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Routing & Packet Forwarding

Routing Protocols & Concepts

Router as a Computer:

Computers that specialize in sending packets over the data network.
Responsible for interconnecting networks.
Routers are the network center.
Generally have a min of 2 connections:
WAN connection (Connection to ISP).
LAN connection.
In addition to packet forwarding, routers provide other services as well.
A router connects multiple networks.
This means that it has multiple interfaces that each belong to a different IP network.
Each network that a router connects to typically requires a separate interface.

Routing Table:

Used to determine the best path.
Incoming packets’ dest IP address is looked up in the table.
The table includes an exit interface or next hop.
Once a match is found, the router encapsulates the IP packet into the L2 frame of the exit interface.

Router as a computer:

Router components and their functions:
CPU – Executes OS instructions.
RAM – Contains the running copy of config, routing table, buffers, etc. Contents lost when pwr is off.
ROM – Holds diagnostic software used @ startup and the bootstrap prgm.
NVRAM – Stores startup config.
Flash – Contains the OS, non-volitile.
Interfaces – usually multiple physical interfaces. Examples of interface types:
Ethernet / Fast Ethernet interfaces.
Serial interfaces.
Management interfaces (concole).

Internetwork Operating System:

IOS
A multitasking OS integrated with routing, switching, internetworking, and telecomm functions.
Stored as an image. Many different IOS images available.
IOS interface is CLI
On boot, the startup-config file in NVRAM is copied into RAM and stored as the running-config file.
Changes entered by the admin are stored in the running-config and are immediately implemented by the IOS.

Bootup Process:

1. Perform POST.
2. Load bootstrap, Main task is to locate the IOS and load it into RAM.
3. Locate and load IOS
Flash, TFTP, ROM.
####’s – IOS image is decompression.
4. Locate and load config file or enter setup mode.
if unable to find, %Error opening tftp://255.255.255.255/net work-config (Timed out).
R1(config)# no service config. (disable router from looking for configs via TFTP),
R1(config)# no ip domain-lookup. (turn off, if you make a typing error)

Verifying Router bootup Process:

Show version command “..# show version” is used to view information about the router during the bootup process. Information includes:
Platform model number.
Image name and IOS version.
Bootstrap version stored in ROM.
Image file name and where it was loaded from.
CPU and Amount of RAM.
Number and type of interfaces.
Amount of NVRAM.
Amount of Flash.
Config register settings (0×2102). ** 0x = indicates HEX **, ** CCNA questions on this **
Could be seen as 0×2142, 0×2101, 0×2100.
0×2142 is used for password recovery.

Router Interfaces:

A router interface is a physical connectior that enables a router to send or receive packets.
Management Ports:
Console port.
Auxiliary Port.

Interfaces:
Each interface connects to a separate network.
Types of router interfaces:
Ethernet, (LAN interface).
Fastethernet, (LAN interface).
Serial, (WAN interface).
T1
DSL.
ISDN.
Frame Relay.
Cable.
Two major grps of routers interfaces.
LAN interfaces.
Are used to connect router to LAN network.
Has a L2 MAC address.
Can be assigned a L3 IP address.
Usually consist of an RJ-45 jack
WAN interfaces.
Used to connect routers to external networks, usually over larger geographical distance.
Can use different L2 encapsulations – PPP, Frame Relay, HDLC.
Each interface has its own IP address and subnet mask.
Use their own L2 addresses depending on the technology.

Routers and the Network Layer:

Routers use destination IP address to forward packets.
The path a packet takes is determined after a router consults the routing table.
After router determines the best path.
Packet is encapsulated into a frame.
Frame is then placed on network medium in form of bits.
Router receives a stream of encoded bits.
Bits are decoded and passed to L2.
Router de-encapsulates the frame.
Remaining packet passed up to L3.
Routing decision made at this layer by examining destination IP address.
Packet is then re-encapsulated and sent out outbound interface.

Configure Devices and Apply Addresses:

Implementing Basic Addressing Schemes.
When designing a new network or mapping an existing network you must provide the following info in the form of a document:
Topology drawing that illustrates physical connectivity.
Address table that provides the following info.
Device name.
Interfaces used.
IP addresses.
Default gateway.

Basic Router Config:

A basic router config should contain the following:
Router hostname – host name should be unique.
Banner motd – At a min, should warn against unauthorized use.
Passwords – #enable secret ____, #line console 0, #line vty 0 4, (Use strong passwords).
Interface configurations – interface type, IP address and subnet mask, description, no shutdown and clock rate.
Should also include a description on all interfaces that are used.

After entering in the basic configuration the following tasks should be completed.
Verify basic conf and router operations.
Save the changes on a router.

Verify Basic Router Config:

R1#show running-config

Save the config changes:

R1#copy running-config startup-config

Additional commands further verify router configuration:
Show running-config – Displays config currently in RAM.
Show startup-config – Displays config file in NVRAM.
Show IP route – displays routing table.
Show interfaces – Displays all interface configs and stats.
Show ip interface brief – summarized interface config info.

Tags: switch, ip, 802.1q, guides, router, study