1. Chapter 1 Introduction to Routing and Packet Forwarding
CIS 82 Routing Protocols and Concepts
Rick Graziani
Cabrillo College
Last Updated: 2/16/2009

2. This Presentation
For detailed information see the notes section within this PowerPoint.
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 me for a username and password.
Web Site:

3. 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.

4. 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:
ISBN-13:

5. Topics 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
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
Path Determination and Switching Function
Packet Fields and Frame Formats
Best Path and Metrics
Equal Cost Load Balancing
Path Determination
Switching Function

6. 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

7. Routers are Computers A router is a computer:
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.

8. Routers forwarding packets (packet switching):
From the original source to the final destination.
Selects best path based on destination IP address
A router connects multiple networks:
Interfaces on different IP networks
Routers forwarding packets (packet switching):
From the original source to the final destination.
Selects best path based on destination IP address
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

9. Router interfaces:
LAN
WAN
Router interfaces:
LAN
WAN

10. Routers Determine the Best Path
The router’s primary responsibility:
Determining the best path to send packets
Forwarding packets toward their destination
The router’s primary responsibility:
Determining the best path
Forwarding packets toward their destination

11. Routers Determine the Best Path
IP Packet enters router’s Ethernet interface.
Router examines the packet’s destination IP address.
Router searches for a best match between packet’s destination IP address and network address in routing table.
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
Using the exit-interface in the route, the packet is forwarded to the next router or the final destination.
Routing table
Determines best path.
Best match between destination IP address and network address in routing table

12. Router CPU and Memory CPU - Executes operating system instructions
Random access memory (RAM)
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
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:
Allocating memory
Managing processes
Security
Managing file systems
Many different IOS images.
An IOS image is a file that contains the entire IOS for that router.
Router model
IOS features
Example IPv6 or a routing protocol such as Intermediate System–to–Intermediate System (IS-IS).
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)
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 (startup-config), 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

16. Bootup Process running-config startup-config IOS Bootup program
IOS (running)
ios (partial)

17. Where is the permanent configuration file stored used during boot-up?
NVRAM
Where is the diagnostics software stored executed by hardware modules?
ROM
Where is the backup (partial) copy of the IOS stored?
ROM
Where is IOS permanently stored before it is copied into RAM?
FLASH
Where are the bootsystem commands stored which are used to locate the IOS?
NVRAM
running-config
startup-config
IOS
Bootup program
IOS (running)
ios (partial)

18. ? ? ? ? ? ? ?
running-config
startup-config
IOS
Bootup program
IOS (running)
ios (partial)

19. startup-config
running-config
Bootup program
IOS (running)
IOS
ios (partial)
running-config
startup-config
IOS
Bootup program
IOS (running)
ios (partial)

20. Router Boot Process – Details (later)
1. ROM
1. POST
2. Bootstrap code executed
3. Check Configuration Register value (NVRAM)
0 = ROM Monitor mode
1 = ROM IOS
= startup-config in NVRAM
2. Check for IOS boot system commands in startup-config file (NVRAM)
If boot system commands in startup-config
a. Run boot system commands in order they appear in startup-config to locate the IOS
b If boot system commands fail, use default fallback sequence to locate the IOS (Flash, TFTP, ROM)
3. Locate and load IOS, Default fallback sequence: No IOS boot system commands in startup-config
a. Flash (sequential)
b. TFTP server (netboot) - The router uses the configuration register value to form a filename from which to boot a default system image stored on a network server.
c. ROM (partial IOS) or keep retrying TFTP depending upon router model
- If no IOS located, get partial IOS version from ROM
4. Locate and load startup-config configuration
a. If startup-config found, copy to running-config
b. If startup-config not found, prompt for setup-mode
c. If setup-mode bypassed, create a “skeleton” default running-config (no startup-config)
1. ROM
1. POST
2. Bootstrap code executed
3. Check Configuration Register value (NVRAM)
0 = ROM Monitor mode
1 = ROM IOS
= startup-config in NVRAM
2. Check for IOS boot system commands in startup-config file (NVRAM)
If boot system commands in startup-config
a. Run boot system commands in order they appear in startup-config to locate the IOS
b If boot system commands fail, use default fallback sequence to locate the IOS (Flash, TFTP, ROM)
3. Locate and load IOS, Default fallback sequence: No IOS boot system commands in startup-config
a. Flash (sequential)
b. TFTP server (netboot) - The router uses the configuration register value to form a filename from which to boot a default system image stored on a network server.
c. ROM (partial IOS) or keep retrying TFTP depending upon router model
- If no IOS located, get partial IOS version from ROM
4. Locate and load startup-config configuration
a. If startup-config found, copy to running-config
b. If startup-config not found, prompt for setup-mode
c. If setup-mode bypassed, create a “skeleton” default running-config (no startup-config)

21. Verify the router boot-up process
show version command is used to view information about the router during the bootup process (later).

22. 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.
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.

23. Management Ports Console port Terminal
PC running terminal emulator software
No need for network access
Used for initial configuration
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.
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.

24. Router Interfaces Interfaces - Receive and forward packets.
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.
Fast Ethernet interfaces for connections to different LANs
Serial interfaces are used for WAN connections including T1, DSL, and ISDN.
Interfaces - Receive and forward packets.
Various types of networks
Different types of media and connectors.
Different types of interfaces.
Fast Ethernet interfaces - LANs
Serial interfaces - WAN connections including T1, DSL, and ISDN

25. Router Interfaces Router Interface: Different network
FastEthernet 0/0
MAC: 0c00-41cc-ae12
/16
FastEthernet 0/0
MAC: 0c00-3a44-190a
/24
Serial 0/0
Serial 0/1
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
/24
Router Interface:
Different network
IP address and subnet mask of that network
Cisco IOS will not allow two active interfaces on the same router to belong to the same network.

26. LAN Interfaces Ethernet and Fast Ethernet interfaces
Connects the router to the LAN
Layer 2 MAC address
Participates in the Ethernet
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: crossover cable
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.

27. WAN Interfaces Point-to-Point, ISDN, and Frame Relay interfaces
Connects routers to external networks.
The Layer 2 encapsulation can be different types including:
PPP
Frame Relay
HDLC (High-Level Data Link Control).
Note: MAC addresses are used only on Ethernet interfaces and are not on WAN interfaces.
Layer 2 WAN encapsulation types and addresses are covered in a later course.
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.

28. 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.
R1 receives the packet
Examines the packet’s destination IP address
Searches the routing table
Forwards the packet onto R2.
R2 receives the packet
Searches its routing table
Forwards the packet out its directly connected Ethernet network to PC2
Layer 3 device because its primary forwarding decision is based on the information in the Layer 3 IP packet (destination IP address).
This is known as routing.

29. 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.
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. Path Determination and Switching Functions
Packet Fields and Frame Formats
Best Path and Metrics
Equal Cost Load Balancing
Path Determination
Switching Function

31. 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
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

32. IPv4 (Internet Protocol)
Ethernet Frame
IPv4 (Internet Protocol)
Layer 2 addresses:
Interface-to-Interface on the same network.
Changes as packet is decapsulated and encapsulated 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)
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)
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)

33. Best Path Router’s best-path to a network: optimum or “shortest” path
Routing protocol dependent
Dynamic routing protocols use their own rules and metrics.
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.
Example, a router will prefer a path that is one hop away over a path that is two hops away.
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.

34. Best Path Comparing Dynamic Routing Protocols: RIP and OSPF
1.5 Mbps
1.5 Mbps
Comparing Dynamic Routing Protocols: RIP and OSPF
RIP uses hop count
R1 to R3
Fewer links but much slower
OSPF uses bandwidth
R1 to R2 to R3
More routers but much faster links
RIP uses hop count
R1 to R3
Fewer links but much slower
OSPF uses bandwidth
R1 to R2 to R3
More routers but much faster links

35. Equal Cost Load Balancing
To reach the /24 network it is 2 hops via R2 and 2 hops via R4.
Equal Cost Load Balancing ? ?
/24
What happens if a routing table has two or more paths with the same metric to the same destination network? (equal-cost metric)
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)
Router will perform equal-cost load balancing.

36. Equal-Cost Paths Versus Unequal-Cost Paths
/24
Can a router use multiple paths if the paths (cost, metric) to reach the destination network are not equal?
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)
Yes, if the routers are using the EIGRP routing protocol which supports unequal cost load balancing.

37. 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.
Packet forwarding involves two functions:
Path determination function
Switching function

38. Path Forwarding Router receives packet.
Destination IP address matches a network on one of its directly connected networks.
Packet is forwarded out that network.
Directly connected network
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.
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
Remote network
No route determined

39. Path Forwarding Router receives packet.
Destination IP address matches a remote network which can only be reached via another router.
Packet is forwarded out that network to the next-hop router.
Remote network
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.
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
Remote network
No route determined

40. Path Forwarding Does this mean the network does not exist?
Router receives packet.
Destination IP address does NOT match any network in the router’s routing table.
Packet is dropped.
No route determined
Does this mean the network does not exist?
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.
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
Remote network
No route determined
No, only that the router does not know about that network. (later)

41. Path Forwarding Switching function is the process used by a router to:
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
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.

42.
Path Forwarding
Layer 2 Data Link Frame
Layer 3 IP Packet
Dest. MAC 0B-31
Source MAC 00-20
Type 800
Trailer
Dest. MAC 00-10
Source MAC 0A-10
Type 800
Trailer
Dest. IP
Source IP
IP fields
Data
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
What does a router do with a packet received from one network and destined for another network?
Decapsulates the Layer 3 packet by removing the Layer 2 frame header and trailer
Examines the destination IP address of the IP packet to find the best path in the routing table
Encapsulates the Layer 3 packet into a new Layer 2 frame and forwards the frame out the exit interface

43. 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
Source Address
Type
Data
Trailer
Current Data Link Address of Host or Router’s exit interface
Next hop Data Link Address of Host or Router’s interface
Now, let’s do an example…

44. The details will be shown next! Now for the details…
Layer 2 Data Link Frame
Layer 3 IP Packet
Dest. MAC 0B-31
Source MAC 00-20
Type 800
Trailer
Dest. Add FF-FF
Source Add
Type 800
Trailer
Dest. MAC 00-10
Source MAC 0A-10
Type 800
Trailer
Dest. IP
Source IP
IP fields
Data
Dest. IP
Source IP
IP fields
Data
This is just a summary.
The details will be shown next!
Now for the details…

45. From Host X to Router RTA
Layer 2 Data Link Frame
Layer 3 IP Packet
Dest. MAC 00-10
Source MAC 0A-10
Type 800
Trailer
Dest. IP
Source IP
IP fields
Data
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
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

46. 3. RTA strips off the Ethernet frame.
Layer 2 Data Link Frame
Layer 3 IP Packet
Dest. MAC 00-10
Source MAC 0A-10
Type 800
Trailer
Dest. MAC 0B-31
Source MAC 00-20
Type 800
Trailer
Dest. IP
Source IP
IP fields
Data
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.
/24 has next-hop-ip address of 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 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 Packet is encapsulated into a new data link (Ethernet) frame.
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.
/24 has next-hop-ip address of 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 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 Packet is encapsulated into a new data link (Ethernet) frame.

47. 3. RTB strips off the Ethernet frame.
Layer 2 Data Link Frame
Layer 3 IP Packet
Dest. MAC 0B-31
Source MAC 00-20
Type 800
Trailer
Dest. Add FF-FF
Source Add
Type 800
Trailer
Dest. IP
Source IP
IP fields
Data
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.
/24 has next-hop-ip address of 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.
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.
/24 has next-hop-ip address of 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.

48. 1. RTC copies in the data link (serial, PPP) frame.
Layer 2 Data Link Frame
Layer 3 IP Packet
Dest. Add FF-FF
Source Add
Type 800
Trailer
Dest. MAC 0B-20
Source MAC 0C-22
Type 800
Trailer
Dest. IP
Source IP
IP fields
Data
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 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
5. Packet is encapsulated into a new data link (Ethernet) frame and sent out the interface.
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 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
5. Packet is encapsulated into a new data link (Ethernet) frame and sent out the interface.

49. If it does not, the packet will be dropped.
Layer 2 Data Link Frame
Layer 3 IP Packet
Dest. MAC 0B-20
Source MAC 0C-22
Type 800
Trailer
Dest. IP
Source IP
IP fields
Data
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?
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?

50. The summary once again! Layer 2 Data Link Frame Layer 3 IP Packet
Dest. MAC 0B-31
Source MAC 00-20
Type 800
Trailer
Dest. Add FF-FF
Source Add
Type 800
Trailer
Dest. MAC 00-10
Source MAC 0A-10
Type 800
Trailer
Dest. IP
Source IP
IP fields
Data
Dest. IP
Source IP
IP fields
Data
The summary once again!

51. CLI Configuration and Addressing
Implementing Basic Addressing Schemes
Basic Router Configuration

52. Learning IOS: Lab 1.5.2 (Cabrillo College Version)
Networking Lab
NetLab
Packet Tracer

53. Establishing a HyperTerminal session (next week)
Router
Console port
Terminal or a PC with terminal emulation software
Rollover cable
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.
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.

54. Establishing a Terminal session
Tera Term
HyperTerminal (comes with Windows)
Putty =
Important: A console connection is not the same as a network connection!

55. When do you need to use a console connection to the router?
When there is not a network connection to the router (can’t use telnet).
What software do you need?
Tera Term, HyperTerminal, Putty, etc.
What cable and ports do you use?
PC: Serial port & Router: Console Port
Rollover or Console Cable
Terminal Connection
No network connection needed
Console Port
Serial

56. When can you use a network connection to the router?
C:\> ping
C:\> telnet
Ethernet Connection
Network connection needed
NIC
When can you use a network connection to the router?
When there is a network connection to the router (telnet).
What software/command do you need?
TCP/IP, Terminal prompt (DOS), Tera Term, etc.
What cable and ports do you use?
PC & Router: Ethernet NIC
Ethernet straight-through cable
When should you not use a network connection to configure the router?
When the change may disconnect the telnet connection.

57. C:\> ping
C:\> telnet
Ethernet Connection
Network connection needed
NIC
Terminal Connection
No network connection needed
Console Port
Serial

58. NetLab

59. NetLab
Basic Router Pod

60. Your Interfaces may differ
R1# show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/ YES manual up up
FastEthernet0/ YES manual up up
Serial0/ YES manual up up
Serial0/ unassigned YES manual up up
FastEthernet 0 = FastEthernet 0/0
FastEthernet 1 = FastEthernet 0/1 = FastEthernet 1/0
Serial 0 = Serial 0/0 = Serial 0/0/0
Serial 1 = Serial 0/1 = Serial 0/0/1

61. Learning IOS: Lab 1.5.2 (Cabrillo College Version)

62. Command Overview (partial list from lab)
Router> user mode
Router> enable
Router# privilege mode
Router# configure terminal
Router(config)# exit
Router# config t
Router(config)# hostname name
Router(config)# enable secret password privilege password
Router(config)# line console 0 console password
Router(config-line)# password password
Router(config-line)# login
Router(config)# line vty telnet password
Router(config)# banner motd # message # banner
Router(config)# interface type number configure interface
Router(config-if)# ip address address mask
Router(config-if)# description description
Router(config-if)# no shutdown

63. Other Commands Router# copy running-config startup-config
Router# show running-config
Router# show ip route
Router# show ip interface brief
Router# show interfaces

64. Different Modes Router# hostname R1 ^
% Invalid input detected at '^' marker.
Router# configure terminal
Router(config)# hostname R1
R1(config)#
IOS commands must be entered in the correct mode.

65. Serial Connectors 2500 have the “older,” larger serial interfaces
Smart Serial
“Older” Serial
2500 have the “older,” larger serial interfaces
Later Cisco routers use the smart serial interfaces which allows more data to be forwarded across fewer cable pins.

66. Serial Connectors DCE Cable DTE Cable
Router is typically a DTE device.
The DTE cable is connected to the serial interface on the router to a CSU/DSU device (DCE).

67. WAN Interface Configuration
R1(config)# interface Serial0/0
R1(config-if)# ip address
R1(config-if)# description Link to R2
R1(config-if)# clock rate DCE Only
R1(config-if)# no shutdown

68. Unsolicited Messages from IOS
R1(config)# interface fastethernet0/0
R1(config-if)# ip address
R1(config-if)# no shutdown
R1(config-if)# descri
*Mar 1 01:16:08.212: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
*Mar 1 01:16:09.214: %LINEPROTO-5-UPDOWN: Line protocol on Interface
FastEthernet0/0, changed state to upption
R1(config-if)#
The IOS often sends unsolicited messages
Does not affect the command
Can cause you to lose your place when typing.

69. Unsolicited Messages from IOS
R1(config)# line console 0
R1(config-line)# logging synchronous
R1(config-if)# descri
*Mar 1 01:28:04.242: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
*Mar 1 01:28:05.243: %LINEPROTO-5-UPDOWN: Line protocol on Interface
FastEthernet0/0, changed state to up
R1(config-if)# description
To keep the unsolicited output separate from your input, enter line configuration mode for the console port and add the logging synchronous

70. LAN Interface Configuration
R1(config)# interface FastEthernet0/0
R1(config-if)# ip address
R1(config-if)# description R1 LAN
R1(config-if)# no shutdown
Fa0/1

71. Each Interface Belongs to a Different Network
R1(config)# interface FastEthernet0/1
R1(config-if)# ip address
overlaps with FastEthernet0/0
R1(config-if)# no shutdown
FastEthernet0/1: incorrect IP address assignment
Fa0/1
/24
/24
Same Network!

72. Each Interface Belongs to a Different Network
R1# show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/ YES manual up up
Serial0/ YES manual up up
FastEthernet0/ YES manual administratively
down down
Serial0/ unassigned YES unset administratively
Fa0/1

73. Verifying Interfaces R1# show interfaces
<some interfaces not shown>
FastEthernet0/0 is up, line protocol is up (connected)
Hardware is Lance, address is 0007.eca (bia 00e0.f7e4.e47e)
Description: R1 LAN
Internet address is /24
MTU 1500 bytes, BW 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 /24
MTU 1500 bytes, BW 1544 Kbit, DLY usec, rely 255/255, load 1/255
Encapsulation HDLC, loopback not set, keepalive set (10 sec)
Last input never, output never, output hang never

74. Verify Router Configuration
R1# show running-config !
version 12.3
hostname R1
interface FastEthernet0/0
description R1 LAN
ip address
interface Serial0/0
description Link to R2
ip address
clock rate 64000
banner motd ^C
******************************************
WARNING!! Unauthorized Access Prohibited!! ^C
line con 0
password cisco
login
line vty 0 4
end
Note: shutdown is the default. no shutdown does not show in the configuration.

75. 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
interface Serial0/0
description Link to R2
ip address
clock rate 64000
banner motd ^C
******************************************
WARNING!! Unauthorized Access Prohibited!! ^C
line con 0
password cisco
login
line vty 0 4
end

76. Building the Routing Table
Introducing the Routing Table
Directly Connected Networks

77. 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 /24 is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0

78. 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 /24 is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0
Routing table is a data file in RAM that is used to store route information about:
Directly connected networks
Remote networks
Routing table is a data file in RAM that is used to store route information about:
Directly connected
Remote networks

79. Introducing the Routing Table
R1# show ip route
<output omitted>
C /24 is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0
Exit Interfaces
Directly connected interfaces contain the exit interface (more later)
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)

80. Introducing the Routing Table
R1# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<output omitted>
C /24 is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0
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.
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.

81. Introducing the Routing Table
R1# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
<output omitted>
C /24 is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0
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: (later)
Dynamic routing protocol
Static routes
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.

82. Chapter 1 Introduction to Routing and Packet Forwarding
CIS 82 Routing Protocols and Concepts
Rick Graziani
Cabrillo College