Chapter Description

This sample chapter from Routing and also Switching Essentials v6 Companion Guide, answer the question, “What does a rexternal perform with a packet received from one network-related and destined for one more network?” Details of the routing table will be examined, including connected, static, and also dynamic routes.

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Routing and Switching Essentials v6 Companion Guide

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Router Operation (1.3)

To make routing decisions, a router extransforms indevelopment via various other routers. Alternatively, the router deserve to additionally be manually configured on exactly how to reach a details netoccupational.

In this section you will define how a router learns around remote netfunctions as soon as operating in a little to medium-sized organization netoccupational.

Analyze the Routing Table (1.3.1)

The routing table is at the heart of making routing decisions. It is vital that you understand the indevelopment presented in a routing table. In this topic, you will learn around routing table entries for directly associated netfunctions.

The Routing Table (

The routing table of a rexternal stores information around the following:

Directly associated routesThese routes come from the active rexternal interencounters. Routers add a straight connected course when an interchallenge is configured via an IP resolve and also is caused.

Remote routesThese are remote netfunctions connected to various other routers. Routes to these netfunctions deserve to be either statically configured or dynamically learned via dynamic routing protocols.

Specifically, a routing table is a data record in RAM that stores route information about straight connected and also remote networks. The routing table consists of netoccupational or next-hop associations. These associations tell a router that a specific location have the right to be optimally reached by sending the packet to a specific router that represents the following hop on the means to the final destination. The next-hop association have the right to additionally be the outgoing or departure interface to the next location.

Figure 1-33 identifies the straight linked networks and also remote networks of rexternal R1.


Figure 1-33 Directly Connected and also Remote Network Routes

Routing Table Sources (

On a Cisco router, the display ip route command is offered to display screen the IPv4 routing table of a rexternal. A router offers extra route information, consisting of how the route was learned, how long the path has actually been in the table, and also which specific interface to use to acquire to a preidentified location.

Entries in the routing table deserve to be included as follows:

Local path interfacesAdded once an interface is configured and also active. This entry is just presented in IOS 15 or more recent for IPv4 routes and also all IOS releases for IPv6 routes.

Directly linked interfacesAdded to the routing table once an interchallenge is configured and energetic.

Static routes—Added as soon as a course is manually configured and the exit interchallenge is energetic.

Dynamic routing protocol—Added once routing protocols that dynamically learn about the network-related, such as EIGRP and OSPF, are imposed and also networks are identified.

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

L—Identifies the resolve assigned to a router’s interconfront. This enables the rexternal to efficiently determine as soon as it receives a packet for the interchallenge rather of being forwarded.

C—Identifies a straight linked netoccupational.

S—Identifies a static course developed to reach a specific network-related.

D—Identifies a dynamically learned netoccupational from one more router using EIGRP.

O—Identifies a dynamically learned network-related from an additional rexternal utilizing the OSPF routing protocol.

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

Example 1-14 Routing Table for R1

Figure 1-34 Remote Netjob-related Entry Identifiers

Table 1-5 describes the components of the routing table entry displayed in Figure 1-34.

Table 1-5 Parts of a Remote Netoccupational Entry





Route Source

Identifies just how the route was learned.


Destination Network

Identifies the IPv4 deal with of the remote netoccupational.


Administrative Distance

Identifies the trustworthiness of the path resource.Lower values indicate desired path resource.



Identifies the worth assigned to reach the remote network-related. Lower worths suggest desired courses.


Next Hop

Identifies the IPv4 deal with of the following rexternal to forward the packet to.


Route Timestamp

Identifies how a lot time has actually passed given that the course was learned.


Outgoing Interface

Identifies the exit interface to use to forward a packet toward the final destination.

Activity Interpret the Content of a Routing Table Entry

Refer to the digital course to complete this activity.

Directly Connected Routes (1.3.2)

In this topic you will learn how a router builds a routing table of straight linked netfunctions.

Directly Connected Interencounters (

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

Example 1-15 Empty Routing Table

R1# show ip routeCodes: L - regional, C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP exterior, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA exterior type 1, N2 - OSPF NSSA outside kind 2 E1 - OSPF outside kind 1, E2 - OSPF outside kind 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candiday default, U - per-user static course, o - ODR P - routine downloaded static routeGateway of last retype is not setR1#Before the interconfront state is considered up/up and also added to the IPv4 routing table, the interchallenge must

Be assigned a valid IPv4 or IPv6 address

Be activated via the no shutdown command

Receive a carrier signal from another gadget (rexternal, switch, hold, and also so on)

When the interchallenge is up, the netjob-related of that interface is included to the routing table as a straight linked netoccupational.

Directly Connected Routing Table Entries (

An energetic, correctly configured, directly connected interchallenge actually creates two routing table entries. Figure 1-35 displays the IPv4 routing table entries on R1 for the straight associated netoccupational


Figure 1-35 Directly Connected Netjob-related Entry Identifiers

The routing table entry for directly connected interdeals with is much easier than the entries for remote netfunctions. Table 1-6 describes the parts of the routing table entry presented in Figure 1-35.

Table 1-6 Parts of a Directly Connected Network-related Entry





Route Source

Identifies just how the network was learned by the router.Directly connected interdeals with have 2 route resource codes. ‘C’ identifies a directly connected netoccupational. ‘L’ identifies the IPv4 address assigned to the router’s interchallenge.


Desticountry Network

Identifies the location network-related and also how it is connected.


Outgoing Interface

Identifies the departure interconfront to use when forwarding packets to the destination netoccupational.


Prior to IOS 15, local route routing table entries (L) were not displayed in the IPv4 routing table. Local path (L) entries have always been component of the IPv6 routing table.

Directly Connected Instances (

Example 1-16 mirrors the procedures to configure and also activate the interfaces attached to R1 in Figure 1-20. Notice the Layer 1 and also 2 informational messages produced as each interconfront is activated.

Example 1-16 Configuring the Directly Connected IPv4 InterfacesExample 1-17 Verifying the Directly Connected Routing Table EntriesExample 1-18 Configuring the Directly Connected IPv6 Interfaces

R1(config)# interface gigabitethernet 0/0R1(config-if)# summary Link to LAN 1R1(config-if)# ipv6 deal with 2001:db8:acad:1::1/64R1(config-if)# no shutdownR1(config-if)# exit*Feb 3 21:38:37.279: %LINK-3-UPDOWN: Interface GigabitEthernet0/0, changed state to down*Feb 3 21:38:40.967: %LINK-3-UPDOWN: Interconfront GigabitEthernet0/0, adjusted state to up*Feb 3 21:38:41.967: %LINEPROTO-5-UPDOWN: Line protocol on Interconfront GigabitEther- net0/0, readjusted state to upR1(config)# interface gigabitethernet 0/1R1(config-if)# description Link to LAN 2R1(config-if)# ipv6 resolve 2001:db8:acad:2::1/64R1(config-if)# no shutdownR1(config-if)# exit*Feb 3 21:39:21.867: %LINK-3-UPDOWN: Interface GigabitEthernet0/1, changed state to down*Feb 3 21:39:24.967: %LINK-3-UPDOWN: Interchallenge GigabitEthernet0/1, changed state to up*Feb 3 21:39:25.967: %LINEPROTO-5-UPDOWN: Line protocol on Interconfront Gigalittle bit Ethernet0/1, changed state to upR1(config)# interface serial 0/0/0R1(config-if)# description Link to R2R1(config-if)# ipv6 resolve 2001:db8:acad:3::1/64R1(config-if)# clock price 128000R1(config-if)# no shutdown*Feb 3 21:39:43.307: %LINK-3-UPDOWN: Interconfront Serial0/0/0, changed state to downR1(config-if)# endR1#The display ipv6 route command presented in Example 1-19 is supplied to verify that IPv6 netfunctions and also specific IPv6 interchallenge addresses have been mounted in the IPv6 routing table. Like IPv4, a ‘C’ beside a path indicates that this is a directly connected network. An ‘L’ suggests the local course. In an IPv6 netoccupational, the regional course has actually a /128 presolve. Local routes are provided by the routing table to effectively process packets via a location attend to of the interchallenge of the router.

Example 1-19 Verifying IPv6 Routing Table

R1# present ipv6 routeIPv6 Routing Table - default - 5 entriesCodes: C - Connected, L - Local, S - Static, U - Per-user Static route B - BGP, R - RIP, H - NHRP, I1 - ISIS L1 I2 - ISIS L2, IA - ISIS interlocation, IS - ISIS summary, D - EIGRP EX - EIGRP outside, ND - ND Default, NDp - ND Predeal with, DCE - Destination NDr - Restraight, O - OSPF Intra, OI - OSPF Inter, OE1 - OSPF ext 1 OE2 - OSPF ext 2, ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2C 2001:DB8:ACAD:1::/64 <0/0> by means of GigabitEthernet0/0, directly connectedL 2001:DB8:ACAD:1::1/128 <0/0> through GigabitEthernet0/0, receiveC 2001:DB8:ACAD:2::/64 <0/0> using GigabitEthernet0/1, straight connectedL 2001:DB8:ACAD:2::1/128 <0/0> via GigabitEthernet0/1, receiveL FF00::/8 <0/0> via Null0, receiveR1#Notice that tbelow is also a route set up to the FF00::/8 netoccupational. This course is compelled for multicast routing.

Example 1-20 displays just how the present ipv6 route command deserve to be merged through a details netoccupational location to display the details of just how the router learned that course.

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Example 1-20 Verifying a Single IPv6 Route Entry

R1# show ipv6 route 2001:db8:acad:1::/64Routing enattempt for 2001:DB8:ACAD:1::/64 Knvery own through "connected", distance 0, metric 0, form connected Route count is 1/1, share count 0 Routing paths: directly associated through GigabitEthernet0/0 Last updated 03:14:56 agoR1#Example 1-21 screens how connectivity to R2 deserve to be showed making use of the ping command. Notice what happens once the G0/0 LAN interchallenge of R2 is the taracquire of the ping command. The pings are unsuccessful. This is bereason R1 does not have actually an entry in the routing table to reach the 2001:DB8:ACAD:4::/64 netoccupational.

Example 1-21 Testing Connectivity to R2