Chapter 1 Introduction to Routing and Packet ForwardingRouting Protocols and Concepts quangkien@gmail.com.Topicsl Inside the Router Ÿ Routers are computers Ÿ Router CPU and Memory Ÿ Internetwork Operating System Ÿ Router Bootup Process Ÿ Router Ports doc

Ÿ Best match between destination IP address and network address in routing table IP Packet enters router’s Ethernet interface.. Router searches for a best match between packet’s destina

Topics

l Inside the Router

Ÿ Routers are computers

Ÿ Router CPU and Memory

Ÿ Internetwork Operating

System

Ÿ Router Bootup Process

Ÿ Router Ports and Interfaces

Ÿ Routers and the Network

Ÿ Best Path and Metrics

Ÿ Equal Cost Load Balancing

Ÿ Basic Router Configuration

l Building the Routing Table

Ÿ Introducing the Routing Table

Ÿ Directly Connected Networks

Ÿ Static Routing

Ÿ Dynamic Routing

Ÿ Routing Table Principles

Inside the Router

l Routers are computers

l Router CPU and Memory

l Internetwork Operating System

l Router Bootup Process

l Router Ports and Interfaces

l Routers and the Network Layer

Routers are Computers

l A router is a computer:

Ÿ CPU, RAM, ROM, Operating System

l The first router: used for the Advanced Research Projects Agency

Network (ARPANET):

Ÿ IMP (Interface Message Processor)

Ÿ Honeywell 516 minicomputer that brought the ARPANET to life

on August 30, 1969.

Leonard Kleinrock and the first IMP.

l Routers forwarding packets (packet switching):

Ÿ From the original source to the final destination.

Ÿ Selects best path based on destination IP address

l A router connects multiple networks:

Ÿ Interfaces on different IP networks

l Router interfaces:

Routers Determine the Best Path

l The router’s primary responsibility:

Ÿ Determining the best path

Ÿ Forwarding packets toward their destination

Routers Determine the Best Path

l Routing table

Ÿ Determines best path

Ÿ Best match between destination IP address and network

address in routing table

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.

Using the exit-interface in the route, the packet is forwarded to the next router or the final destination.

Router

CPU and

Memory

l CPU - Executes operating system instructions

Ÿ running copy of configuration file

Ÿ routing table

Ÿ ARP cache

Ÿ Diagnostic software used when router is powered up

Ÿ Router’s bootstrap program

Ÿ Scaled down version of operating system IOS

Ÿ Stores startup configuration (including IP addresses, Routing protocol)

l Flash memory - Contains the operating system (Cisco IOS)

l Interfaces - There exist multiple physical interfaces that are used to connect

network Examples of interface types:

Ÿ Ethernet / fast Ethernet interfaces

Ÿ Serial interfaces

Ÿ Management interfaces

10Router physical characteristics

Ÿ Managing file systems

l Many different IOS images

l An IOS image is a file that contains the entire IOS for that router

Ÿ Router model

Ÿ IOS features

l Example IPv6 or a routing protocol such as Intermediate System–

to–Intermediate System (IS-IS).

Router Bootup Process (more in later course)

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

IOS

Bootup program

ios (partial) running-config

IOS (running)

1 POST

2 Bootstrap code executed

3 Check Configuration Register value (NVRAM)

0 = ROM Monitor mode

1 = ROM IOS

2 - 15 = 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)

Router Boot Process –

Details (later)

Verify the router boot-up process

l show version command is used to view information about the

router during the bootup process (later)

Ports and Interfaces

l Port - normally means one of the management ports used for

administrative access

l Interface normally refers to interfaces that are capable of sending

and receiving user traffic.

l Note: However, these terms are often used interchangeably in the

industry and even with IOS output.

Ÿ PC running terminal emulator software

l No need for network access

l Used for initial configuration

Auxiliary (AUX) port

l Not all routers have auxiliary ports.

Ÿ At times, can be used similarly to a console port

Ÿ Can also be used to attach a modem

l Note: Auxiliary ports will not be used in this curriculum.

Router Interfaces

l Interfaces - Receive and forward packets

Ÿ Various types of networks

Ÿ Different types of media and connectors

Ÿ Different types of interfaces

l Fast Ethernet interfaces - LANs

l Serial interfaces - WAN connections including T1, DSL, and ISDN

Router Interfaces

l Router Interface:

Ÿ Different network

Ÿ IP address and subnet mask of that network

l Cisco IOS will not allow two active interfaces on the same

router to belong to the same network.

FastEthernet 0/0 MAC: 0c00-3a44-190a 192.168.1.1/24

FastEthernet 0/0 MAC: 0c00-41cc-ae12 10.1.0.1/16

Serial 0/0 172.16.1.1/24

Serial 0/1 172.16.1.2/24

LAN Interfaces

l Ethernet and Fast Ethernet interfaces

l Connects the router to the LAN

Ÿ Layer 2 MAC address

Ÿ Participates in the Ethernet

Ÿ Address Resolution Protocol (ARP):

Ÿ Maintains ARP cache for that interface

Ÿ Sends ARP requests when needed

Ÿ Responds with ARP replies when required

l Typically an RJ-45 jack (UTP)

Ÿ Router to switch: straight-through cable

Ÿ Router to router: crossover cable

WAN Interfaces

l Point-to-Point, ISDN, and Frame Relay interfaces

l Connects routers to external networks.

l The Layer 2 encapsulation can be different types including:

Ÿ Frame Relay

Ÿ HDLC (High-Level Data Link Control)

l Note: MAC addresses are used only on Ethernet interfaces and are

not on WAN interfaces

l Layer 2 WAN encapsulation types and addresses are covered in a

later course.

l Layer 3 device because its primary forwarding

decision is based on the information in the Layer 3 IP

packet (destination IP address)

l This is known as routing.

26Routers Operate at Layers 1, 2, and 3

Path Determination and

Switching Functions

l Packet Fields and Frame Formats

l Best Path and Metrics

l Equal Cost Load Balancing

l Path Determination

l Switching Function

Path Determination and Switching Functions

l The following sections focus on exactly what happens to data as it

moves from source to destination

Ÿ Review the packet and frame field specifications

Ÿ Discuss in detail how the frame fields change from hop to hop,

whereas the packet fields remain unchanged

Ethernet Frame

l Layer 2 addresses:

Ÿ Interface-to-Interface on the same network.

Ÿ Changes as packet is decapsulated and encapsulated from

network to network

l Layer 3 addresses:

Ÿ Original source layer 3 address (IP)

Ÿ Final destination layer 3 address (IP)

Ÿ Does not change (except with NAT, but this is not a concern of

IP but an internal network process)

IPv4 (Internet Protocol)

Best Path

l Router’s best-path to a network:

Ÿ optimum or “shortest” path

l Routing protocol dependent

l Dynamic routing protocols use their own rules and metrics

l A metric is the quantitative value used to measure the distance to a

given route

l The best path to a network is the path with the lowest metric

l Example, a router will prefer a path that is one hop away over a path

that is two hops away.

Best Path

1.5 Mbps

1.5 Mbps

l Comparing Dynamic Routing Protocols: RIP and OSPF

l RIP uses hop count

Ÿ R1 to R3

Ÿ Fewer links but much slower

l OSPF uses bandwidth

Ÿ R1 to R2 to R3

Ÿ More routers but much faster links

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.

192.168.1.0/24

Can a router use multiple paths if the paths

(cost, metric) to reach the destination

network are not equal?

Yes, if the routers are using the EIGRP routing

protocol which supports unequal cost load

balancing.

192.168.1.0/24

Path Forwarding

l Packet forwarding involves two functions:

Ÿ Path determination function

Ÿ Switching function

Path Forwarding

l Path determination function is the process of how the router determines

which path to use when forwarding a packet.

l To determine the best path, the router searches its routing table for a

network address that matches the packet’s destination IP address.

l One of three path determinations results from this search:

Directly connected network

Router receives packet.

Destination IP address matches a network on one

of its directly connected networks.

Packet is forwarded out that network.

Path Forwarding

l Path determination function is the process of how the router determines

which path to use when forwarding a packet.

l To determine the best path, the router searches its routing table for a

network address that matches the packet’s destination IP address.

l One of three path determinations results from this search:

Remote

network

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.

Path Forwarding

l Path determination function is the process of how the router determines

which path to use when forwarding a packet.

l To determine the best path, the router searches its routing table for a

network address that matches the packet’s destination IP address.

l One of three path determinations results from this search:

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?

No, only that the router does not know

about that network (later)

Path Forwarding

l Switching function is the process used by a router to:

Ÿ Accept a packet on one interface and

Ÿ Forward it out another interface

l 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

Trailer Type

800

Source MAC 00-20

Source IP 192.168.1.10

Dest IP 192.168.4.10

Layer 3 IP Packet

Trailer Type

800

Source MAC 0A-10

Remember: Encapsulation

l Now, let’s do an example…

Data Other IP

fields

Source IP Address

Destination IP Address

Trailer Data

Type

Source Address

Destination

Address

Layer 3 IP Packet

Layer 2 Data Link Frame

Current Data Link

Address of Host or Router’s exit interface

Next hop Data

Link Address of

Host or Router’s

interface

These change from

host to router, router to

router, and router to

host.

These addresses

do not change!

l This is just a summary.

l The details will be shown next!

l Now for the details…

Trailer Type

800

Source MAC 0A-10

Source IP 192.168.1.10

Dest IP 192.168.4.10

Layer 3 IP Packet

Trailer Type

800

Source MAC 00-20

Dest MAC

0B-31

Data

IP fields

Source IP 192.168.1.10

Dest IP 192.168.4.10

Trailer Type

800

Source Add Dest Add

FF-FF

From Host X to Router RTA

l 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.

l How does Host X know to forward to packet to RTA and not directly to Host

Y?

Ÿ IP Source and IP Destination Addresses are on different networks

l How does Host X know or get RTA’s Ethernet address?

Ÿ Checks ARP Table for Default Gateway IP Address and associated

MAC Address.

l What if it there is not an entry in the ARP Table?

Ÿ Host X sends an ARP Request and RTA sends an ARP Reply

Trailer Type

800

Source MAC 0A-10

Source IP 192.168.1.10

Dest IP 192.168.4.10

Layer 3 IP Packet

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.

l 192.168.4.0/24 has next-hop-ip address of 192.168.2.2 and an exit-interface of e1

l 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

l 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

Trailer Type

800

Source MAC 00-20

Source IP 192.168.1.10

Dest IP 192.168.4.10

Layer 3 IP Packet

RTA Routing Table Network Hops Next-hop-ip Exit-interface 192.168.1.0/24 0 Dir.Conn e0

192.168.2.0/24 0 Dir.Conn e1 192.168.3.0/24 1 192.168.2.2 e1 192.168.4.0/24 2 192.168.2.2 e1

RTA ARP Cache

IP Address MAC Address

192.168.2.2 0B-31

Trailer Type

800

Source MAC 0A-10

Dest MAC

00-10

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.

l 192.168.4.0/24 has next-hop-ip address of 192.168.3.2 and an exit-interface of Serial0

l 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

l 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.)

l 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

Trailer Type

800

Source Add Dest Add

FF-FF

Layer 2 Data Link Frame

Data

IP fields

Source IP 192.168.1.10

Dest IP 192.168.4.10

Layer 3 IP Packet

RTB Routing Table Network Hops Next-hop-ip Exit-interface 192.168.1.0/24 1 192.168.2.1 e0

192.168.2.0/24 0 Dir.Conn e0 192.168.3.0/24 0 Dir.Conn s0 192.168.4.0/24 1 192.168.3.2 s0

Trailer Type

800

Source MAC 00-20

Dest MAC

0B-31