CCNA R&S-20-Configuring IPv4 Addresses and Routes

©2015 Amir Jafari – www.amir-Jafari.com
Routing and Switching 200-120
20 - Configuring IPv4 Addresses and Routes

Configuring IPv4 Addresses and Routes
Agenda
IP Routing
Configuring Connected Routes
Configuring Static Routes

IP Routing
IP Routing
Routers route IPv4 packets
For routers to route packets, routers follow a routing process
That routing process relies on information called IP routes
Each IP route lists a destination—an IP network, IP subnet, or some other group of IP
addresses
Each route also lists instructions that tell the router where to forward packets sent to
addresses in that IP network or subnet

IP Routing
IP Routing
Routers use three methods to add IPv4 routes to their IPv4 routing tables:
1. Routers first learn connected routes, which are routes for subnets attached to a router
interface
2. Routers can also use static routes, which are routes created through a configuration
command (ip route) that tells the router what route to put in the IPv4 routing table
3. Routers can use a routing protocol, in which routers tell each other about all their
known routes, so that all routers can learn and build routes to all networks and subnets

IP Routing
IP Routing
The process requires Layer 3 logic on the routers as well, by which the routers compare
the destination address in the packet to their routing tables, to decide where to
forward the IP packet next
The routing process also relies on data link and physical details at each link
These lower-layer devices and protocols move the IP packets around the TCP/IP
network by encapsulating and transmitting the packets inside data link layer frames
The term “IP routing” refers to the packet-forwarding process only

IP Routing
IPv4 Routing Process Reference
Based on its own opinion of the range of addresses in the local subnet, a LAN-based host
acts as follows:
Step 1. If the destination is local, send directly:
A. Find the destination host’s MAC address. Use the already-known Address Resolution
Protocol (ARP) table entry, or use ARP messages to learn the information
B. Encapsulate the IP packet in a data link frame, with the destination data link address
of the destination host
Step 2. If the destination is not local, send to the default gateway:
A. Find the default gateway’s MAC address. Use the already-known ARP table entry,
or use ARP messages to learn the information
B. Encapsulate the IP packet in a data link frame, with the destination data link address
of the default gateway

IP Routing
Host Routing Logic Summary

IP Routing
Routers have a little more routing work to do as compared with hosts. The five-step
summary of a router’s routing logic are as follow:
1. For each received data link frame, choose whether or not to process the frame.
Process it if:
A. The frame has no errors (per the data link trailer Frame Check Sequence, or FCS,
field)
B. The frame’s destination data link address is the router’s address (or an appropriate
multicast or broadcast address)
2. If choosing to process the frame at Step 1, deencapsulate the packet from inside the
data link frame

IP Routing
3. Make a routing decision. To do so, compare the packet’s destination IP address to the
routing table and find the route that matches the destination address. This route identifies
the outgoing interface of the router and possibly the next-hop router
4. Encapsulate the packet into a data link frame appropriate for the outgoing interface.
When forwarding out LAN interfaces, use ARP as needed to find the next device’s MAC
address
5. Transmit the frame out the outgoing interface, as listed in the matched IP route

IP Routing
Router Routing Logic Summary

IP Routing
An Example of IP Routing
IPv4 Network Used to Show Five-Step Routing Example
Host A (172.16.1.9) sends a packet to host B (172.16.2.9)

IP Routing
Host Forwards the IP Packet to the Default Router (Gateway)
Host A Sends Packet to Host B
To send the packet, encapsulate it in an Ethernet frame. Make the destination MAC
address be R1’s G0/0 MAC address (host A’s default gateway)

IP Routing
Routing Step 1: Decide Whether to Process the Incoming Frame
The first step in the routing process begins with a decision of whether a router should
process the frame or silently discard (ignore) the frame
The router makes no attempt at error recovery; that is, the router does not ask the
sender to retransmit the data
Routing Step 1, on Router R1: Checking FCS and Destination MAC

IP Routing
Routing Step 2: Deencapsulation of the IP Packet
Routing Step 2 on Router R1: Deencapsulating the Packet

IP Routing
Routing Step 3: Choosing Where to Forward the Packet
Routes for remote subnets typically list both an outgoing interface and next-hop router IP
address. Routes for subnets that connect directly to the router list only the outgoing
interface, because packets to these destinations do not need to be sent to another router
Routing Step 3 on Router R1: Matching the Routing Table

IP Routing
Routing Step 4: Encapsulating the Packet in a New Frame
Routing Step 4 on Router R1: Encapsulating the Packet

IP Routing
Routing Step 4: Encapsulating the Packet in a New Frame
Routing Step 4 on Router R1 with a LAN Outgoing Interface

IP Routing
Routing Step 5: Transmitting the Frame
After the frame has been prepared, the router simply needs to transmit the frame
The router might have to wait, particularly if other frames are already waiting their
turn to exit the interface

IP Routing
Potential Routing Performance Issues
Routers barely spend any processing time to route a single IP packet. In fact, even
slower routers need to forward tens of thousands of packets per second (pps); to do
that, the routers cannot spend a lot of effort processing each packet
The process of matching a packet’s destination address with the IP routing table can
actually take a lot of CPU time.
Routers in the core of the Internet have hundreds of thousands of routes. Now think
about a router CPU that needs to search a list 100,000 entries long, for every packet,
for a router that needed to forward hundreds of thousands of packets per second!
And what if the router had to do subnetting math each time, calculating the range of
addresses in each subnet for each route? Those actions would take too many CPU
cycles

IP Routing
Potential Routing Performance Issues
Over the years, Cisco has created several ways to optimize the internal process of
how routers forward packets.
Some methods tie to a specific model series of router. Layer 3 switches do the
forwarding in Application Specific Integrated Circuits (ASIC), which are computer
chips built for the purpose of forwarding frames or packets.
All these optimizations take the basic logic from the fivestep list here, but work
differently inside the router hardware and software, in an effort to use fewer CPU
cycles and reduce the overhead of forwarding IP packets

IP Routing
Cisco Router Fast Switching and CEF
Historically speaking, Cisco has had three major variations of internal routing logic that
apply across the entire router product family:
1. First, Cisco routers used internal logic called process switching which works basically
like the routing process detailed so far in this slide, without any of the extra optimizations.
2. Next, Cisco introduced alternate internal routing logic called fast switching which made
a couple of optimizations compared to the older process-switching logic.
A. First, it kept another list in addition to the routing table, listing specific IP addresses
for recently forwarded packets.
B. This fast-switching cache also kept a copy of the new data link headers used when
forwarding packets to each destination, so rather than build a new data link header
for each packet destined for a particular IP address, the router saved a little effort
by copying the old data link header.

IP Routing
3. Cisco improved over fast switching with the introduction of Cisco Express Forwarding
(CEF)
CEF uses additional tables for faster searches, and it saves outgoing data link headers.
However, CEF organizes its tables for all routing table destinations, ahead of time,
not just for some of the specific destination IP addresses.
CEF also uses much more sophisticated search algorithms and binary tree structures
as compared to fast switching.
As a result, the CEF table lookups that replace the routing table matches take even less
time than with fast switching. And it caches the data link headers as well. Today, current
models of Cisco routers, and current IOS versions, use CEF by default.

IP Routing
Comparisons of Packet Switching, Fast Switching, and CEF

Cisco routers enable IPv4 routing globally, by default
Then, to make the router be ready to route packets on a particular interface, the router
must be configured with an IP address and the interface must be configured such that it
comes up, reaching a “line status up, line protocol up” state
Only at that point can routers route IP packets in and out a particular interface
After a router can route IP packets out one or more interfaces, the router needs some
routes

Routers can add routes to their routing tables through three methods:
1. Connected routes: Added because of the configuration of the ip address interface
subcommand on the local router
2. Static routes: Added because of the configuration of the ip route global command on
the local router
3. Routing protocols: Added as a function by configuration on all routers, resulting in a
process by which routers dynamically tell each other about the network so that they all
learn routes

Connected Routes and the ip address Command
A Cisco router automatically adds a route to its routing table for the subnet connected to
each interface, assuming that the following two facts are true:
The interface is in a working state—in other words, the interface status in the show
interfaces command lists a line status of up and a protocol status of up
The interface has an IP address assigned through the ip address interface subcommand

Sample Network to Show Connected Routes

Connected and Local Routes on Router R1

The router also automatically produces a different kind of route, called a local route
The local routes define a route for the one specific IP address configured on the router
interface
Each local route has a /32 prefix length, defining a host route, which defines a route
just for that one IP address
Routers use these local routes that list their own local IP addresses to more efficiently
forward packets sent to the router itself

Routing Between Subnets on VLANs
To route IP packets in and out of those VLANs, some router needs to have an IP address
in each subnet and have a connected route to each of those subnets
Then the hosts in each subnet can use the router IP addresses as their default gateways,
respectively
Three options exist for connecting a router to each subnet on a VLAN:
1. Use a router, with one router LAN interface and cable connected to the switch for
each and every VLAN (typically not used)
2. Use a router, with a VLAN trunk connecting to a LAN switch
3. Use a Layer 3 switch

Routing Between Subnets on VLANs
Subinterfaces on Router R1

Configuring Routing to VLANs using 802.1Q on Routers
When discussing how to route packets to subnets associated with VLANs connected to a
router 802.1Q trunk, the networking world has name for this feature: Router on a Stick
(ROAS)
ROAS uses router VLAN trunking configuration to give the router a logical router
interface connected to each VLAN, and therefore each subnet that sits on a separate
VLAN
The router needs to have an IP address/mask associated with each VLAN on the trunk
However, the router uses only one physical interface on which to configure the ip
address command
Cisco solves this problem by creating multiple virtual router interfaces, one associated
with each VLAN on that trunk. Cisco calls these virtual interfaces subinterfaces
That trunking configuration revolves around subinterfaces

Subinterfaces on Router B1

Cisco routers do not attempt to negotiate trunking, so both the router and switch need
to manually configure trunking.
These steps detail how to configure 802.1Q trunking on a router:
Step 1. Create a unique subinterface for each VLAN that needs to be routed
(interface type number.subint)
Step 2. Enable 802.1Q, and associate one specific VLAN with the subinterface in
subinterface config mode (encapsulation dot1q vlan_id)
Step 3. Configure IP settings (address and mask) in subinterface config mode (ip
address address mask)

Router Configuration for the 802.1Q Encapsulation

The subinterface number begins with the period, like .10 and .20 in this case. These
numbers can be any number from 1 up through a very large number (over 4 billion)
The number just needs to be unique among all subinterfaces associated with this one
physical interface
In fact, the subinterface number does not even have to match the associated VLAN ID
If the physical Ethernet interface reaches an up/up state, the subinterface should as
well, which would then let the router add the connected routes
The ISL configuration on the same router would be practically identical. Just substitute
the keyword isl instead of dot1q in each case

Each 802.1Q trunk has one native VLAN, and when used, the configuration to use that
native VLAN differs, with two options for the router side of the configuration:
Configure the ip address command on the physical interface, but without an
encapsulation command; the router considers this physical interface to be using the
native VLAN
Configure the ip address command on a subinterface, and use the
encapsulation...native subcommand.
Besides just scanning the configuration, the show vlans command on a router spells out
which router trunk interfaces use which VLANs, which VLAN is the native VLAN, plus
some packet statistics

Router Configuration Using Native VLAN 10 on Router B1

show vlans Command to Match Sample Router Trunking Configuration

Configuring Routing to VLANs Using a Layer 3 Switch
A Layer 3 switch is one device that does two primary functions: Layer 2 LAN switching
and Layer 3 IP routing
The configuration of a Layer 3 switch mostly looks like the Layer 2 switching
configuration with a small bit of configuration added for the Layer 3 functions
The Layer 3 switching function needs a virtual interface connected to each VLAN
internal to the switch
These VLAN interfaces act like router interfaces, with an IP address and mask
The Layer 3 switch has an IP routing table, with connected routes off each of these
VLAN interfaces. (These interfaces are also referred to as Switched Virtual Interfaces
[SVI].)

Routing on VLAN Interfaces in a Layer 3 Switch

The following steps show how to configure Layer 3 switching:
Step 1. Enable hardware support for IPv4 routing. For example, on 2960 switches,
configure the sdm prefer lanbase-routing global command and reload the switch
Step 2. Enable IPv4 routing globally (ip routing)
Step 3. Create VLAN interfaces for each VLAN for which the Layer 3 switch is routing
packets (interface vlan vlan_id)
Step 4. Configure an IP address and mask on the VLAN interface (in interface
configuration mode for that interface), enabling IPv4 on that VLAN interface (ip address
address mask)
Step 5. If the switch defaults to place the VLAN interface in a disabled (shutdown) state,
enable the interface (no shutdown)

VLAN Interface Configuration for Layer 3 Switching

To support the routing of packets, the switch adds connected IP routes. Each route
is listed as being connected to a different VLAN interface.
Connected Routes on a Layer 3 Switch

Secondary IP Addressing
Imagine that you planned your IP addressing scheme for a network. Later, a particular
subnet grows, and you have used all the valid IP addresses in the subnet. Three main
options exist which you can do:
1. Make the existing subnet larger, by choosing a mask with more host bits. Existing hosts
have to change their subnet mask settings, and new hosts can use IP addresses from the
expanded address range
2. Migrate to a completely new (larger) subnet. All existing devices change their IP
addresses
3. Add a second subnet in the same location, using secondary addressing. The third
option uses a Cisco router feature called secondary IP addressing. Secondary
addressing uses multiple networks or subnets on the same data link

TCP/IP Network with Secondary Addresses

Secondary addressing allows some hosts to have addresses in one IP subnet, others to
have addresses in a second IP subnet, and the router to have addresses in both
Both IP subnets would be in the same Layer 2 broadcast domain (VLAN). As a result,
the router will have connected routes for both the subnets, so the router can route
packets to both subnets and even between both subnets
Secondary addressing does have one negative: Traffic between hosts on the same
VLAN, but in different subnets, requires a trip through the router

Secondary IP Addressing Configuration and the show ip route Command

Supporting Connected Routes to Subnet Zero
IOS can restrict a router from configuring an ip address command with an address
inside the zero subnet
The zero subnet (or subnet zero) is the one subnet in each classful network that has all
binary 0s in the subnet part of the binary version of the subnet number
In decimal, the zero subnet happens to be the same number as the classful network
number
IOS allows the network engineer to tell a router to either allow addresses in the zero
subnet or not
With the ip subnet-zero command configured, IOS allows the zero subnet with no
restrictions

With the no ip subnet-zero command configured, the router rejects any ip address
command that uses an address/mask combination for the zero subnet
For many of the more recent IOS versions, IOS allows the use of the zero subnet
no ip subnet-zero command affects the local router’s ip address commands, as well as
the local router’s ip route commands (which define static routes)
However, it does not affect the local router’s routes as learned with a routing protocol
For example, R1 could be configured with no ip subnet-zero, but still learn a route for a
zero subnet using a routing protocol

Effects of [no] ip subnet-zero on a Local Router

Static Route Configuration
IOS allows the definition of individual static routes using the ip route global
configuration command
Every ip route command defines a destination that can be matched, usually with a
subnet ID and mask
The command also lists the forwarding instructions, typically listing either the outgoing
interface or the next-hop router’s IP address
IOS then takes that information and adds that route to the IP routing table

Static Route Configuration Concept

Sample Network Used in Static Route Configuration Examples

Static Routes Added to R1

IOS adds and removes these static routes dynamically over time, based on whether
the outgoing interface is working or not.
Later, when the interface comes up again, IOS adds the route back to the routing table.
Also, note that the ip route command also supports the permanent keyword, which tells
IOS to leave the static route in the routing table, even when the associated interface
fails.
Finally, if using static routes, and not using any dynamic routing protocols at all, all
routers would need to have some static routes configured

Static Default Routes
When a router tries to route a packet, the router might not match the packet’s
destination IP address with any route. When that happens, the router normally just
discards the packet
Routers can be configured so that they use either a statically configured or
dynamically learned default route
The default route matches all packets, so that if a packet does not match any other
more specific route in the routing table, the router can at least forward the packet
based on the default route

One classic example in which companies might use static default routes in their
enterprise TCP/IP networks is when the company has many remote sites, each with a
single, relatively slow WAN connection.
Each remote site has only one possible physical route to use to send packets to the rest
of the network.
So, rather than use a routing protocol, which sends messages over the WAN and uses
precious WAN bandwidth, each remote route might use a default route that sends all
traffic to the central site
IOS allows the configuration of a static default route by using special values for the
subnet and mask fields in the ip route command: 0.0.0.0 and 0.0.0.0

Use of Static Default Routes at 1000 Low-Speed Remote Sites

The output of the show ip route command lists a few new and interesting facts.
First, it lists the route with a code of “S,” meaning static, but also with a *, meaning it is
a candidate default route
A router can learn about more than one default route, and the router then has to
choose which one to use
The * means that it is at least a candidate to become the default route
Just above, the “Gateway of Last Resort” refers to the chosen default route, which in
this case is the just-configured static route with outgoing interface S0/0/1

Command Reference
Command Reference

Configuring IPv4 Addresses and Routes
References
1) Cisco Systems, Inc, www.cisco.com/
2) Wendell Odom ,”Cisco CCENT/CCNA ICND1 100-101 Official Cert Guide”, Cisco
Press, USA, 2013

CCNA R&S-20-Configuring IPv4 Addresses and Routes

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