.0 is directly connected, FastEthernet0/0 C 172.16.2.0 is directly connected, Serial0/0/0 S 172.16.3.0 is directly connected, Serial0/0/0 C 192.168.1.0/24 is directly connected, Serial0/0/1 S 192.168.2.0/24 [1/0] via 192.168.1.1 Static route: default AD = 1 (never 0) Exit-interface: AD = 1 Next-hop IP address: AD = 1 Exit Interface: ip route 172.16.3.0 255.255.255.0 serial 0/0/0 Next-hop: ip route 192.168.2.0 255.255.255.0 192.168.1.1
49 49 Static Routes and Administrative Distance R2# show ip route 172.16.0.0/24 is subnetted, 3 subnets C 172.16.1.0 is directly connected, FastEthernet0/0 C 172.16.2.0 is directly connected, Serial0/0/0 S 172.16.3.0 is directly connected, Serial0/0/0 C 192.168.1.0/24 is directly connected, Serial0/0/1 S 192.168.2.0/24 [1/0] via 192.168.1.1 The static route to 172.16.3.0 is listed as “directly connected”. It is common misconception to assume that the AD value of this route must be 0 because it states “directly connected a” - false assumption. Exit Interface: ip route 172.16.3.0 255.255.255.0 serial 0/0/0 Next-hop: ip route 192.168.2.0 255.255.255.0 192.168.1.1