Download Introduction to Routing and Packet Forwarding

Introduction to Routing
and Packet Forwarding
Routing Protocols and
Concepts – Chapter 1
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Objectives

Functions of a router

Configuring Route

Structure of a routing table.

Dynamic Routing and Routing Protocols
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Router as a Computer
•Routers are responsible for interconnecting networks by
selecting the best path for a packet to travel and forwarding
packets to their destination using IP address
•Routers generally have 2 connections:
-WAN connection (Connection to ISP)
-LAN connection
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Configure Devices and Apply Addresses
 Basic Router Configuration
 A basic router configuration should contain the following:
-Router name - Host name should be unique
-Banner - At a minimum, banner should warn against
unauthorized use
-Passwords - Use strong passwords
-Interface configurations –
•Specify interface type,
•IP address and subnet mask.
•Issue no shutdown command.
•If DCE serial interface issue clock rate command.
Refer to worksheet on basic router configuration
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Configure Devices and Apply Addresses
 Verify Basic Router Configuration
-Issue the show running-config command
-Save the basic router configuration by Issuing the copy
running-config startup-config command
-Additional commands that will enable you to further verify
router configuration are:
Show running-config - Displays configuration currently in
RAM
Show startup-config - Displays configuration file NVRAM
Show IP route - Displays routing table
Show interfaces - Displays all interface configurations
Show IP int brief - Displays abbreviated interface
configuration information
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Routing Table Structure
 Routing Table is stored in ram and contains information
about:
Directly connected networks - this occurs when a device is
connected to another router interface
Remotely connected networks - this is a network that is not
directly connected to a particular router
Detailed information about the networks include source of
information, network address & subnet mask, and Ip address of
next-hop router
 Show ip route command is used to view a routing table
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Routing Table Structure
 Adding a connected network to the routing table
-Router interfaces
Each router interface is a member of a different network
Activated using the no shutdown command
In order for static and dynamic routes to exist in routing
table you must have directly connected networks
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Routing Table Structure
 Static routes in the routing table
-Includes: network address and subnet mask and IP address of
next hop router or exit interface
-Denoted with the code S in the routing table
-Routing tables must contain directly connected networks used
to connect remote networks before static or dynamic routing
can be used
 When to use static routes
-When network only consists of a few routers
-Network is connected to internet only through one ISP
-Hub & spoke topology is used on a large network
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Routing Table Structure
 Connected and Static routes
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Routing Table Structure
 Dynamic routing protocols
-Used to add remote networks to a routing table
-Are used to discover networks
-Are used to update and maintain routing tables
 Automatic network discovery
-Routers are able discover new networks by sharing routing
table information
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Routing Table Structure
 Maintaining routing tables
-Dynamic routing protocols are used to share routing information with
other router & to maintain and up date their own routing table.
 IP routing protocols. Example of routing protocols include:
-RIP
-IGRP
-EIGRP
-OSPF
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Router Paths and Packet Switching
 A Metric is a numerical value
used by routing protocols help
determine the best path to a
destination
–The smaller the metric value
the better the path
 2 types of metrics used by routing
protocols are:
-Hop count - this is the number
of routers a packet must travel
through to get to its destination
-Bandwidth - this is the
“speed” of a link also known as
the data capacity of a link
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Routing Table Structure
 Routing table entries come from the following sources
-Directly connected networks
-Static routes
-Dynamic routing protocols
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Routing Table Structure
 Parent and Child Routes
-A parent route is a level 1
route, i.e., it does not
contain any next-hop IP
address or exit interface
information
 Level 2 child routes contain
route source & the network
address of the route
 Diagram illustrates 2 child
networks belonging to the
parent route 172.16.0.0 / 24
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Routing Table Lookup Process
 Longest Match: Level 1 Network Routes
–Best match is also known as the longest match
–The best match is the one that has the most number of left
most bits matching between the destination IP address and the
route in the routing table.
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Classful and Classless IP Addressing
 Classful Routing Updates
classful routing protocols
(i.e. RIPv1) do not send
subnet masks in their
routing updates as the
Subnet mask is directly
related to the network
address
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Routing Behavior
 Classful Routing Behavior – Search Process
 An example of when classful routing behavior is in effect and
why the router drops the Packet
-The destination’s subnet mask is a /24 and none of the child
routes left most bits match the first 24 bits. This means packet
is dropped
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Routing Behavior
 Classless Routing Behavior: ip classless
 Route lookup process when ip classless is in use
-If classless routing behavior in effect then
Search level 1 routes
Supernet routes Checked first
-If a match exists then forward packet
Default routes Checked second
If there is no match or no default
route then the
Packet is dropped
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Routing Behavior
 Classless Routing Behavior – Search Process
 If no match is found in child routes of previous slide then
Router continues to search the routing table for a
match that may have fewer bits in the match
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Classless Inter-Domain Routing (CIDR)
 Route summarization done by
CIDR
Routes are summarized with
masks that are less than that
of the default classful mask
Example:
172.16.0.0 / 13 is the
summarized route for the
172.16.0.0 / 16 to 172.23.0.0 /
16 classful networks
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Classless Inter-Domain Routing (CIDR)
 Steps to calculate a route
summary
-List networks in binary
format
-Count number of left
most matching bits to
determine summary
route’s mask
-Copy the matching
bits and add zero bits
to determine the
summarized
network address
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Default Route
 Default routes
Packets that are not defined specifically in a routing
table will go to the specified interface for the default
route
Example: Customer routers use default routes to
connect to an ISP router.
Command used to configure a default route is
#ip route 0.0.0.0 0.0.0.0 s0/0/1
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Default Route and Static route
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