Networking Chapter 8

The Wonderful World of Routing

Chapter 8

Objectives
• Explain how routers work
• Describe dynamic routing technologies
• Install and configure a router successfully

Three Parts to Chapter 8
• How Routers Work
• Dynamic Routing
• Working with Routers

• Router
– Hardware or software
– Forwards packet based on destination IP
address
– Layer 3, the Network layer
– Classically, dedicated boxes
• At least two connections
– Popular router: CISCO 2600

• Home router
– First exposure to a router for most techs
– Connect PC to DSL receiver or cable modem
– More than it appears to be
– LAN side may connect internally to a built-in
switch

Figure 8.2 Business end of a typical home router

Figure 8.3 Cisco router diagram

Figure 8.4 Linksys home router diagram

• All routers
– Examine packets
– Send packets to proper destination

• Routing Tables
– Router strips off Layer 2 information
– Drops IP packet into a queue
• Source address is not important
• Every packet dropped into the same queue based
on time of arrival
– Router inspects the destination IP address
– Router send IP packet out correct port
– Routing table tells router where to send packets

Figure 8.6 All incoming packets stripped of Layer 2
data and dropped into a common queue

• Routing Tables (cont.)
– Example home router (Figure 8.7)
• Two ports
• Each row in table defines a single route
• Each column identifies specific criteria
– Destination LAN IP
– Subnet Mask
• If packet’s network ID matches a Destination LAN IP,
router uses other information in row to determine
where to send it

• Third and fourth columns
– Gateway
» IP address for the next hop router
» Packet sent to Gateway if network ID does not
match one of the directly connected ports
– Interface
» Tells router which port to use
» “LAN” or “WAN” on sample router
» Other routing tables use IP address or other info

• First Router compares destination IP address to
every listing in the routing table
• Then router makes a decision

• Every router (with two exceptions) has a default route
• Default route in Figure 8.7
– Destination LAN IP: 0.0.0.0
– Subnet Mask: 0.0.0.0
– Gateway: 76.30.4.1
– Interface: WAN
• Router sends incoming packet to default route unless
another line gives another route

• Destination LAN IP: 10.12.14.0 (any packet for this
network)
• Subnet Mask: 255.255.255.0 (using /24 network ID)
• Gateway: 0.0.0.0 (don’t use a gateway)
• Interface: LAN (ARP on the LAN interface to get MAC
address and send directly to host)

• Destination LAN IP: 76.30.4.0 (any packet for this
network)
• Subnet Mask: 255.255.254.0 (using /23 network ID)
• Gateway: 0.0.0.0 (don’t use a gateway)
• Interface: WAN (ARP on the WAN interface to get MAC
address and send directly to host)

Figure 8.7 Routing table from a home
router

Figure 8.8 Electronic diagram of the router

Figure 8.9 The network based on the routing table

– Every node on the network has a routing table
• Some computers (multi-homed) have more
than one NIC
• IP uses a routing table for every packet it sends
– Send directly to a host on a LAN or …
– Send to the default gateway

– Computer routing table (next slide)
• More routes than example home router
• Computer IP address: 10.12.14.201/24
• Computer loopback: 127.0.0.1
• Metric: a relative value defining the “cost” of using a
route
– When more than one route to a destination,
lower metric is used
– When route with lower metric goes down,
other route used

Figure 8.10 Two routes to the same network

Figure 8.11 When a route no longer works, the
router automatically switches

Routing table on an XP computer connected to Figure 8.7 router

IPv4 portion of Vista route print command

• Freedom from Layer 2
– Routers can connect different network
technologies
– Routers strip off all Layer 2 data
– Routers can connect almost anything that
stores IP packets

Figure 8.12 Modular Cisco router

Figure 8.15 Redone network IDs; nodes in the LAN
use private IP addressees internally

• Inside the router
– Host 192.168.10.202 sends a packet to 12.43.65.223
– Packet is sent to the gateway router
– Router replaces the sending host’s IP with its
own public IP address
– Router then adds the destination IP address
and the source ephemeral port to the
NAT translation table
– On receiving response, router reverses the IP
addresses and ports

Figure 8.17 Updating the packet

• Inside the router (cont.)
– Router compares incoming destination
port and source address to entries in NAT
translation table
– Determines which local IP address to put
back on the packet
– Sends packet to the correct computer on
the LAN

• Port forwarding
– Hides a port number from the Internet
– Enables public servers to work behind a
NAT router
– Gives servers the protection of NAT while
allowing access to a local server from the
Internet

A. Background
1. Routers have static routes
I. Manually entered
II. Detected at setup by the router

• Background (cont.)
2. Dynamic routing protocols defined
I. Routers communicate among themselves
with change information
II. Update each other on changes about direct
connections and distant routers
III. A passage of a packet through a single router
is a hop

Figure 8.23 Hopping through a WAN

Dynamic Routing Makes the Internet
1. Internet depends on dynamic routing for
self-healing
2. Manual updating impossible with so many
routes coming up and going down

Table 8.1 Dynamic Routing Protocols

Protocol Type IGP(Intergateway protocol)or Notes
BGP(Border Gateway)?
RIPv1 Distance vector IGP Old; only used classful subnets
RIPv2 Distance vector IGP Supports CIDR
BGP-4 Distance vector BGP Used on the Internet, connects
Autonomous Systems
OSPF Link state IGP Fast, popular, uses Area IDs
(Area 0/backbone)
IS-IS Link state IGP Alternative to OSPF
EIGRP Hybrid IGP Cisco proprietary

A. Simple physical installation
1. Home router
I. Give it power
II. Plug in connections
2. Business-class router
I. Insert it into a rack
II. Give it power
III. Plug in connections

B. Connecting to Routers
1. Each router must be configured
2. Yost cable
I. Oldest method for connecting to router for
configuration
II. Almost unique to Cisco-brand routers
III. Also called rollover cable

Figure 8.39 Cisco console cable

3. Managed devices include both routers
and advanced switches that can be
configured
4. Plug the rollover into console port on
Cisco router; plug other end into serial
port on a PC (may need USB-to-serial
adapter)

5. Use a terminal emulation program to talk
to the router from the PC over this
connection
I. PuTTY
II. HyperTerminal
III. Serial port settings: 9600 baud, 8 data bits,
1 stop bit, no parity

6. Once connected and running terminal
emulator
I. You will see the initial router prompt
II. On Cisco router, this is the Cisco IOS prompt
III. Working with IOS commands
a. Type enable and press ENTER
b. Prompt changes to Router#
c. IOS is complex
d. Newer CISCO routers will lead you through initial
configuration for basic setup

Figure 8.42 Initial router prompt

7. Normally, you will access a router
through Web access or network
management software (both explored
next)
8. Web access
I. Most routers have a built-in Web server
II. Can do everything you need to do

8. Web access
III. Easier than working with Cisco
command-line IOS
IV. Web access only works if router has a built-in
IP address from the factory, or you must
enable Web interface after assigning an IP
address to the router
V. To access the Web interface, you must know
the IP address

8. Web access
VI. Most techs use a laptop and a special cable
(Yost or rollover or crossover) to connect
directly to router for initial configuration
a. Know the IP address of the router
b. Assign to the laptop an IP address with the same
network ID of the router
c. Connect to router

Figure 8.43 Default IP address

8. Web access
d. Check the link lights to verify proper connection
e. Open the Web browser and enter the IP address
of the router
f. You will need to enter the default user name and
password (check the router’s documentation)
g. Once logged in, find the settings you need

Figure 8.44 Entering the IP address

Figure 8.45 User name and password

9. Network Management Software
III. Often a Web site
IV.Administrators manage network and
make necessary changes
V. Proprietary tools (OEM)
a. By manufacturers of managed devices
b. Usually very powerful and easy to use
c. Only work on that OEM’s devices

Figure 8.46 Cisco Network Assistant

9. Network Management Software
VI.Third-party NMS tools
a. Some free
b. Usually harder to configure
c. Must constantly be updated to work with as
many devices as possible
d. Usually lack the amount of detail in an OEM
NMS

9.Network Management Software
VI.Third-party NMS tools
e. While CiscoWorks enables you to change the
IP address of a port, third-party tools only let
you see the IP settings
f. OpenNMS is a popular open-source NMS

10.Other connection methods
I. Most routers have more than one way
to connect
II. Home router may come with a USB port
and configuration software
a. More powerful routers may allow connection
with Telnet protocol or newer Secure Shell
(SSH)

10.Other connection methods
a. Terminal emulations protocols that look like
the terminal emulators seen earlier, but that
use the network rather than a serial cable
b. More on terminal emulators in Chapter 9,
“TCP/IP Applications”

C. Basic Router
Configuration
1.Must have at least two connections
2.You must properly configure every port on

3.Make sure the routing table sends packets

C. Basic Router Configuration
4. STEP 1: Set up the WAN side
I. WAN side in home or small business router
connects to an ISP
II. Get setup information from ISP
III. Most home routers use DHCP on the WAN
side and just need to be configured to use
DHCP

4. STEP 1: Set up the WAN side
i. May need to enter a static address
ii. You can buy a single static IP address
iii. If static address, ISP will tell you what to
enter into the router

5. STEP 2: Set up the LAN
I. You usually have
total control over the

LAN side
II. Choose a network ID
from the private
range
III. Assign the correct IP

Figure 8.50 Entering a static IP

Figure 8.51 Setting up an IP address for LAN side

6. STEP 3: Establish routes
I. Router will usually build a routing table based
on information you provided
II. You may add more routes if needed
III. Use IOS command line on Cisco routers

7. STEP 4 (Optional): Configure a Dynamic
Protocol
I. Dynamic routing protocols tied to
individual NICs
II. When you connect two routers together,
make sure the NICs are configured to use
the same dynamic routing protocol

7. STEP 4 (Optional): Configure a Dynamic
Protocol
III. Unless you are in charge of two or more
routers, you will never use a dynamic
routing protocol
IV. Once a dynamic routing protocol is turned
on, it is all automatic

8. Document and back up
I.Document what you’ve done to configure each

II.Back up the configuration using whatever

D. Router Problems
1.Consider non-router issues first, because

I. Check NICs, computer, and switches
before router
II. Routers are more reliable than other
equipment

D. Router Problems
2. Keep in mind what your router is
supposed to do
I. Does it just route traffic?
II. Does it also perform NAT?
III. Is routing failing, or is another function
of the router failing?

D. Router Problems
3. Know how to use a few basic tools
that can help you check the router
I. TRACEROUTE
a. Records the route between any two hosts
b. Like PING, it sends out a single packet to
another host
c. Unlike PING, it returns information about
every router between them

D. Router Problems
3. Know how to use a few basic tools
that can help you check the router
I. TRACEROUTE
d. Tells you when things are not working
e. Gives you an idea of where to look for a problem
f. Windows – TRACERT
g. UNIX/Linux – TRACEROUTE
h. UNIX/Linux – My TRACEROUTE (MTR)
i.Dynamic, continually updating the route

Networking Chapter 8

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