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Advanced Routing
Lesson 2 – Fundamental components and processes of routers

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Lesson Content

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Why Routing?
Routers are Computers
Internal components of a typical cisco router
Routers Interconnect Networks
Routers choose best paths
Packet Forwarding Methods
Routing Decisions
Path Determination
The Router Boot Sequence
Managing Configuration Register
Software configuration meanings
Checking the current config register value
Changing the config register
Recovering Passwords

Why Routing?
Routers are responsible for routing of traffic between networks
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• Routers have multiple interfaces, each on a different IP network
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• Routers use routing tables to determine the best path to send
packets
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Routers choose best paths
(Cont…)

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Packet Forwarding Methods

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• Process switching – An older
packet forwarding mechanism
still available for Cisco routers
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• Cisco Express Forwarding
(CEF) – The most recent,
fastest, and preferred Cisco
IOS packet-forwarding
mechanism
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Routing Decisions

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Path Determination
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 value used to measure the distance to a given
network
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Dynamic routing protocols use their own rules and metrics to build
and update routing tables:
• Routing Information Protocol (RIP) - Hop count
• Open Shortest Path First (OSPF) - Cost based on cumulative bandwidth
from source to destination
• Enhanced Interior Gateway Routing Protocol (EIGRP) - Bandwidth, delay,
load, reliability
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It consists of the following steps:
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Bootstrap looks for and loads the Cisco IOS software
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If startup-config is present, the file is copied from NVRAM to RAM
and will use this as the running-config
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For example, the POST checks
for the different interfaces on the router
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Step 2 –Locate and load Cisco IOS
• The bootstrap is a program in ROM that is used to execute
programs
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• The default order of an IOS loading from a router is: flash, TFTP
server, then ROM
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This file is called startupconfig and is only there if an administrator copies the running-config
file into NVRAM
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• The router will use this file to run the router
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If this fails, it will start the setup mode configuration
process
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• The 16 bits (2 bytes) of the configuration register are read from 15
to 0, from left to right
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Configuration
Register
Bit number
Binary

18

15
0

2
14
0

13
1

12
0

11
0

1
10
0

9
0

8
1

7
0

0
6
0

5
0

4
0

3
0

2
2
0

1
1

0
0

Software configuration meanings

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Checking the current config
register value
• The current value of the configuration register can be identified by
using the show version command in user mode
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e
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To
recover a password and tell the router to ignore NVRAM contents,
bit 6 needs to be on by setting the configuration register to 0x2142
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Boot the router and interrupt the boot sequence by performing a
break, which will take the router into ROM monitor mode
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Change the configuration register to turn on bit 6 (with the value
0x2142)
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Reload the router
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Say "no" to entering Setup mode, the enter privileged mode
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Copy the startup-config file to running-config, and don't forget to
verify that your interfaces are re-enabled
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Change the password
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Reset the configuration register to the default value
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Save the router configuration
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Reload the router (optional)
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Answer no to entering setup mode, press
Enter to go into user mode, and then type enable to go into privileged
mode

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