Routing Concepts

Module 14: Routing Concepts

Instructor Planning Guide

• This PowerPoint deck is divided into two parts:
  • Instructor Planning Guide: Information to help instructors become familiar with the module and teaching aids.
  • Instructor Class Presentation: Optional slides for classroom use, starting from slide #9.
• The Planning Guide should be removed before sharing the presentation.
• Additional help and resources are available on the Instructor Home Page, Course Resources, netacad.com, the Cisco Networking Academy Facebook page, and the Instructor Only FB group.

Module Features

The GUI in this module may include:

• Animations: To expose learners to new skills and concepts.
• Videos: To expose learners to new skills and concepts.
• Check Your Understanding (CYU): Online quizzes to gauge content understanding.
• Interactive Activities: Various formats to help learners gauge content understanding.
• Syntax Checker: Small simulations for practicing Cisco command line configurations.
• PT Activity: Simulation and modeling activities to explore, acquire, reinforce, and expand skills.
• Hands-On Labs: Labs designed for working with physical equipment.
• Class Activities: Found on the Instructor Resources page, designed to facilitate learning, class discussion, and collaboration.
• Module Quizzes: Self-assessments to integrate concepts and skills learned.
• Module Summary: Briefly recaps module content.

Check Your Understanding (CYU) Activities

• CYU activities help students quickly determine if they understand the content and can proceed, or if they need to review.
• These activities do not affect student grades.

Module 14 Activities

• 14.1.7: Check Your Understanding - Path Determination (Recommended)
• 14.2.4: Check Your Understanding - Packet Forwarding (Recommended)
• 14.3.5: Packet Tracer - Basic Router Configuration Review (Recommended)
• 14.4.13: Check Your Understanding - IP Routing Table (Recommended)
• 14.5.6: Check Your Understanding - Dynamic and Static Routing (Recommended)

Best Practices for Instructors

• Review the activities and assessments for this module.
• Include as many questions as possible to keep students engaged.

Topic 14.1

• Discussion questions:
  • How might we use the longest-match rules to our advantage and reduce the routing table size?
  • Why do you think directly-connected networks are added to the routing table first?

Topic 14.2

• Discussion questions:
  • What in the packet/frame must change every time a packet moves through a router?
  • What is the router’s primary responsibility in the packet forwarding process?

Topic 14.3

• Discussion questions:
  • What is the difference in the information the show interface and show ip interface commands give you?

Topic 14.4

• Discussion questions:
  • Ask the students for their own analogy of what Administrative Distance is.
  • Have the students explain the /0 designation for a default route in their own words.

Topic 14.5

• Discussion questions:
  • Routing protocols are generally categorized as IGP or EGP. What’s the difference?
  • Have the students explain remote network discovery in their own words.

Module Objectives

• Module Title: Routing Concepts
• Module Objective: Explain how routers use information in packets to make forwarding decisions.

Topic Objectives:

• Path Determination: Explain how routers determine the best path.
• Packet Forwarding: Explain how routers forward packets to the destination.
• Basic Router Configuration Review: Configure basic settings on a router.
• IP Routing Table: Describe the structure of a routing table.
• Static and Dynamic Routing: Compare static and dynamic routing concepts.

14.1 Path Determination

Two Functions of a Router

• Routing: When a router receives an IP packet, it determines which interface to use to forward the packet to the destination.
• The interface may be the final destination or a network connected to another router.
• Each network a router connects to typically requires a separate interface.
• The primary functions are:
  • Determine the best path to forward packets based on the routing table.
  • Forward packets toward their destination.

Router Functions Example

• Routers use their IP routing tables to determine the best path to forward a packet.
• R1 and R2 use their respective IP routing tables to determine the best path and then forward the packet.

Best Path Equals Longest Match

• The best path in the routing table is also known as the longest match.
• The routing table contains route entries consisting of a prefix (network address) and prefix length.
• For a match, a minimum number of far-left bits must match between the destination IP address and the route in the routing table.
• The prefix length determines the minimum number of matching far-left bits.
• The route with the greatest number of matching far-left bits is the longest match and is always the preferred route.
• Prefix length refers to the network portion of both IPv4 and IPv6 addresses.

IPv4 Longest Match Example

• IPv4 packet with destination IP address: 172.16.0.10
• Router has three route entries:
  • 172.16.0.0/12
  • 172.16.0.0/18
  • 172.16.0.0/26
• 172.16.0.0/26 has the longest match and would be chosen to forward the packet.
• For any of these routes to be considered a match there must be at least the number of matching bits indicated by the subnet mask of the route.

IPv6 Longest Match Example

• IPv6 packet with destination IPv6 address 2001:db8:c000::99.
• Route entries:
  • 2001:db8:c000::/40 (Match of 40 bits)
  • 2001:db8:c000::/48 (Match of 48 bits - longest match)
  • 2001:db8:c000:5555::/64 (Does not match - requires 64 bits)

Building the Routing Table

• Directly Connected Networks: Added to the routing table when a local interface is configured with an IP address and subnet mask (prefix length) and is active (up and up).
• Remote Networks: Networks not directly connected to the router.
  • Static Routes: Manually configured and added to the routing table.
  • Dynamic Routing Protocols: Routing protocols dynamically learn about the remote network and add it to the routing table.
• Default Route: Specifies a next-hop router to use when the routing table does not contain a specific route that matches the destination IP address.
  • Can be entered manually as a static route or learned automatically from a dynamic routing protocol.
  • Has a /0 prefix length, meaning no bits need to match the destination IP address.
  • Used if no routes have a match longer than 0 bits.
  • Sometimes referred to as a gateway of last resort.

14.2 Packet Forwarding

Packet Forwarding Decision Process

1. Data link frame with encapsulated IP packet arrives on the ingress interface.
2. Router examines the destination IP address in the packet header and consults its IP routing table.
3. Router finds the longest matching prefix in the routing table.
4. Router encapsulates the packet in a data link frame and forwards it out the egress interface. Destination could be a device or next-hop router.
5. If no matching route entry is found, the packet is dropped.

Forwarding to a Directly Connected Network

• If the route entry indicates that the egress interface is a directly connected network, the packet can be forwarded directly to the destination device, typically an Ethernet LAN.
• To encapsulate the packet, the router needs to determine the destination MAC address associated with the destination IP address. The process varies based on the packet type (IPv4 or IPv6).

Forwarding to a Next-Hop Router

• If the route entry indicates that the destination IP address is on a remote network, the packet must be forwarded to the next-hop router, indicated in the route entry.
• If the forwarding router and the next-hop router are on an Ethernet network, a similar process (ARP and ICMPv6 Neighbor Discovery) will occur for determining the destination MAC address of the packet.
• The router will search for the IP address of the next-hop router in its ARP table or neighbor cache, instead of the destination IP address of the packet.
• This process will vary for other types of Layer 2 networks.

Drop the Packet

• If there is no match between the destination IP address and a prefix in the routing table and no default route, the packet will be dropped.

End-to-End Packet Forwarding

• The primary responsibility of the packet forwarding function is to encapsulate packets in the appropriate data link frame type for the outgoing interface.
• For example, the data link frame format for a serial link could be Point-to-Point (PPP) protocol, High-Level Data Link Control (HDLC) protocol, or some other Layer 2 protocol.

Packet Forwarding Mechanisms

• The more efficiently a router can perform the encapsulation task, the faster packets can be forwarded.
• Routers support the following three packet forwarding mechanisms:
  • Process switching
  • Fast switching
  • Cisco Express Forwarding (CEF)

Process Switching

• An older packet forwarding mechanism, still available on Cisco routers.
• When a packet arrives, it is forwarded to the control plane, where the CPU matches the destination address with an entry in its routing table and determines the exit interface.
• The router does this for every packet, even if the destination is the same for a stream of packets.

Fast Switching

• Another older packet forwarding mechanism, successor to process switching.
• Uses a fast-switching cache to store next-hop information.
• When a packet arrives, the CPU searches for a match in the fast-switching cache. If not found, it is process-switched.
• The flow information is stored in the fast-switching cache.
• Subsequent packets to the same destination reuse the next-hop information in the cache without CPU intervention.

Cisco Express Forwarding (CEF)

• The most recent and default Cisco IOS packet-forwarding mechanism.
• Builds a Forwarding Information Base (FIB) and an adjacency table.
• Table entries are change-triggered, not packet-triggered like fast switching.
• When a network has converged, the FIB and adjacency tables contain all information needed for forwarding packets.

14.3 Basic Router Configuration Review

Topology

• The topology in the figure will be used for configuration and verification examples.

Configuration Commands

• Example configuration commands for Router R1:

Router> enable
Router# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)# hostname R1
R1(config)# enable secret class
R1(config)# line console 0
R1(config-line)# logging synchronous
R1(config-line)# password cisco
R1(config-line)# login
R1(config-line)# exit
R1(config)# line vty 0 4
R1(config-line)# password cisco
R1(config-line)# login
R1(config-line)# transport input ssh telnet
R1(config-line)# exit
R1(config)# service password-encryption
R1(config)# banner motd #
Enter TEXT message.  End with a new line and the character '#'.
***********************************************
WARNING: Unauthorized access is prohibited!
***********************************************
#
R1(config)# ipv6 unicast-routing
R1(config)# interface gigabitethernet 0/0/0
R1(config-if)# description Link to LAN 1
R1(config-if)# ip address 10.0.1.1 255.255.255.0
R1(config-if)# ipv6 address 2001:db8:acad:1::1/64
R1(config-if)# ipv6 address fe80::1:a link-local
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)# interface gigabitethernet 0/0/1
R1(config-if)# description Link to LAN 2
R1(config-if)# ip address 10.0.2.1 255.255.255.0
R1(config-if)# ipv6 address 2001:db8:acad:2::1/64
R1(config-if)# ipv6 address fe80::1:b link-local
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)# interface serial 0/1/1
R1(config-if)# description Link to R2
R1(config-if)# ip address 10.0.3.1 255.255.255.0
R1(config-if)# ipv6 address 2001:db8:acad:3::1/64
R1(config-if)# ipv6 address fe80::1:c link-local
R1(config-if)# no shutdown
R1(config-if)# exit
R1# copy running-config startup-config
Destination filename [startup-config]?
Building configuration...
[OK]
R1#

Verification Commands

• Common verification commands include:
  • show ip interface brief
  • show running-config interface interface-type number
  • show interfaces
  • show ip interface
  • show ip route
  • ping
• Replace ip with ipv6 for the IPv6 version of the command.

Filter Command Output

• Filtering commands can be used to display specific sections of output.
• To enable filtering, enter a pipe (|) character after the show command, then enter a filtering parameter and expression.
• Filtering parameters include:
  • section: Displays the entire section starting with the filtering expression.
  • include: Includes all output lines that match the filtering expression. Uses regex.
  • exclude: Excludes all output lines that match the filtering expression. Uses regex.
  • begin: Displays all output lines from a certain point. Uses regex.
• Output filters can be used in combination with any show command.

Packet Tracer - Basic Router Configuration Review

• In this Packet Tracer, you will:
  • Configure Devices and Verify Connectivity
  • Display Router Information

14.4 IP Routing Table

Route Sources

• A routing table contains a list of routes to known networks (prefixes and prefix lengths).
• The source of this information is derived from:
  • Directly connected networks
  • Static routes
  • Dynamic routing protocols
• The source for each route is identified by a code:
  • L: Identifies the address assigned to a router interface.
  • C: Identifies a directly connected network.
  • S: Identifies a static route.
  • O: Identifies a dynamically learned network from OSPF.
  • *: This route is a candidate for a default route.

Routing Table Principles

• Every router makes its decision alone, based on its own routing table.
  • Router R1 can only forward packets using its own routing table.
  • R1 does not know what routes are in the routing tables of other routers (e.g., R2).
• The information in a routing table of one router does not necessarily match the routing table of another router.
  • Just because R1 has a route to a network via R2, that does not mean R2 knows about that same network.
• Routing information about a path does not provide return routing information.
  • If R1 knows to forward a packet out its G0/0/0 interface to PC1, it doesn’t necessarily mean it knows how to forward packets from PC1 back to the remote network.

Routing Table Entries

• Information included in a routing table entry:
  • Route source: How the route was learned.
  • Destination network (prefix and prefix length): Address of the remote network.
  • Administrative distance (AD): Trustworthiness of the route source. Lower values indicate preferred route source.
  • Metric: Value assigned to reach the remote network. Lower values indicate preferred routes.
  • Next-hop: IP address of the next router to which the packet would be forwarded.
  • Route timestamp: How much time has passed since the route was learned.
  • Exit interface: Egress interface to use for outgoing packets to reach their final destination.
• The prefix length specifies the minimum number of far-left bits that must match between the IP address of the packet and the destination network (prefix) for the route to be used.

Directly Connected Networks

• To learn about any remote networks, the router must have at least one active interface configured with an IP address and subnet mask (prefix length). This is a directly connected network or route.
• Routers add a directly connected route to its routing table when an interface is configured with an IP address and is activated.
• A directly connected network is denoted by a status code of C in the routing table.
• The routing table also contains a local route for each of its directly connected networks, indicated by the status code of L.
• For IPv4 local routes, the prefix length is /32, and for IPv6 local routes, the prefix length is /128.
• The purpose of the local route is to efficiently determine when it receives a packet for the interface instead of a packet that needs to be forwarded.

Static Routes

• After directly connected interfaces are configured and added to the routing table, static or dynamic routing can be implemented for accessing remote networks.
• Static routes are manually configured and define an explicit path between two networking devices.
• They are not automatically updated and must be manually reconfigured if the network topology changes.
• Uses:
  • Ease of routing table maintenance in smaller networks.
  • A single default route to represent a path to any network without a more specific match.
  • Routing to and from stub networks (accessed by a single route, and the router has only one neighbor).

Static Routes in the IP Routing Table

• Example shows IPv4 and IPv6 static routes configured on R1 to reach the 10.0.4.0/24 and 2001:db8:acad:4::/64 networks on R2.

Dynamic Routing Protocols

• Dynamic routing protocols are used by routers to automatically share information about the reachability and status of remote networks.
• They perform several activities, including network discovery and maintaining routing tables.

Dynamic Routes in the Routing Table

• OSPF is used in the sample topology to dynamically learn all the networks connected to R1 and R2.
• The routing table entries use the status code of O to indicate the route was learned by OSPF.
• Both entries also include the IP address of the next-hop router, via the ip-address.
• IPv6 routing protocols use the link-local address of the next-hop router.

Default Route

• The default route specifies a next-hop router to use when the routing table does not contain a specific route that matches the destination IP address.
• A default route can be either a static route or learned automatically from a dynamic routing protocol.
• A default route has an IPv4 route entry of 0.0.0.0/0 or an IPv6 route entry of ::/0.
• This means that zero or no bits need to match between the destination IP address and the default route.

Structure of an IPv4 Routing Table

• IPv4 was standardized using the now obsolete classful addressing architecture.
• The IPv4 routing table is organized using this same classful structure.
• Although the lookup process no longer uses classes, the structure of the IPv4 routing table still retains in this format.
• An indented entry is known as a child route.
• A route entry is indented if it is the subnet of a classful address (class A, B, or C network).
• Directly connected networks will always be indented (child routes) because the local address of the interface is always entered in the routing table as a /32.
• The child route will include the route source and all the forwarding information such as the next-hop address.
• The classful network address of this subnet will be shown above the route entry, less indented, and without a source code.
• That route is known as a parent route.

Structure of an IPv6 Routing Table

• The concept of classful addressing was never part of IPv6, so the structure of an IPv6 routing table is very straightforward.
• Every IPv6 route entry is formatted and aligned the same way.

Administrative Distance

• A route entry for a specific network address (prefix and prefix length) can only appear once in the routing table.
• However, it is possible that the routing table learns about the same network address from more than one routing source.
• Except for very specific circumstances, only one dynamic routing protocol should be implemented on a router.
• Each routing protocol may decide on a different path to reach the destination based on the metric of that routing protocol.
• Cisco IOS uses the administrative distance (AD) to determine the route to install into the IP routing table.
• The AD represents the trustworthiness of the route. The lower the AD, the more trustworthy the route source.

Administrative Distance Values

14.5 Static and Dynamic Routing

Static or Dynamic?

• Static and dynamic routing are not mutually exclusive.
• Most networks use a combination of dynamic routing protocols and static routes.
• Static routes are commonly used in the following scenarios:
  • As a default route forwarding packets to a service provider
  • For routes outside the routing domain and not learned by the dynamic routing protocol
  • When the network administrator wants to explicitly define the path for a specific network
  • For routing between stub networks
• Static routes are useful for smaller networks with only one path to an outside network.
• They also provide security in a larger network for certain types of traffic, or links to other networks that need more control.

Dynamic Routing Protocols

• Dynamic routing protocols are implemented in any type of network consisting of more than just a few routers.
• Dynamic routing protocols are scalable and automatically determine better routes if there is a change in the topology.
• Dynamic routing protocols are commonly used in the following scenarios:
  • In networks consisting of more than just a few routers
  • When a change in the network topology requires the network to automatically determine another path
  • For scalability. As the network grows, the dynamic routing protocol automatically learns about any new networks.

Comparison of Dynamic and Static Routing

Dynamic Routing Evolution

• Dynamic routing protocols have been used in networks since the late 1980s.
• One of the first routing protocols was RIP. RIPv1 was released in 1988, but some of the basic algorithms within the protocol were used on the Advanced Research Projects Agency Network (ARPANET) as early as 1969.
• As networks evolved and became more complex, new routing protocols emerged.

Interior Gateway Protocols (IGPs) vs. Exterior Gateway Protocols (EGPs)

• Interior Gateway Protocols (IGPs) are routing protocols used to exchange routing information within a routing domain administered by a single organization.
• BGP is the only EGP.
• BGP is used to exchange routing information between different organizations, known as autonomous systems (AS).
• BGP is used by ISPs to route packets over the internet.
• Distance vector, link-state, and path vector routing protocols refer to the type of routing algorithm used to determine the best path.

Dynamic Routing Protocol Concepts

• 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 choice of best paths.
• The purpose of dynamic routing protocols includes the following:
  • Discovery of remote networks
  • Maintaining up-to-date routing information
  • Choosing the best path to destination networks
  • Ability to find a new best path if the current path is no longer available

Main Components of Dynamic Routing Protocols

• Data structures: Routing protocols typically use tables or databases for their operations. This information is kept in RAM.
• Routing protocol messages: Routing protocols use various types of messages to discover neighboring routers, exchange routing information, and other tasks to learn and maintain accurate information about the network.
• Algorithm: An algorithm is a finite list of steps used to accomplish a task. Routing protocols use algorithms for facilitating routing information and for the best path determination.
• Routing protocols determine the best path, or route, to each network. That route is then offered to the routing table. The route will be installed in the routing table if there is not another routing source with a lower AD.

Best Path Selection

• The best path is selected by a routing protocol based on the value or metric it uses to determine the distance to reach a network.
• A metric is the quantitative value used to measure the distance to a given network.
• The best path to a network is the path with the lowest metric.
• Dynamic routing protocols typically use their own rules and metrics to build and update routing tables.

Common Dynamic Protocols and Their Metrics

• Each router along a path adds a hop to the hop count.
• A maximum of 15 hops allowed. |
  | Open Shortest Path First (OSPF) | * The metric is “cost” which is based on the cumulative bandwidth from source to destination.
• Faster links are assigned lower costs compared to slower (higher cost) links. |
  | Enhanced Interior Gateway Routing Protocol (EIGRP) | * It calculates a metric based on the slowest bandwidth and delay values.
• It could also include load and reliability into the metric calculation. |

Load Balancing

• When a router has two or more paths to a destination with equal cost metrics, then the router forwards the packets using both paths equally. This is called equal cost load balancing.
• The routing table contains the single destination network, but has multiple exit interfaces, one for each equal cost path. The router forwards packets using the multiple exit interfaces listed in the routing table.
• If configured correctly, load balancing can increase the effectiveness and performance of the network.
• Equal cost load balancing is implemented automatically by dynamic routing protocols.
• It is enabled with static routes when there are multiple static routes to the same destination network using different next-hop routers.
• Only EIGRP supports unequal cost load balancing.

14.6 Module Practice and Quiz

What Was Learned in This Module?

• The primary functions of a router are to determine the best path to forward packets based on the information in its routing table, and to forward packets toward their destination.
• The best path in the routing table is also known as the longest match. The longest match is the route in the routing table that has the greatest number of far-left matching bits with the destination IP address of the packet.
• Directly connected networks are networks that are configured on the active interfaces of a router. A directly connected network is added to the routing table when an interface is configured with an IP address and subnet mask (prefix length) and is active (up and up).
• Routers learn about remote networks in two ways: static routes and with dynamic routing protocols.
• After a router determines the correct path, it can forward the packet on a directly connected network, it can forward the packet to a next-hop router, or it can drop the packet.
• Routers support three packet forwarding mechanisms: process switching, fast switching, and CEF.
• There are several configuration and verification commands for routers, including show ip route, show ip interface, show ip interface brief and show running-config.
• A routing table contains a list of routes known networks (prefixes and prefix lengths). The source of this information is derived from directly connected networks, static routes, and dynamic routing protocols.
• Every router makes its decision alone, based on the information it has in its own routing table. The information in a routing table of one router does not necessarily match the routing table of another router.
• Routing information about a path does not provide return routing information.
• Routing table entries include the route source, destination network, AD, metric, next-hop, route timestamp, and exit interface.
• Static routes are manually configured and define an explicit path between two networking devices.
• Dynamic routing protocols can discover a network, maintain routing tables, select a best path, and automatically discover a new best path if the topology changes.
• The default route specifies a next-hop router to use when the routing table does not contain a specific route that matches the destination IP address. A default route can be either a static route or learned automatically from a dynamic routing protocol.
• IPv4 routing tables still have a structure based on classful addressing represented by levels of indentation. IPv6 routing tables do not use the IPv4 routing table structure.
• Cisco IOS uses what is known as the administrative distance (AD) to determine the route to install into the IP routing table. The AD represents the trustworthiness of the route. The lower the AD, the more trustworthy the route source.
• Static routes are commonly used as a default route forwarding packets to a service provider, for routes outside the routing domain and not learned by the dynamic routing protocol, when the network administrator wants to explicitly define the path for a specific network, or for routing between stub networks.
• Dynamic routing protocol are commonly used in networks consisting of more than just a few routers, when a change in the network topology requires the network to automatically determine another path, and for scalability. As the network grows, the dynamic routing protocol automatically learns about any new networks.
• Current routing protocols include IGPs and EGPs. IGPs exchange routing information within a routing domain administered by a single organization. The only EGP is BGP. BGP exchanges routing information between different organizations. BGP is used by ISPs to route packets over the internet.
• Distance vector, link-state, and path vector routing protocols refer to the type of routing algorithm used to determine best path.
• The main components of dynamic routing protocols are data structures, routing protocol messages, and algorithms.
• The best path is selected by a routing protocol based on the value or metric it uses to determine the distance to reach a network. The best path to a network is the path with the lowest metric.
• When a router has two or more paths to a destination with equal cost metrics, then the router forwards the packets using both paths equally. This is called equal cost load balancing.

New Terms and Commands

• best path
• longest match
• prefix length
• next-hop router
• process switching
• fast switching
• Cisco Express Forwarding (CEF)
• route sources
• static routes
• dynamic routing protocols
• ip route
• Default Route
• ip route 0.0.0.0 0.0.0.0 [ exit-if | next- hop-ip ]
• ipv6 route ::/0 [ exit-if | next-hop-ipv6 ]
• Administrative Distance
• RIPv2
• OSPFv2
• EIGRP
• EIGRP for IPv6
• OSPFv3
• IS-IS
• IS-IS for IPv6
• BGP
• BGP-MP
• EGP
• load balancing
• equal-cost load balancing
• unequal-cost load balancing