What address does a router use when deciding to send a packet?

This section explains how routers use information in data packets to make forwarding decisions in a small to medium-sized business network.

Switching Packets Between Networks (1.2.1)

This topic explains the encapsulation and de-encapsulation process that routers use when switching packets between interfaces.

Router Switching Function (1.2.1.1)

A primary function of a router is to forward packets toward their destination. This is accomplished by using a switching function, which is the process used by a router to accept a packet on one interface and forward it out another interface. A key responsibility of the switching function is to encapsulate packets in the appropriate data link frame type for the outgoing data link.

After the router has determined the exit interface using the path determination function, the router must encapsulate the packet into the data link frame of the outgoing interface.

What does a router do with a packet received from one network and destined for another network? Refer to Figure 1-23.

Figure 1-23 Encapsulating and De-Encapsulating Packets

The router performs the following three major steps:

• Step 1. De-encapsulates the Layer 2 frame header and trailer to expose the Layer 3 packet.

• Step 2. Examines the destination IP address of the IP packet to find the best path in the routing table.

• Step 3. If the router finds a path to the destination, it encapsulates the Layer 3 packet into a new Layer 2 frame and forwards the frame out the exit interface.

As shown in Figure 1-23, devices have Layer 3 IPv4 addresses, and Ethernet interfaces have Layer 2 data link addresses. For example, PC1 is configured with IPv4 address 192.168.1.10 and an example MAC address of 0A-10. As a packet travels from the source device to the final destination device, the Layer 3 IP addresses do not change. However, the Layer 2 data link addresses change at every hop as the packet is de-encapsulated and re-encapsulated in a new Layer 2 frame by each router.

It is common for packets to require encapsulation into a different type of Layer 2 frame than the one in which it was received. For example, a router might receive an Ethernet encapsulated frame on a FastEthernet interface and then process that frame to be forwarded out of a serial interface.

Notice in Figure 1-23 that the ports between R2 and R3 do not have associated MAC addresses. This is because it is a serial link. MAC addresses are only required on Ethernet multiaccess networks. A serial link is a point-to-point connection and uses a different Layer 2 frame that does not require the use of a MAC address. In this example, when Ethernet frames are received on R2 from the Fa0/0 interface, destined for PC2, they are de-encapsulated and then re-encapsulated for the serial interface, such as a PPP encapsulated frame. When R3 receives the PPP frame, it is de-encapsulated again and then re-encapsulated into an Ethernet frame with a destination MAC address of 0B-20, prior to being forwarded out the Fa0/0 interface.

Send a Packet (1.2.1.2)

In Figure 1-24, PC1 is sending a packet to PC2. PC1 must determine if the destination IPv4 address is on the same network. PC1 determines its own subnet by doing an AND operation on its own IPv4 address and subnet mask. This produces the network address that PC1 belongs to. Next, PC1 does this same AND operation using the packet destination IPv4 address and the PC1 subnet mask.

Figure 1-24 PC1 Sends a Packet to PC2

If the destination network address is the same network as PC1, then PC1 does not use the default gateway. Instead, PC1 refers to its Address Resolution Protocol (ARP) cache for the MAC address of the device with that destination IPv4 address. If the MAC address is not in the cache, then PC1 generates an ARP request to acquire the address to complete the packet and send it to the destination. If the destination network address is on a different network, then PC1 forwards the packet to its default gateway.

To determine the MAC address of the default gateway, PC1 checks its ARP table for the IPv4 address of the default gateway and its associated MAC address.

If an ARP entry does not exist in the ARP table for the default gateway, PC1 sends an ARP request. Router R1 sends back an ARP reply. PC1 can then forward the packet to the MAC address of the default gateway, the Fa0/0 interface of router R1.

A similar process is used for IPv6 packets. However, instead of the ARP process, IPv6 address resolution uses ICMPv6 Neighbor Solicitation and Neighbor Advertisement messages. IPv6-to-MAC address mappings are kept in a table similar to the ARP cache, called the neighbor cache.

Forward to the Next Hop (1.2.1.3)

Figure 1-25 shows the processes that take place when R1 receives the Ethernet frame from PC1.

Figure 1-25 R1 Looks Up Route to Destination

1. R1 examines the destination MAC address, which matches the MAC address of the receiving interface on R1, FastEthernet 0/0. R1, therefore, copies the frame into its buffer.

2. R1 identifies the Ethernet Type field as 0 × 800, which means that the Ethernet frame contains an IPv4 packet in the data portion of the frame.

3. R1 de-encapsulates the Ethernet frame to examine the Layer 3 information.

4. Because the destination IPv4 address of the packet does not match any of the directly connected networks of R1, R1 consults its routing table to route this packet. R1 searches the routing table for a network address that would include the destination IPv4 address of the packet as a host address within that network. In this example, the routing table has a route for the 192.168.4.0/24 network. The destination IPv4 address of the packet is 192.168.4.10, which is a host IPv4 address on that network.

The route that R1 finds to the 192.168.4.0/24 network has a next-hop IPv4 address of 192.168.2.2 and an exit interface of FastEthernet 0/1. This means that the IPv4 packet is encapsulated in a new Ethernet frame with the destination MAC address of the IPv4 address of the next-hop router.

Figure 1-26 show the processes that take place when R1 forwards the packet to R2.

Figure 1-26 R1 Forwards Packet to R2

Because the exit interface is on an Ethernet network, R1 must resolve the next-hop IPv4 address with a destination MAC address using ARP:

1. R1 looks up the next-hop IPv4 address of 192.168.2.2 in its ARP cache. If the entry is not in the ARP cache, R1 would send an ARP request out of its FastEthernet 0/1 interface and R2 would return an ARP reply. R1 would then update its ARP cache with an entry for 192.168.2.2 and the associated MAC address.

2. The IPv4 packet is now encapsulated into a new Ethernet frame and forwarded out the FastEthernet 0/1 interface of R1.

Packet Routing (1.2.1.4)

Figure 1-27 shows the processes that take place when R2 receives the frame on its Fa0/0 interface.

Figure 1-27 R2 Looks Up Route to Destination

1. R2 examines the destination MAC address, which matches the MAC address of the receiving interface, FastEthernet 0/0. R2, therefore, copies the frame into its buffer.

2. R2 identifies the Ethernet Type field as 0 × 800, which means that the Ethernet frame contains an IPv4 packet in the data portion of the frame.

3. R2 de-encapsulates the Ethernet frame.

Figure 1-28 shows the processes that take place when R2 forwards the packet to R3.

Figure 1-28 R2 Forwards Packet to R3

1. Because the destination IPv4 address of the packet does not match any of the interface addresses of R2, R2 consults its routing table to route this packet. R2 searches the routing table for the destination IPv4 address of the packet using the same process R1 used.

The routing table of R2 has a route to the 192.168.4.0/24 network, with a next-hop IPv4 address of 192.168.3.2 and an exit interface of Serial 0/0/0. Because the exit interface is not an Ethernet network, R2 does not have to resolve the next-hop IPv4 address with a destination MAC address.

2. The IPv4 packet is now encapsulated into a new data link frame and sent out the Serial 0/0/0 exit interface.

When the interface is a point-to-point (P2P) serial connection, the router encapsulates the IPv4 packet into the proper data link frame format used by the exit interface (HDLC, PPP, and so on). Because there are no MAC addresses on serial interfaces, R2 sets the data link destination address to an equivalent of a broadcast.

Reach the Destination (1.2.1.5)

The following processes take place when the frame arrives at R3:

1. R3 copies the data link PPP frame into its buffer.

2. R3 de-encapsulates the data link PPP frame.

3. R3 searches the routing table for the destination IPv4 address of the packet. The routing table has a route to a directly connected network on R3. This means that the packet can be sent directly to the destination device and does not need to be sent to another router.

Figure 1-29 shows the processes that take place when R3 forwards the packet to PC2.

Figure 1-29 R3 Forwards Packet to PC2

Because the exit interface is a directly connected Ethernet network, R3 must resolve the destination IPv4 address of the packet with a destination MAC address:

1. R3 searches for the destination IPv4 address of the packet in its ARP cache. If the entry is not in the ARP cache, R3 sends an ARP request out of its FastEthernet 0/0 interface. PC2 sends back an ARP reply with its MAC address. R3 then updates its ARP cache with an entry for 192.168.4.10 and the MAC address that is returned in the ARP reply.

2. The IPv4 packet is encapsulated into a new Ethernet data link frame and sent out the FastEthernet 0/0 interface of R3.

3. When PC2 receives the frame, it examines the destination MAC address, which matches the MAC address of the receiving interface, its Ethernet network interface card (NIC). PC2, therefore, copies the rest of the frame into its buffer.

4. PC2 identifies the Ethernet Type field as 0 × 800, which means that the Ethernet frame contains an IPv4 packet in the data portion of the frame.

5. PC2 de-encapsulates the Ethernet frame and passes the IPv4 packet to the IPv4 process of its operating system.

Path Determination (1.2.2)

A router refers to its routing table when making best path decisions. In this topic, we will examine the path determination function of a router.

Routing Decisions (1.2.2.1)

A primary function of a router is to determine the best path to use to send packets. To determine the best path, the router searches its routing table for a network address that matches the destination IP address of the packet.

The routing table search results in one of three path determinations:

• Directly connected network—If the destination IP address of the packet belongs to a device on a network that is directly connected to one of the interfaces of the router, that packet is forwarded directly to the destination device. This means that the destination IP address of the packet is a host address on the same network as the interface of the router.

• Remote network—If the destination IP address of the packet belongs to a remote network, then the packet is forwarded to another router. Remote networks can only be reached by forwarding packets to another router.

• No route determined—If the destination IP address of the packet does not belong to either a connected or a remote network, the router determines if there is a Gateway of Last Resort available. A Gateway of Last Resort is set when a default route is configured or learned on a router. If there is a default route, the packet is forwarded to the Gateway of Last Resort. If the router does not have a default route, then the packet is discarded.

The logic flowchart in Figure 1-30 illustrates the router packet-forwarding decision process.

Figure 1-30 Packet-Forwarding Decision Process

Best Path (1.2.2.2)

Determining the best path involves the evaluation of multiple paths to the same destination network and selecting the optimum or shortest path to reach that network. Whenever multiple paths to the same network exist, each path uses a different exit interface on the router to reach that network.

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. The routing algorithm generates a value, or a metric, for each path through the network. Metrics can be based on either a single characteristic or several characteristics of a path. Some routing protocols can base route selection on multiple metrics, combining them into a single metric.

The following lists some dynamic protocols and the metrics they use:

• Routing Information Protocol (RIP)—Hop count

• Open Shortest Path First (OSPF)—Cisco’s cost based on cumulative bandwidth from source to destination

• Enhanced Interior Gateway Routing Protocol (EIGRP)—Bandwidth, delay, load, reliability

Figure 1-31 highlights how the path may be different depending on the metric being used.

Figure 1-31 Hop Count Versus Bandwidth as a Metric

Load Balancing (1.2.2.3)

What happens if a routing table has two or more paths with identical metrics to the same destination network?

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 can be configured to use both dynamic routing protocols and static routes.

Figure 1-32 provides an example of equal cost load balancing.

Figure 1-32 Equal Cost Load Balancing

Administrative Distance (1.2.2.4)

It is possible for a router to be configured with multiple routing protocols and static routes. If this occurs, the routing table may have more than one route source for the same destination network. For example, if both RIP and EIGRP are configured on a router, both routing protocols may learn of the same destination network. However, each routing protocol may decide on a different path to reach the destination based on the metrics of that routing protocol. RIP chooses a path based on hop count, whereas EIGRP chooses a path based on its composite metric. How does the router know which route to use?

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. For example, a static route has an AD of 1, whereas an EIGRP-discovered route has an AD of 90. Given two separate routes to the same destination, the router chooses the route with the lowest AD. When a router has the choice of a static route and an EIGRP route, the static route takes precedence. Similarly, a directly connected route with an AD of 0 takes precedence over a static route with an AD of 1.

Table 1-4 lists various routing protocols and their associated ADs.

Table 1-4 Default Administrative Distances

Page 2

To make routing decisions, a router exchanges information with other routers. Alternatively, the router can also be manually configured on how to reach a specific network.

In this section you will explain how a router learns about remote networks when operating in a small to medium-sized business network.

Analyze the Routing Table (1.3.1)

The routing table is at the heart of making routing decisions. It is important that you understand the information presented in a routing table. In this topic, you will learn about routing table entries for directly connected networks.

The Routing Table (1.3.1.1)

The routing table of a router stores information about the following:

• Directly connected routes—These routes come from the active router interfaces. Routers add a directly connected route when an interface is configured with an IP address and is activated.

• Remote routes—These are remote networks connected to other routers. Routes to these networks can be either statically configured or dynamically learned through dynamic routing protocols.

Specifically, a routing table is a data file in RAM that stores route information about directly connected and remote networks. The routing table contains network or next-hop associations. These associations tell a router that a particular destination can be optimally reached by sending the packet to a specific router that represents the next hop on the way to the final destination. The next-hop association can also be the outgoing or exit interface to the next destination.

Figure 1-33 identifies the directly connected networks and remote networks of router R1.

Figure 1-33 Directly Connected and Remote Network Routes

Routing Table Sources (1.3.1.2)

On a Cisco router, the show ip route command is used to display the IPv4 routing table of a router. A router provides additional route information, including how the route was learned, how long the route has been in the table, and which specific interface to use to get to a predefined destination.

Entries in the routing table can be added as follows:

• Local route interfaces—Added when an interface is configured and active. This entry is only displayed in IOS 15 or newer for IPv4 routes and all IOS releases for IPv6 routes.

• Directly connected interfaces—Added to the routing table when an interface is configured and active.

• Static routes—Added when a route is manually configured and the exit interface is active.

• Dynamic routing protocol—Added when routing protocols that dynamically learn about the network, such as EIGRP and OSPF, are implemented and networks are identified.

The sources of the routing table entries are identified by a code. The code identifies how the route was learned. For instance, common codes include the following:

• L—Identifies the address assigned to a router’s interface. This allows the router to efficiently determine when it receives a packet for the interface instead of being forwarded.

• C—Identifies a directly connected network.

• S—Identifies a static route created to reach a specific network.

• D—Identifies a dynamically learned network from another router using EIGRP.

• O—Identifies a dynamically learned network from another router using the OSPF routing protocol.

Example 1-14 shows the routing table for the R1 router in Figure 1-20.

Example 1-14 Routing Table for R1

Remote Network Routing Entries (1.3.1.3)

As a network administrator, it is imperative to know how to interpret the content of IPv4 and IPv6 routing tables. Figure 1-34 displays an IPv4 routing table entry on R1 for the route to remote network 10.1.1.0.

Figure 1-34 Remote Network Entry Identifiers

Table 1-5 describes the parts of the routing table entry shown in Figure 1-34.

Table 1-5 Parts of a Remote Network Entry

Directly Connected Routes (1.3.2)

In this topic you will learn how a router builds a routing table of directly connected networks.

Directly Connected Interfaces (1.3.2.1)

A newly deployed router, without configured interfaces, has an empty routing table, as shown in Example 1-15.

Example 1-15 Empty Routing Table

Before the interface state is considered up/up and added to the IPv4 routing table, the interface must

• Be assigned a valid IPv4 or IPv6 address

• Be activated with the no shutdown command

• Receive a carrier signal from another device (router, switch, host, and so on)

When the interface is up, the network of that interface is added to the routing table as a directly connected network.

Directly Connected Routing Table Entries (1.3.2.2)

An active, properly configured, directly connected interface actually creates two routing table entries. Figure 1-35 displays the IPv4 routing table entries on R1 for the directly connected network 192.168.10.0.

Figure 1-35 Directly Connected Network Entry Identifiers

The routing table entry for directly connected interfaces is simpler than the entries for remote networks. Table 1-6 describes the parts of the routing table entry shown in Figure 1-35.

Table 1-6 Parts of a Directly Connected Network Entry

Directly Connected Examples (1.3.2.3)

Example 1-16 shows the steps to configure and activate the interfaces attached to R1 in Figure 1-20. Notice the Layer 1 and 2 informational messages generated as each interface is activated.

Example 1-16 Configuring the Directly Connected IPv4 Interfaces

As each interface is added, the routing table automatically adds the connected (‘C’) and local (‘L’) entries. Example 1-17 provides an example of the routing table with the directly connected interfaces of R1 configured and activated.

Example 1-17 Verifying the Directly Connected Routing Table Entries

Directly Connected IPv6 Example (1.3.2.4)

Example 1-18 shows the configuration steps for the directly connected interfaces of R1 in Figure 1-21 with the indicated IPv6 addresses. Notice the Layer 1 and Layer 2 informational messages generated as each interface is configured and activated.

Example 1-18 Configuring the Directly Connected IPv6 Interfaces

The show ipv6 route command shown in Example 1-19 is used to verify that IPv6 networks and specific IPv6 interface addresses have been installed in the IPv6 routing table. Like IPv4, a ‘C’ next to a route indicates that this is a directly connected network. An ‘L’ indicates the local route. In an IPv6 network, the local route has a /128 prefix. Local routes are used by the routing table to efficiently process packets with a destination address of the interface of the router.

Example 1-19 Verifying IPv6 Routing Table

Notice that there is also a route installed to the FF00::/8 network. This route is required for multicast routing.

Example 1-20 displays how the show ipv6 route command can be combined with a specific network destination to display the details of how the router learned that route.

Example 1-20 Verifying a Single IPv6 Route Entry

Example 1-21 displays how connectivity to R2 can be verified using the ping command. Notice what happens when the G0/0 LAN interface of R2 is the target of the ping command. The pings are unsuccessful. This is because R1 does not have an entry in the routing table to reach the 2001:DB8:ACAD:4::/64 network.

Example 1-21 Testing Connectivity to R2

R1 requires additional information to reach a remote network. Remote network route entries can be added to the routing table using either of the following:

• Static routing

• Dynamic routing protocols

Statically Learned Routes (1.3.3)

In this topic you will learn how a router builds a routing table using static routes.

Static Routes (1.3.3.1)

After directly connected interfaces are configured and added to the routing table, static or dynamic routing can be implemented.

Static routes are manually configured. They define an explicit path between two networking devices. Unlike a dynamic routing protocol, static routes are not automatically updated and must be manually reconfigured if the network topology changes. The benefits of using static routes include improved security and resource efficiency. Static routes use less bandwidth than dynamic routing protocols, and no CPU cycles are used to calculate and communicate routes. The main disadvantage to using static routes is the lack of automatic reconfiguration if the network topology changes.

There are two common types of static routes in the routing table:

• Static route to a specific network

• Default static route

A static route can be configured to reach a specific remote network. IPv4 static routes are configured using the following command:

A static route is identified in the routing table with the code ‘S.’

A default static route is similar to a default gateway on a host. The default static route specifies the exit point to use when the routing table does not contain a path for the destination network. A default static route is useful when a router has only one exit point to another router, such as when the router connects to a central router or service provider.

To configure an IPv4 default static route, use the following command:

Figure 1-36 provides a simple scenario of how default and static routes can be applied.

Figure 1-36 Static and Default Route Scenario

Static Route Examples (1.3.3.2)

Example 1-22 shows the configuration and verification of an IPv4 default static route on R1 from Figure 1-20. The static route is using Serial 0/0/0 as the exit interface. Notice that the configuration of the route generated an ‘S*’ entry in the routing table. The ‘S’ signifies that the route source is a static route, whereas the asterisk (*) identifies this route as a possible candidate to be the default route. In fact, it has been chosen as the default route as evidenced by the line that reads, “Gateway of Last Resort is 0.0.0.0 to network 0.0.0.0.”

Example 1-22 Configuring and Verifying a Default Static IPv4 Route

Example 1-23 shows the configuration and verification of two static routes from R2 to reach the two LANs on R1. The route to 192.168.10.0/24 has been configured using the exit interface while the route to 192.168.11.0/24 has been configured using the next-hop IPv4 address. Although both are acceptable, there are some differences in how they operate. For instance, notice how different they look in the routing table. Also notice that because these static routes were to specific networks, the output indicates that the Gateway of Last Resort is not set.

Example 1-23 Configuring and Verifying Static IPv4 Routes

Static IPv6 Route Examples (1.3.3.3)

Like IPv4, IPv6 supports static and default static routes. They are used and configured like IPv4 static routes.

To configure a default static IPv6 route, use the ipv6 route ::/0 {ipv6-address | interface-type interface-number} global configuration command.

Example 1-24 shows the configuration and verification of a default static route on R1 from Figure 1-21. The static route is using Serial 0/0/0 as the exit interface.

Example 1-24 Configuring and Verifying a Default Static IPv6 Route

Notice in the output that the default static route configuration generated an ‘S’ entry in the routing table. The ‘S’ signifies that the route source is a static route. Unlike the IPv4 static route, there is no asterisk (*) or Gateway of Last Resort explicitly identified.

Like IPv4, static routes are routes explicitly configured to reach a specific remote network. Static IPv6 routes are configured using the ipv6 route ipv6-prefix/prefix-length {ipv6-address|interface-type interface-number} global configuration command.

Example 1-25 shows the configuration and verification of two static routes from R2 to reach the two LANs on R1. The route to the 2001:0DB8:ACAD:2::/64 LAN is configured with an exit interface, whereas the route to the 2001:0DB8:ACAD:1::/64 LAN is configured with the next-hop IPv6 address. The next-hop IPv6 address can be either an IPv6 global unicast or a link-local address.

Example 1-25 Configuring and Verifying Static IPv6 Routes

Example 1-26 confirms remote network connectivity to the 2001:0DB8:ACAD:4::/64 LAN on R2 from R1.

Example 1-26 Verify Connectivity to Remote Network

Dynamic Routing Protocols (1.3.4)

In this topic you will learn how a router builds a routing table using dynamic routes.

Dynamic Routing (1.3.4.1)

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

Network discovery is the ability of a routing protocol to share information about the networks that it knows about with other routers that are also using the same routing protocol. Instead of depending on manually configured static routes to remote networks on every router, a dynamic routing protocol allows the routers to automatically learn about these networks from other routers. These networks, and the best path to each, are added to the routing table of the router and identified as a network learned by a specific dynamic routing protocol.

During network discovery, routers exchange routes and update their routing tables. Routers have converged after they have finished exchanging and updating their routing tables. Routers then maintain the networks in their routing tables.

Figure 1-37 provides a simple scenario of how two neighboring routers would initially exchange routing information. In this simplified exchange, R1 introduces itself and the networks it can reach. R2 responds with its list of networks.

Figure 1-37 Dynamic Routing Scenario

IPv4 Routing Protocols (1.3.4.2)

A router running a dynamic routing protocol does not only make a best path determination to a network; it also determines a new best path if the initial path becomes unusable (or if the topology changes). For these reasons, dynamic routing protocols have an advantage over static routes. Routers that use dynamic routing protocols automatically share routing information with other routers and compensate for any topology changes without involving the network administrator.

Cisco routers can support a variety of dynamic IPv4 routing protocols, including these:

• EIGRP—Enhanced Interior Gateway Routing Protocol

• OSPF—Open Shortest Path First

• IS-IS—Intermediate System-to-Intermediate System

• RIP—Routing Information Protocol

To determine which routing protocols the IOS supports, use the router ? command in global configuration mode, as shown in Example 1-27.

Example 1-27 IPv4 Routing Protocols

IPv4 Dynamic Routing Examples (1.3.4.3)

In this dynamic routing example, assume that R1 and R2 have been configured to support the dynamic routing protocol EIGRP. R2 now has a connection to the Internet, as shown in Figure 1-38. The routers also advertise directly connected networks. R2 advertises that it is the default gateway to other networks.

Figure 1-38 IPv4 Topology with Connection to the Internet

The output in Example 1-28 displays the routing table of R1 after the routers have exchanged updates and converged.

Example 1-28 Verify Dynamic IPv4 Routes

Along with the connected and link-local interfaces, there are three ‘D’ entries in the routing table.

• The entry beginning with ‘D*EX’ identifies that the source of this entry was EIGRP (‘D’). The route is a candidate to be a default route (‘*’), and the route is an external route (‘*EX’) forwarded by EIGRP.

• The other two ‘D’ entries are routes installed in the routing table based on the update from R2 advertising its LANs.

IPv6 Routing Protocols (1.3.4.4)

ISR devices support the dynamic IPv6 routing protocols shown in Example 1-29.

Example 1-29 IPv6 Routing Protocols

Support for dynamic IPv6 routing protocols is dependent on hardware and IOS version. Most of the modifications in the routing protocols are to support the longer IPv6 addresses and different header structures.

IPv6 routing is not enabled by default. Therefore, to enable IPv6 routers to forward traffic, you must configure the ipv6 unicast-routing global configuration command.

IPv6 Dynamic Routing Examples (1.3.4.5)

Routers R1 and R2 in Figure 1-21 have been configured with the dynamic routing protocol EIGRP for IPv6. (This is the IPv6 equivalent of EIGRP for IPv4.)

To view the routing table on R1, enter the show ipv6 route command, as shown in Example 1-30.

Example 1-30 Verify Dynamic IPv6 Routes

The output shows the routing table of R1 after the routers have exchanged updates and converged. Along with the connected and local routes, there are two ‘D’ entries (EIGRP routes) in the routing table.

Page 3

There are many key structures and performance-related characteristics referred to when discussing networks: topology, speed, cost, security, availability, scalability, and reliability.

Cisco routers and Cisco switches have many similarities. They support a similar modal operating system, similar command structures, and many of the same commands. One distinguishing feature between switches and routers is the type of interfaces supported by each. Once an interface is configured on both devices, the appropriate show commands need to be used to verify a working interface.

The main purpose of a router is to connect multiple networks and forward packets from one network to the next. This means that a router typically has multiple interfaces. Each interface is a member or host on a different IP network.

Cisco IOS uses what is known as the administrative distance (AD) to determine the route to install into the IP routing table. The routing table is a list of networks the router knows. The routing table includes network addresses for its own interfaces, which are the directly connected networks, as well as network addresses for remote networks. A remote network is a network that can only be reached by forwarding the packet to another router.

Remote networks are added to the routing table in two ways: either by the network administrator manually configuring static routes or by implementing a dynamic routing protocol. Static routes do not have as much overhead as dynamic routing protocols; however, static routes can require more maintenance if the topology is constantly changing or is unstable.

Dynamic routing protocols automatically adjust to changes without intervention from the network administrator. Dynamic routing protocols require more CPU processing and use a certain amount of link capacity for routing updates and messages. In many cases, a routing table will contain both static and dynamic routes.

Routers make their primary forwarding decision at Layer 3, the network layer. However, router interfaces participate in Layers 1, 2, and 3. Layer 3 IP packets are encapsulated into a Layer 2 data link frame and encoded into bits at Layer 1. Router interfaces participate in Layer 2 processes associated with their encapsulation. For example, an Ethernet interface on a router participates in the ARP process like other hosts on that LAN.

The Cisco IP routing table is not a flat database. The routing table is actually a hierarchical structure that is used to speed up the lookup process when locating routes and forwarding packets.

Components of the IPv6 routing table are similar to the IPv4 routing table. For instance, it is populated using directly connected interfaces, static routes, and dynamically learned routes.

Page 4

The following activities provide practice with the topics introduced in this chapter. The Labs and Class Activities are available in the companion Routing and Switching Essentials v6 Labs and Study Guide (ISBN 9781587134265). The Packet Tracer Activities PKA files are found in the online course.

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