Chapter 17: Dynamic routing
Introduces dynamic routing by contrasting automatic route learning between routers with manually configured static routes.
After this chapter you will be able to
- Dynamic and static routing contrast. Distinguish static routing, where an administrator manually configures routes, from dynamic routing, where routers communicate to build their routing tables automatically.
- Dynamic routing information exchange. Explain how routers use a routing protocol to share information about known networks and build their routing tables without an administrator manually configuring each route.
- Static routes after link failure. Trace what happens when the R2–R3 link fails: R2 loses its route to 192.168.30.0/24 because it cannot reach the next hop, while R1 keeps forwarding packets toward R2 on a static route that has not adjusted to the failure.
- Dynamic routing failure recovery. Explain how dynamic routing responds to the failed link and to its restoration, and why that adaptability improves network resilience.
- Dynamic routing scalability. Explain why dynamic routing is more practical than configuring static routes as a network grows, using the six-router example.
- IGP and EGP scope. Distinguish IGPs from EGPs by where they exchange routing information: within one autonomous system or between different autonomous systems.
- Distance-vector route knowledge. Explain what distance-vector routers share, what they know about a destination, and why R1 selects the route to LAN A through R5.
- Link-state connectivity maps. Describe what link-state routers share and how each router uses the resulting connectivity map to calculate routes.
- BGP path-vector routing. Explain that BGP is the only widely used exterior gateway protocol and that its path-vector approach considers the series of autonomous systems a route crosses, rather than the series of individual routers. Describe why the shortest AS sequence alone does not determine a BGP route.
- Two meanings of route selection. Distinguish selecting routes for the routing table from selecting a route in the table to forward a packet. Explain that a route must be in the table before it can be used for forwarding.
- Best route for table entry. Explain that when a router learns multiple routes to the same destination, it inserts only the best one into its routing table, comparing metric and administrative distance to determine which is best.
- Metric-based route selection. Explain why R1 selects the lower-metric route and what happens when that route becomes invalid. Identify the metric in the example routing-table entry.
- Administrative distance route preference. Explain why a router uses administrative distance (AD), rather than metrics, to choose between routes to the same destination learned from different routing protocols. Describe how lower AD determines the preferred route in the EIGRP–OSPF example.
- Floating static backup routes. Explain how to make a static route a backup for a dynamically learned route and what happens when the primary route is lost.
- Distinct destination prefixes. Explain why routes with the same network address but different prefix lengths can all enter a routing table, even when their subnets overlap.
- Competing route preferences. Describe how a router chooses among routes to the same destination: compare administrative distance when they come from different routing protocols, compare metrics when they come from the same protocol, and retain equal-cost routes in the example.
- Matching route for forwarding. Trace how a router selects a route for a packet by finding routes whose prefixes match the destination IP address, then choosing the matching route with the longest prefix.
- Interface selection and advertisement. Explain how to enter OSPF router configuration mode and what a network command causes the router to do. Distinguish the interfaces it selects from the network prefixes those interfaces advertise.
- Wildcard mask matching rules. Describe how zero and one bits in a network command’s wildcard mask determine which interface IP addresses match, and distinguish that matching purpose from a netmask’s purpose.
- Precise interface activation. Apply the wildcard matching rule to explain how a broad network command can select multiple interfaces and why specifying an interface’s exact IP address with 0.0.0.0 is recommended.
What it builds on
- Static routing (chapter 9) · Introduces static routing as the manually configured alternative.
- The routing table (chapter 9) · Routers use routing tables to determine routes.
- Configuring static routes (chapter 9) · Static routes can be configured manually.
- Route selection (chapter 9) · Packet-forwarding route selection using the most specific matching route
- Attributes of an IPv4 network (chapter 7) · Understanding an interface’s network prefix.
- The structure of an IPv4 address (chapter 7) · Recognizing individual interface IPv4 addresses.
Section by section
Introduction
Introduces dynamic routing by contrasting automatic route learning between routers with manually configured static routes.
- Dynamic and static routing contrast: Distinguish static routing, where an administrator manually configures routes, from dynamic routing, where routers communicate to build their routing tables automatically.
Why it matters: This contrast explains what changes when a network uses dynamic rather than static routing.
Key terms: dynamic routing, static routing, routing table
17.1 Dynamic routing vs. static routing
Introduces dynamic routing as routers exchanging information about known networks to build routing tables without manually configured routes.
- Dynamic routing information exchange: Explain how routers use a routing protocol to share information about known networks and build their routing tables without an administrator manually configuring each route.
Why it matters: This is the basic distinction between dynamic routing and manually configuring routes.
Key terms: dynamic routing, routing protocol, routing information, advertisement, routing table
17.1.1 Adaptability
This subsection contrasts how static and dynamic routes respond to a failed link and explains why dynamic routing improves network resilience.
- Static routes after link failure: Trace what happens when the R2–R3 link fails: R2 loses its route to 192.168.30.0/24 because it cannot reach the next hop, while R1 keeps forwarding packets toward R2 on a static route that has not adjusted to the failure.
Why it matters: This explains why a configured static route can continue sending traffic toward a path that no longer works. - Dynamic routing failure recovery: Explain how dynamic routing responds to the failed link and to its restoration, and why that adaptability improves network resilience.
Why it matters: Automatic route changes help traffic recover from a failure without waiting for someone to replace static routes manually.
Key terms: static route, next-hop IP address, routing table, dynamic routing, adaptability, resilience
17.1.2 Scalability
The section explains why dynamic routing scales better than static routing while noting when static routes remain useful.
- Dynamic routing scalability: Explain why dynamic routing is more practical than configuring static routes as a network grows, using the six-router example.
Why it matters: This explains why growing networks commonly benefit from dynamic routing. - Static route control: Explain why static routes may still be useful and whether they can be combined with dynamic routing.
Why it matters: Dynamic routing's scalability does not mean static routes must be abandoned.
Key terms: scalability, static routing, dynamic routing protocols, routing information, static routes, next hop, predictability, control
17.2 Types of routing protocols
This section distinguishes routing protocols by where they exchange routing information and introduces algorithm type as another way to categorize them.
- IGP and EGP scope: Distinguish IGPs from EGPs by where they exchange routing information: within one autonomous system or between different autonomous systems.
Why it matters: The distinction tells you whether a routing protocol is being used inside an organization’s network or between organizations. - Routing protocol classification axes: Explain that routing protocols can be categorized both as IGPs or EGPs and by algorithm type, which describes how routers share information and calculate routes.
Why it matters: These categories describe different aspects of a routing protocol rather than competing labels.
Key terms: Interior Gateway Protocols (IGP), Exterior Gateway Protocols (EGP), autonomous system (AS), IGP, EGP, algorithm type
17.2.1 Interior gateway protocols
This section contrasts how distance-vector and link-state interior gateway protocols learn routes and explains the resource cost of building a network map.
- Distance-vector route knowledge: Explain what distance-vector routers share, what they know about a destination, and why R1 selects the route to LAN A through R5.
Why it matters: This explains how a router can choose a route without knowing the entire network. - Link-state connectivity maps: Describe what link-state routers share and how each router uses the resulting connectivity map to calculate routes.
Why it matters: The shared map is the defining contrast with a distance-vector router’s neighbor-based view. - Link-state resource cost: Explain why building and using a link-state connectivity map can demand more router resources than distance-vector routing.
Why it matters: It identifies a practical tradeoff of giving each router a network-wide map.
Key terms: distance-vector, metric, next-hop router, RIP, EIGRP, link-state, connected links, connectivity map, OSPF, IS-IS
17.2.2 Exterior gateway protocols
This section explains how BGP uses path-vector routing to choose paths between autonomous systems.
- BGP path-vector routing: Explain that BGP is the only widely used exterior gateway protocol and that its path-vector approach considers the series of autonomous systems a route crosses, rather than the series of individual routers. Describe why the shortest AS sequence alone does not determine a BGP route.
Why it matters: It distinguishes routing between autonomous systems from routing based on individual routers.
Key terms: exterior gateway protocol, BGP, path-vector, autonomous system, path attributes
17.3 Route selection
The section distinguishes selecting a route to forward a packet from selecting which learned or configured routes enter the routing table.
- Two meanings of route selection: Distinguish selecting routes for the routing table from selecting a route in the table to forward a packet. Explain that a route must be in the table before it can be used for forwarding.
Why it matters: The distinction keeps you from confusing how a route reaches the table with how a packet uses it. - Best route for table entry: Explain that when a router learns multiple routes to the same destination, it inserts only the best one into its routing table, comparing metric and administrative distance to determine which is best.
Why it matters: This explains why learning a route does not guarantee that it appears in the routing table.
Key terms: route selection, routing table population, packet forwarding, metric, administrative distance, routing table
17.3.1 The metric parameter
This subsection explains how a routing protocol’s metric helps a router choose a route, how several protocols calculate metrics, and how to identify a metric in a routing-table entry.
- Metric-based route selection: Explain why R1 selects the lower-metric route and what happens when that route becomes invalid. Identify the metric in the example routing-table entry.
Why it matters: It lets you explain both R1’s initial choice and the change visible in its routing table after an interface goes down. - Routing protocol metric calculations: Describe the different information RIP, OSPF, and EIGRP use to calculate their metrics.
Why it matters: It clarifies what a metric represents when you encounter routes learned through different protocols.
Key terms: metric, routing table, administrative distance, RIP, hop count, OSPF, cost, EIGRP, bandwidth
17.3.2 The administrative distance parameter
This section explains how administrative distance selects among routes learned in different ways, when equal-cost routes can share traffic, and how a static route can serve as a backup.
- Administrative distance route preference: Explain why a router uses administrative distance (AD), rather than metrics, to choose between routes to the same destination learned from different routing protocols. Describe how lower AD determines the preferred route in the EIGRP–OSPF example.
Why it matters: AD explains why the route with the numerically smaller metric does not necessarily win when protocols differ. - Same destination comparison boundary: State when metrics and AD are used to compare routes, and distinguish routes whose network addresses match but prefix lengths differ.
Why it matters: It prevents treating two different prefixes as competing routes just because their network addresses look alike. - Equal cost multipath routing: Describe what happens when multiple routes to the same destination are learned through the same routing protocol with the same metric.
Why it matters: An equal-cost tie can result in multiple usable paths rather than one winning route. - Floating static backup routes: Explain how to make a static route a backup for a dynamically learned route and what happens when the primary route is lost.
Why it matters: A floating static route provides a predictable backup path without overriding the preferred dynamic route.
Key terms: administrative distance, AD, metric, destination network address, prefix length, equal-cost multi-path, ECMP, load-balanced, floating static route, ip route
17.3.3 Route selection examples
This section uses examples to distinguish how routers choose routes for the routing table from how they choose a route to forward a packet.
- Distinct destination prefixes: Explain why routes with the same network address but different prefix lengths can all enter a routing table, even when their subnets overlap.
Why it matters: This prevents you from using longest-prefix matching to discard routes while building the routing table. - Competing route preferences: Describe how a router chooses among routes to the same destination: compare administrative distance when they come from different routing protocols, compare metrics when they come from the same protocol, and retain equal-cost routes in the example.
Why it matters: Using the right comparison tells you which learned routes actually appear in the table. - Matching route for forwarding: Trace how a router selects a route for a packet by finding routes whose prefixes match the destination IP address, then choosing the matching route with the longest prefix.
Why it matters: The word “matching” is essential: the longest prefix in the table is not necessarily the route a packet takes.
Key terms: destination network, prefix length, routing table population, administrative distance, AD, metric, ECMP, most specific matching route, packet forwarding
17.4 The network command
Explains how the OSPF network command uses wildcard masks to select router interfaces, why the command does not specify the prefixes advertised, and how to avoid activating OSPF on unintended interfaces.
- Interface selection and advertisement: Explain how to enter OSPF router configuration mode and what a network command causes the router to do. Distinguish the interfaces it selects from the network prefixes those interfaces advertise.
Why it matters: It prevents you from mistaking an interface-selection command for a declaration of the route that will be advertised. - Wildcard mask matching rules: Describe how zero and one bits in a network command’s wildcard mask determine which interface IP addresses match, and distinguish that matching purpose from a netmask’s purpose.
Why it matters: The matching rules let you tell which interfaces a command will activate, even when they belong to different subnets. - Precise interface activation: Apply the wildcard matching rule to explain how a broad network command can select multiple interfaces and why specifying an interface’s exact IP address with 0.0.0.0 is recommended.
Why it matters: Exact matching helps prevent OSPF from being activated on an interface you did not intend to include.
Key terms: router ospf process-id, network command, network prefix, wildcard mask, netmask, 0 bit, 1 bit, 0.0.0.0
17.5 Summary
Where this is used later
- OSPF foundations (chapter 18) · Introduces dynamic routing as an alternative to static routing.
- The link-state database (chapter 18) · Assumes the basic idea that dynamic routing protocols exchange information to calculate routes.
- (chapter intro) (chapter 18) · The earlier distinction between interior and exterior gateway protocols.
- OSPF areas (chapter 18) · Areas are introduced as a way to address the resource concern.
- Route selection (chapter 21) · The earlier discussion of metric and administrative distance in route selection.
- OSPF cost (chapter 18) · The role of a metric in route selection
- Floating static routes (chapter 21) · Revisits floating static routes for IPv6 routing.
- Advertising a default route (chapter 18) · The earlier route-selection examples and ECMP discussion referenced by this section.
Concepts explained in this chapter
- equal-cost multipath routingEqual-cost multipath routing forwards or distributes traffic across multiple routes to the same destination when those routes have equal routing cost.
- dynamic routingDynamic routing is a routing method in which routers exchange reachability information through routing protocols and automatically build routing tables.
- Enhanced Interior Gateway Routing ProtocolEnhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco-developed interior gateway routing protocol with distance-vector characteristics and support for metric-based route selection.
- administrative distanceAdministrative distance is a router metric that expresses the relative trustworthiness of a route source.
- network commandThe network command is a Cisco IOS routing-protocol configuration command that selects interfaces for protocol activation based on IP-address matching and a wildcard mask.
- floating static routeA floating static route is a static route configured with an administrative distance higher than the preferred route source so it can serve as a backup route.
- interior gateway protocolAn interior gateway protocol is a routing protocol that exchanges reachability information within a single autonomous system.
- distance-vector routing protocolDistance-vector routing calculates paths from route information received from neighboring routers rather than from a complete network topology map.
- link-state routing protocolLink-state routing builds a topology map of the network and calculates routes using that map.
- network portionThe network portion is the leading part of an IP address that identifies the network to which the addressed host belongs.
- wildcard maskA wildcard mask is a 32-bit bit mask in which zero bits require corresponding address bits to match and one bits ignore them during address matching.
- routing table populationRouting table population is the process of selecting which learned or configured routes a router enters into its routing table.
- autonomous systemAn autonomous system is a collection of networks and routers under a common administrative control that participates in interdomain routing.
- exterior gateway protocolAn exterior gateway protocol is a routing protocol that exchanges routing information between different autonomous systems.
- most specific matching routeThe most specific matching route is the route whose network prefix has the greatest number of bits matching a packet's destination IP address.
- switchingA forwarding decision is a router's selection of the next forwarding action for a packet based on its destination and routing information.
- BGPBGP is an exterior gateway routing protocol that calculates routes between autonomous systems using path-vector information.
- link-state databaseA link-state database is an in-memory data structure containing the network topology information used by a link-state routing protocol to calculate routes.
- static routeIPv6 static routing is a routing method in which IPv6 routes are manually configured on routers.
- routing tableA routing table is a data structure containing routes that describe reachable network destinations and how to forward packets toward them.
- router configuration modeRouter configuration mode is a Cisco IOS CLI state in which routing-protocol configuration commands can be entered for a router process.
- OSPF processAn OSPF process is a locally running instance of OSPF on a router that independently maintains routing information and calculates routes.
- router ospfrouter ospf is a Cisco IOS configuration command that creates or selects an OSPF process and enters OSPF router configuration mode.
- Open Shortest Path FirstOSPFv2 is the version of OSPF primarily used to route IPv4 networks.
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