Transcript Layer 3 IP Packet

Chapter 1
Introduction to Routing and
Packet Forwarding
CIS 82 Routing Protocols and Concepts
Rick Graziani
Cabrillo College
[email protected]
Last Updated: 1/16/2008
This Presentation
 This presentation is based on the Exploration course/book, Routing
Protocols and Concepts.
 For a copy of this presentation and access to my web site for other
CCNA, CCNP, and Wireless resources please email me for a
username and password.
 Email: [email protected]
 Web Site: www.cabrillo.edu/~rgraziani
2
Note
 This chapter contains mostly introductory material.
 Most of not all of this information will be explained in more detail
in later chapters or later courses.
 The bootup process and the IOS are examined in a later
course.
 Do not worry or focus too much on the details for now.
 This will all be examined and explained in the following chapters.
 The audio of the lecture for this presentation will be available on
my web site after February 11, 2008
 My web site is www.cabrillo.edu/~rgraziani.
 For access to these PowerPoint presentations and other
materials, please email me at [email protected].
3
For further information
 This presentation is an
overview of what is
covered in the
curriculum/book.
 For further explanation
and details, please read
the chapter/curriculum.
 Book:
 Routing Protocols
and Concepts
 By Rick Graziani and
Allan Johnson
 ISBN: 1-58713-206-0
 ISBN-13: 978-58713206-3
4
Topics


Inside the Router
 Routers are computers
 Router CPU and Memory
 Internetwork Operating
System
 Router Bootup Process
 Router Ports and
Interfaces
 Routers and the Network
Layer
CLI Configuration and
Addressing
 Implementing Basic
Addressing Schemes
 Basic Router
Configuration


Building the Routing Table
 Introducing the Routing
Table
 Directly Connected
Networks
 Static Routing
 Dynamic Routing
 Routing Table Principles
Path Determination and
Switching Function
 Packet Fields and Frame
Formats
 Best Path and Metrics
 Equal Cost Load
Balancing
 Path Determination
 Switching Function
5
Inside the Router
 Routers are computers
 Router CPU and Memory
 Internetwork Operating System
 Router Bootup Process
 Router Ports and Interfaces
 Routers and the Network Layer
Routers are Computers
Leonard Kleinrock and the first IMP.
 A router is a computer:
 CPU, RAM, ROM, Operating System
 The first router: used for the Advanced Research Projects Agency
Network (ARPANET):
 IMP (Interface Message Processor)
 Honeywell 516 minicomputer that brought the ARPANET to life
on August 30, 1969.
7
 Routers forwarding packets:
 From the original source
 To the final destination.
 A router connects multiple networks:
 Interfaces on different IP networks
 Receives a packet on one interface and determines which
interface to forward it towards its destination.
 The interface that the router uses to forward the packet can be:
 The network of the final destination of the packet
 The destination IP address of this packet
 A network connected to another router
8
 Router interfaces:
 LAN
 WAN
9
Routers Determine the Best Path
 The router’s primary responsibility:
 Determining the best path to send packets
 Forwarding packets toward their destination
10
Routers Determine the Best Path
 The routing table is used to determine the best path.
 Examines the destination IP address
 searches for the best match with a network address in the
router’s routing table.
 The routing table includes the exit interface to forward the packet.
 Router encapsulates the IP packet into the data-link frame of the
outgoing or exit interface
 Packet is the forwarded toward its destination
11
Routers Determine the Best Path
 R1 receives the packet encapsulated in an Ethernet frame.
 After decapsulating the packet, the router uses the destination IP
address of the packet to search the routing table for a matching
network address.
 R1 (typo: R2 in book) found the static route 192.168.3.0/24, which
can be reached out its Serial 0/0/0 interface.
 R1 (typo: R2 in book) will encapsulate the packet in a frame format
appropriate for the outbound interface and then forward the packet.
 Note: More later on static and dynamic routes.
12
Router
CPU and
Memory
 CPU - Executes operating system instructions
 Random access memory (RAM) (RAM contents lost when power is off)
 running copy of configuration file.
 routing table
 ARP cache
 Read-only memory (ROM)
 Diagnostic software used when router is powered up.
 Router’s bootstrap program
 Scaled down version of operating system IOS
 Non-volatile RAM (NVRAM)
 Stores startup configuration. (including IP addresses, Routing protocol)
 Flash memory - Contains the operating system (Cisco IOS)
 Interfaces - There exist multiple physical interfaces that are used to connect
network. Examples of interface types:
 Ethernet / fast Ethernet interfaces
 Serial interfaces
 Management interfaces
13
Router physical characteristics
14
Cisco IOS - Internetwork
Operating System
 Responsible for managing the hardware and software resources
of the router, including:
 Allocating memory
 Managing processes
 Security
 Managing file systems
 There are many different IOS images.
 An IOS image is a file that contains the entire IOS for that router.
 depending on the model and the features within the IOS.
 For example, some features can include the ability to run Internet
Protocol version 6 (IPv6) or a routing protocol such as Intermediate
System–to–Intermediate System (IS-IS).
15
Router Bootup Process (more in later course)
16
Bootup Process
Step 1: POST (Power On Self Test)
 Executes diagnostics from ROM on
several hardware components,
including the CPU,RAM, NVRAM
Step 2: Loading Bootstrap Program
 Copied from ROM into RAM
 Executed by CPU
 Main task is to locate the Cisco IOS
and load it into RAM
Step 3: Locating the IOS
 Typically stored in flash memory, but
it can be stored in other places such
as a TFTP server.
 If a full IOS image cannot be
located, a scaled-down version of
the IOS is copied from ROM
 This version of IOS is used to help
diagnose any problems and to try to
load a complete version of the IOS
into RAM.
Step 4: Loading the IOS
 Some of the older Cisco routers ran
the IOS directly from flash
 Current models copy
 the IOS into RAM for execution
 Might see a string of pound signs
(#) while the image decompresses.
Step 5: Locating the Config File
 Bootstrap program searches for the
startup configuration file (startupconfig), in NVRAM.
 This file has the previously saved
configuration commands and
parameters,
Step 6: Loading the Config File
 If a startup configuration file is
found in NVRAM, the IOS loads it
into RAM as the running-config file
and executes the commands.
 If the startup configuration file
cannot be located, prompt the user
to enter setup mode
 If setup mode not used, a default
running-config file is created
17
Bootup Process
running-config
IOS (running)
startup-config
IOS
Bootup program
ios (partial)
18
Verify the router boot-up process
 show version command is used to view information about the
router during the bootup process.
 Information includes:
 IOS version
 ROM bootstrap program
 Location of IOS
 CPU and amount of RAM
 Interfaces
 Amount of NVRAM
 Amount of flash
 Configuration register information
19
Verify the router boot-up process
20
Ports and
Interfaces
 Port - normally means one of the management ports used for
administrative access
 Interface normally refers to interfaces that are capable of sending
and receiving user traffic.
 Note: However, these terms are often used interchangeably in the
industry and even with IOS output.
21
Management
Ports
 Console port - Most common of the management ports
 Used to connect a terminal,
 Or most likely a PC running terminal emulator software,
 No need for network access to that router.
 The console port must be used during initial configuration of the router.
 Auxiliary (AUX) port
 Not all routers have auxiliary ports.
 At times, can be used similarly to a console port
 Can also be used to attach a modem.
 Note: Auxiliary ports will not be used in this curriculum.
22
Router
Interfaces
 Interface on Cisco routers refers to a physical connector on the
router whose main purpose is to receive and forward packets.
 Routers have multiple interfaces used to connect to multiple
networks which may mean:
 Various types of networks
 Different types of media and connectors.
 Different types of interfaces.
 For example, Fast Ethernet interfaces for connections to different
LANs and also have different types of WAN interfaces used to
connect a variety of serial links, including T1, DSL, and ISDN.
23
Router Interfaces
 Every interface on the router:
 Belongs to a different network
 Is a host on a different IP network
 Have an IP address and subnet mask of a different network
 Cisco IOS will not allow two active interfaces on the same
router to belong to the same network.
 Note: A single interface on a router can be used to connect to
multiple networks; however, this is beyond the scope of this course
and is discussed in a later course.
24
LAN Interfaces
 Examples: Ethernet and Fast Ethernet interfaces.
 Used to connect the router to the LAN, similar to how a PC’s Ethernet NIC.
 Layer 2 MAC address
 Participates in the Ethernet LAN the same way as any other hosts on
that LAN.
 Example: Address Resolution Protocol (ARP):
 Maintains ARP cache for that interface
 Sends ARP requests when needed
 Responds with ARP replies when required
 Typically an RJ-45 jack (UTP).
 Router to switch: straight-through cable.
 Router to router via Ethernet interfaces, or PC’s NIC to router’s Ethernet
interface: crossover cable.
25
WAN Interfaces
 Example: serial, ISDN, and Frame Relay interfaces.
 Used to connect routers to external networks, usually over a larger
geographical distance.
 The Layer 2 encapsulation can be different types including:
 PPP
 Frame Relay
 HDLC (High-Level Data Link Control).
 Similar to LAN interfaces, each WAN interface has its own IP address and
subnet mask, making it a member of a specific network.
 Note: MAC addresses are used only on Ethernet interfaces and are not on
WAN interfaces.
 However, WAN interfaces use their own Layer 2 addresses depending on the
technology.
 Layer 2 WAN encapsulation types and addresses are covered in a later
course.
26
Routers at the Network
Layer
 A router is considered a Layer 3 device because its primary
forwarding decision is based on the information in the Layer 3 IP
packet, specifically the destination IP address. \
 This is known as routing.
 When a router receives a packet, it
 examines the destination IP address.
 If the destination IP address does not belong to any of the
router’s directly connected networks, the router must forward
this packet to another router.
27
Routers at the
Network Layer
Sequence of events is
explained in more
detail later in this
chapter.




R1 receives the packet
Examines the packet’s destination IP address
Searches the routing table
Forwards the packet onto R2.




R2 receives the packet
Examines the packet’s destination IP address
Searches its routing table
Forwards the packet out its directly connected Ethernet network
to PC2
28
Routers Operate
at Layers 1, 2,
and 3
 A router makes its primary forwarding decision at Layer 3,
 But also participates in Layer 1 and Layer 2 processes.
 After a router has examined the destination IP address and
consulted its routing table to make its forwarding decision, then
 forward that packet out the appropriate interface toward its
destination.
 Encapsulate the Layer 3 IP packet into the data portion of a Layer 2
data-link frame appropriate for the exit interface.
 The Layer 2 frame will then be encoded into the Layer 1 physical
signals used to represent these bits over the physical link.
29
Routers Operate
at Layers 1, 2,
and 3
 R1 receives the stream of bits on its interface.
 The bits passed up to Layer 2.
 R1 examines data-link frame’ s destination address to determine
whether it matches the receiving interface.
 If match, the data portion of the frame, the IP packet, is then
passed up to Layer 3
 R1 makes its routing decision.
 R1 then reencapsulates the packet into a new Layer 2 data-link
frame and forwards it out the outbound interface (bits).
 The new Layer 2 data-link address is associated with that of the
interface of the next-hop router (or final destination IP address).
30
CLI Configuration and
Addressing
 Implementing Basic Addressing Schemes
 Basic Router Configuration
CLI Configuration
 This is a review from CIS 81 (Networking Fundamentals Exploration 1)
 Basic Router Configuration:
 Naming the router
 Setting passwords
 Configuring interfaces
 Configuring a banner
 Saving changes on a router
 Verifying basic configuration and router operations
32
Establishing a HyperTerminal session (next week)
Router
Console port
Rollover cable
Terminal or a
PC with
terminal
emulation
software
Com1 or Com2 serial port
Take the following steps to connect a terminal to the console port on the router:
 Connect the terminal using the RJ-45 to RJ-45 rollover cable and an RJ-45 to DB-9
or RJ-45 to DB-25 adapter.
 Configure the terminal or PC terminal emulation software for 9600 baud, 8 data bits,
no parity, 1 stop bit, and no flow control.
33
Establishing a HyperTerminal session
=
 Important: A console connection is not the same as a network
connection!
34
NetLab
35
Command Overview
Router>
Router> enable
Router#
Router# configure terminal
Router(config)# exit
Router# config t
user mode
privilege mode
Router(config)# hostname name
Router(config)# enable secret password
Router(config)# line console 0
Router(config-line)# password password
Router(config-line)# login
Router(config)# line vty 0 4
Router(config-line)# password password
Router(config-line)# login
privilege password
console password
Router(config)# banner motd # message #
banner
Router(config)# interface type number
Router(config-if)# ip address address mask
Router(config-if)# description description
Router(config-if)# no shutdown
configure interface
telnet password
36
Other Commands
Router# copy running-config startup-config
Router#
Router#
Router#
Router#
show
show
show
show
running-config
ip route
ip interface brief
interfaces
37
Example
38
Hostname and Privilege Password
Router# config t
Router(config)# hostname R1
R1(config)# enable secret class
39
Passwords
R1(config)# line
R1(config-line)#
R1(config-line)#
R1(config-line)#
R1(config)# line
R1(config-line)#
R1(config-line)#
console 0
password cisco
login
exit
vty 0 4
password cisco
login
40
Banner
R1(config)# banner motd #
Enter TEXT message. End with the character ‘#’.
******************************************
WARNING!! Unauthorized Access Prohibited!!
******************************************
#
R1(config)#
41
WAN Interface Configuration
R1(config)# interface Serial0/0/0
R1(config-if)# ip address 192.168.2.1 255.255.255.0
R1(config-if)# description Link to R2
R1(config-if)# clock rate 64000
DCE Only
R1(config-if)# no shutdown
42
LAN Interface Configuration
R1(config)# interface FastEthernet0/0
R1(config-if)# ip address 192.168.1.1 255.255.255.0
R1(config-if)# description R1 LAN
R1(config-if)# no shutdown
43
Each Interface Belongs to a Different Network
R1(config)# interface FastEthernet0/1
R1(config-if)# ip address 192.168.1.2 255.255.255.0
192.168.1.0 overlaps with FastEthernet0/0
R1(config-if)# no shutdown
192.168.1.0 overlaps with FastEthernet0/0
FastEthernet0/1: incorrect IP address assignment
44
Each Interface Belongs to a Different Network
R1# show ip interface brief
Interface
IP-Address
FastEthernet0/0
192.168.1.1
Serial0/0
192.168.2.1
FastEthernet0/1
192.168.1.2
OK?
YES
YES
YES
Serial0/1
YES
unassigned
Method
manual
manual
manual
Status Protocol
up
up
up
up
administratively
down down
unset administratively
down down
45
Verify Router Configuration
R1# show running-config
!
version 12.3
!
hostname R1
!
interface FastEthernet0/0
description R1 LAN
ip address 192.168.1.1 255.255.255.0
!
interface Serial0/0
description Link to R2
ip address 192.168.2.1 255.255.255.0
clock rate 64000
!
banner motd ^C
******************************************
WARNING!! Unauthorized Access Prohibited!!
******************************************
^C
!
line con 0
password cisco
login
line vty 0 4
password cisco
login
!
end
46
Save Configuration
R1# copy running-config startup-config
R1# show startup-config
Using 728 bytes
!
version 12.3
!
hostname R1
!
interface FastEthernet0/0
description R1 LAN
ip address 192.168.1.1 255.255.255.0
!
interface Serial0/0
description Link to R2
ip address 192.168.2.1 255.255.255.0
clock rate 64000
!
banner motd ^C
******************************************
WARNING!! Unauthorized Access Prohibited!!
******************************************
^C
line con 0
password cisco
login
line vty 0 4
password cisco
login
!
end
47
Show Routing Table
R1# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is not set
C
C
192.168.1.0/24 is directly connected, FastEthernet0/0
192.168.2.0/24 is directly connected, Serial0/0/0
48
Verifying Interfaces
R1# show interfaces
<some interfaces not shown>
FastEthernet0/0 is up, line protocol is up (connected)
Hardware is Lance, address is 0007.eca7.1511 (bia 00e0.f7e4.e47e)
Description: R1 LAN
Internet address is 192.168.1.1/24
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec, rely 255/255, load 1/255
Encapsulation ARPA, loopback not set
ARP type: ARPA, ARP Timeout 04:00:00,
Last input 00:00:08, output 00:00:05, output hang never
Last clearing of “show interface” counters never
Queueing strategy: fifo
Output queue :0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
<output omitted>
Serial0/0 is up, line protocol is up (connected)
Hardware is HD64570
Description: Link to R2
Internet address is 192.168.2.1/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255
Encapsulation HDLC, loopback not set, keepalive set (10 sec)
Last input never, output never, output hang never
<output omitted>
49
Building the Routing Table
 Introducing the Routing Table
 Directly Connected Networks
 Static Routing
 Dynamic Routing
 Routing Table Principles
Introducing the Routing Table
R1# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is not set
C
C
192.168.1.0/24 is directly connected, FastEthernet0/0
192.168.2.0/24 is directly connected, Serial0/0/0
 Routing table is a data file in RAM that is used to store route
information about:
 Directly connected
 Remote networks
51
Introducing the Routing Table
R1# show ip route
<output omitted>
C
C
192.168.1.0/24 is directly connected, FastEthernet0/0
192.168.2.0/24 is directly connected, Serial0/0/0
Exit Interfaces
 The routing table contains network/next-hop associations
 The “next hop” is the IP address of a next-hop router. (coming)
 May also include an outgoing or exit interface (more later)
52
Introducing the Routing Table
R1# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<output omitted>
C
C
192.168.1.0/24 is directly connected, FastEthernet0/0
192.168.2.0/24 is directly connected, Serial0/0/0
Directly Connected
Networks
 directly connected network is a network that is directly attached to
one of the router interfaces.
 When a router’s interface is configured with an IP address and
subnet mask, the interface becomes a host on that attached
network.
 Active directly connected networks are added to the routing table.
53
Introducing the Routing Table
R1# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<output omitted>
C
C
192.168.1.0/24 is directly connected, FastEthernet0/0
192.168.2.0/24 is directly connected, Serial0/0/0
Directly Connected
Networks
 directly connected network is a network that is directly attached to
one of the router interfaces.
 When a router’s interface is configured with an IP address and
subnet mask, the interface becomes a host on that attached
network.
 Active directly connected networks are added to the routing table.
54
Introducing the Routing Table
R1# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<output omitted>
C
C
192.168.1.0/24 is directly connected, FastEthernet0/0
192.168.2.0/24 is directly connected, Serial0/0/0
Remote Network
 A remote network is a network that is not directly connected to the router.
 A remote network is a network that can only be reached by sending the
packet to another router.
 Remote networks are added to the routing table using
 a dynamic routing protocol or
 by configuring static routes.
 Dynamic routes are routes to remote networks that were learned
automatically by the router, using a dynamic routing protocol.
 Static routes are routes to networks that a network administrator manually
configured.
55
Directly Connected Networks
R1# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<output omitted>
C
C
192.168.1.0/24 is directly connected, FastEthernet0/0
192.168.2.0/24 is directly connected, Serial0/0/0
Directly Connected
Networks
 C: Source of the route information, directly connected network, static route,
or a dynamic routing protocol.
 The C represents a directly connected route.
 192.168.1.0/24: The network address and subnet mask of the directly
connected or remote network.
 In this example, 192.168.1.0/24 is the directly connected network.
 FastEthernet 0/0: The exit interface and/or the IP address of the next-hop
router.
 In this example, both FastEthernet 0/0 is the exit interfaces used to
reach these networks.
56
Directly Connected Networks
R1# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<output omitted>
C
C
192.168.1.0/24 is directly connected, FastEthernet0/0
192.168.2.0/24 is directly connected, Serial0/0/0
Directly Connected
Networks
 Before any static or dynamic routing is configured on a router, the
router only knows about its own directly connected networks.
 These are the only networks that are displayed in the routing table
until static or dynamic routing is configured.
 Static and dynamic routes cannot exist in the routing table without a
router’s own directly connected networks.
 The router cannot send packets out an interface if that interface is
not enabled with an IP address and subnet mask, just as a PC
cannot send IP packets out its Ethernet interface if that interface is
not configured with an IP address and subnet mask.
57
Static Routes
R1# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<output omitted>
Gateway of last resort is not set
C
C
S
192.168.1.0/24 is directly connected, FastEthernet0/0
192.168.2.0/24 is directly connected, Serial0/0/0
192.168.3.0/24 [1/0] via 192.168.2.2c
Static Route
 Static route includes the network address and subnet mask of the
remote network, along with the IP address of the next-hop router or
exit interface.
 Note: Configuration of the static route is not shown.
 Static routes are denoted with the code S in the routing table,
 Static routes are examined in detail in the next chapter.
58
Dynamic Routes
R1# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is not set
C
C
S
R
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.4.0/24
is directly connected, FastEthernet0/0
is directly connected, Serial0/0/0
[1/0] via 192.168.2.2
[120/1] via 192.168.2.2, 00:00:20, Serial0/0/0
 R1 has automatically learned about the 192.168.4.0/24 network from R2
through the dynamic routing protocol RIP (Routing Information Protocol).
 RIP was one of the first IP routing protocols and will be fully discussed in
later chapters.
 Note: Configuration of RIP not shown.
59
Routing Table Principles
 These principles, listed as follows, are from Alex Zinin’s book, Cisco
IP Routing:
 Every router makes its decision alone, based on the information
it has in its own routing table.
 The fact that one router has certain information in its routing
table does not mean that other routers have the same
information.
 Routing information about a path from one network to another
does not provide routing information about the reverse, or return,
path.
60
Asymmetric Routing
 Asymmetric routing - Because routers do not necessarily have the
same information in their routing tables, packets can traverse the
network in one direction, using one path, and return through another
path.
 Asymmetric routing is more common in the Internet, which uses the
BGP routing protocol, than it is in most internal networks.
61
Path Determination and
Switching Functions
 Packet Fields and Frame Formats
 Best Path and Metrics
 Equal Cost Load Balancing
 Path Determination
 Switching Function
Path Determination and Switching Functions
 The following sections focus on exactly what happens to data as it
moves from source to destination.
 Review the packet and frame field specifications
 Discuss in detail how the frame fields change from hop to hop,
whereas the packet fields remain unchanged
63
Ethernet Frame
IPv4 (Internet
Protocol)
 Layer 2 addresses:
 Interface-to-Interface on the same network.
 Used to send to the next hop router or final destination.
 Layer 2 source address: sending interface layer 2 address (if applicable)
 Layer 3 destination address: destination interface layer 2 address (if
applicable).
 Changes from network to network.
 Layer 3 addresses:
 Original source layer 3 address (IP)
 Final destination layer 3 address (IP)
 Does not change (except with NAT, but this is not a concern of IP but an
internal network process)
64
Router Paths and
Packet Switching
 As a packet travels from one networking device to another
 The Source and Destination IP addresses NEVER change
 The Source & Destination Layer 2 (MAC) addresses CHANGE
as packet is forwarded from one router to the next.
 TTL field decrement by one until a value of zero is reached at
which point router discards packet (prevents packets from
endlessly traversing the network)
65
Best Path
 Router’s best-path determination involves evaluating multiple paths
to the same destination network and selecting the optimum or
“shortest” path to reach that network.
 Depends upon routing protocol.
 RIP uses hop count whereas OSPF uses bandwidth (Cisco’s
implementation of OSPF).
 Dynamic routing protocols use their own rules and metrics to build
and update routing tables.
 A metric is the quantitative value used to measure the distance to a
given route.
 The best path to a network is the path with the lowest metric.
 For example, a router will prefer a path that is five hops away over a
path that is ten hops away.
66
Best Path
 RIP uses hop count
1.5 Mbps
 R1 to R3
 Fewer links but much slower
 OSPF uses bandwidth
 R1 to R2 to R3
 More routers but much faster links
1.5 Mbps
67
Equal Cost Load
Balancing
 What happens if a routing table has two or more paths with the
same metric to the same destination network? (equal-cost metric)
 Router will perform equal-cost load balancing.
 The router will forward packets using the multiple exit interfaces as
listed in the routing table.
 Static routes and all dynamic routing protocols perform equal cost
load balancing.
 (More later)
68
Equal-Cost Paths
Versus Unequal-Cost
Paths
 Just in case you are wondering, a router can send packets over
multiple networks even when the metric is not the same if it is using
a routing protocol that has this capability.
 This is known as unequal-cost load balancing.
 EIGRP and IGRP are the only routing protocols that can be
configured for unequal-cost load balancing.
 (More in CCNP courses)
69
Path Forwarding
 Packet forwarding involves
two functions:
 Path determination
function
 Switching function
 Path determination function is the process of how the router determines
which path to use when forwarding a packet.
 To determine the best path, the router searches its routing table for a
network address that matches the packet’s destination IP address.
 One of three path determinations results from this search:
 Directly connected network: Packet is forwarded directly to the device
with the packet’s destination IP address.
 Remote network: Packet is forwarded to another router. Remote
networks can only be reached by forwarding packets to another router.
 No route determined: If the router does not have a default route, the
packet is discarded. The router sends an Internet Control Message
Protocol (ICMP) Unreachable message to the source IP address of the
packet.
70
Path Forwarding
 Packet forwarding involves
two functions:
 Path determination
function
 Switching function
 Switching function is the process used by a router to accept a packet on
one interface and forward it out another interface.
 A key responsibility of the switching function is to encapsulate packets in the
appropriate data-link frame type for the outgoing data link.
 What does a router do with a packet received from one network and
destined for another network?
1. Decapsulates the Layer 3 packet by removing the Layer 2 frame header
and trailer
2. Examines the destination IP address of the IP packet to find the best
path in the routing table
3. Encapsulates the Layer 3 packet into a new Layer 2 frame and forwards
the frame out the exit interface
71
Remember: Encapsulation
These addresses
do not change!
Layer 3 IP Packet
These change from
host to router, router to
router, and router to
host.
Destination IP
Address
Source IP
Address
Other IP
fields
Data
Layer 2 Data Link Frame
Destination
Address
Next hop Data
Link Address of
Host or Router’s
interface

Source
Address
Type
Data
Trailer
Current Data Link
Address of Host or
Router’s exit interface
Now, let’s do an example…
72
Layer 2 Data Link Frame
Dest.
Dest.Add
MAC
MAC
0B-31
FF-FF
00-10
Source Add
MAC
0A-10
00-20
Layer 3 IP Packet
Type
800
Dest. IP
192.168.4.10
Source IP
192.168.1.10
IP
fields
Data
Trailer
 This is just a summary.
 The details will be shown next!
 Now for the details…
73
Layer 2 Data Link Frame
Dest. MAC
00-10
Source MAC
0A-10
Layer 3 IP Packet
Type
800
Dest. IP
192.168.4.10
Source IP
192.168.1.10
IP
fields
Data
Trailer
From Host X to Router RTA
 Host X begins by encapsulating the IP packet into a data link frame (in this
case Ethernet) with RTA’s Ethernet 0 interface’s MAC address as the data
link destination address.
 How does Host X know to forward to packet to RTA and not directly to Host
Y?
 IP Source and IP Destination Addresses are on different networks
 How does Host X know or get RTA’s Ethernet address?
 Checks ARP Table for Default Gateway IP Address and associated
MAC Address.
 What if it there is not an entry in the ARP Table?
 Host X sends an ARP Request and RTA sends an ARP Reply
74
Layer 2 Data Link Frame
Dest. MAC
0B-31
Source MAC
00-20
Layer 3 IP Packet
Type
800
RTA ARP Cache
IP Address
MAC Address
192.168.2.2
0B-31
Dest. IP
192.168.4.10
Source IP
192.168.1.10
Network
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.4.0/24
IP
fields
Data
Trailer
RTA Routing Table
Hops Next-hop-ip Exit-interface
0
Dir.Conn.
e0
0
Dir.Conn
e1
1
192.168.2.2
e1
2
192.168.2.2
e1
RTA
1. RTA examines Destination MAC address, which matches the E0 MAC address, so it copies in the frame.
2. RTA sees the Type field is 0x800, IP packet in the data field, a packet which needs to be routed.
3. RTA strips off the Ethernet frame.
RTA looks up the Destination IP Address in its routing table.
 192.168.4.0/24 has next-hop-ip address of 192.168.2.2 and an exit-interface of e1.
 Since the exit interface is on an Ethernet network, RTA must resolve the next-hop-ip address with a
destination MAC address.
4. RTA looks up the next-hop-ip address of 192.168.2.2 in its ARP cache.
 If the entry was not in the ARP cache, the RTA would need to send an ARP request out e1. RTB
would send back an ARP reply, so RTA can update its ARP cache with an entry for 192.168.2.2. 5.
Packet is encapsulated into a new data link (Ethernet) frame.
75
Layer 2 Data Link Frame
Dest. Add
FF-FF
Source Add
Layer 3 IP Packet
Type
800
Dest. IP
192.168.4.10
Source IP
192.168.1.10
Network
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.4.0/24
IP
fields
Data
Trailer
RTB Routing Table
Hops Next-hop-ip Exit-interface
1
192.168.2.1
e0
0
Dir.Conn
e0
0
Dir.Conn
s0
1
192.168.3.2
s0
RTB
1. RTB examines Destination MAC address, which matches the E0 MAC address, and copies in the frame.
2. RTB sees Type field, 0x800, IP packet in the data field, a packet which needs to be routed.
3. RTB strips off the Ethernet frame.
RTB looks up the Destination IP Address in its routing table.
 192.168.4.0/24 has next-hop-ip address of 192.168.3.2 and an exit-interface of Serial0.
 Since the exit interface is not an Ethernet network, RTB does not have to resolve the next-hop-ip address
with a destination MAC address.
 When the interface is a point-to-point serial connection, (like a pipe), RTB encapsulates the IP packet into
the proper data link frame, using the proper serial encapsulation (HDLC, PPP, etc.).
 The data link destination address is set to a broadcast (there’s only one other end of the pipe).
5. Packet is encapsulated into a new data link (serial, PPP) frame and sent out the link.
76
Layer 2 Data Link Frame
Dest. MAC
0B-20
Source MAC
0C-22
Layer 3 IP Packet
Type
800
RTC ARP Cache
IP Address
MAC Address
192.168.4.10
0B-20
Dest. IP
192.168.4.10
Source IP
192.168.1.10
IP
fields
Data
Trailer
RTC Routing Table
Network
Hops Next-hop-ip Exit-interface
192.168.1.0/24 2
192.168.3.1
s0
192.168.2.0/24 1
192.168.3.1
s0
192.168.3.0/24 0
Dir.Conn
s0
192.168.4.0/24 0
Dir.Conn
e0
RTC
1. RTC copies in the data link (serial, PPP) frame.
2. RTC sees the Type field is 0x800, IP packet in the data field, a packet which needs to be routed.
3. RTC strips off the data link, serial, frame.
RTC looks up the Destination IP Address in its routing table.

RTC realizes that this Destination IP Address is on the same network as one of its interfaces and it can sent the packet
directly to the destination and not another router.

Since the exit interface is on an directly connected Ethernet network, RTC must resolve the destination ip address with
a destination MAC address.
2. RTC looks up the destination ip address of 192.168.4.10 in its ARP cache.

If the entry was not in the ARP cache, the RTC would need to send an ARP request out e0. Host Y would send back an
ARP reply, so RTC can update its ARP cache with an entry for 192.168.4.10.
5. Packet is encapsulated into a new data link (Ethernet) frame and sent out the interface.
77
Layer 2 Data Link Frame
Dest. MAC
0B-20
Source MAC
0C-22
Layer 3 IP Packet
Type
800
Dest. IP
192.168.4.10
Source IP
192.168.1.10
IP
fields
Data
Trailer
Host Y
Layer 2: Data Link Frame
1. Host Y examines Destination MAC address, which matches its Ethernet interface MAC address, and
copies in the frame.
2. Host Y sees the Type field is 0x800, IP packet in the data field, which needs to be sent to its IP process.
3. Host Y strips off the data link, Ethernet, frame and sends it to its IP process.
Layer 3: IP Packet
4. Host Y’s IP process examines the Destination IP Address to make sure it matches its own IP Address..

If it does not, the packet will be dropped.
5. The packet’s protocol field is examined to see where to send the data portion of this IP packet: TCP,
UDP or other?
Layer 4: TCP, UDP or other?
78
Layer 2 Data Link Frame
Dest.
Dest.Add
MAC
MAC
0B-31
FF-FF
00-10
Source Add
MAC
0A-10
00-20
Layer 3 IP Packet
Type
800
Dest. IP
192.168.4.10
Source IP
192.168.1.10
IP
fields
Data
Trailer
 The summary once again!
79
Chapter 1
Introduction to Routing and
Packet Forwarding
CIS 82 Routing Protocols and Concepts
Rick Graziani
Cabrillo College
[email protected]