Topic 7(Network Layer)

Memorize this

1. De-encapsulation: The frame enters via the Gigabit Ethernet 0/0/0 interface, where R1 strips off the Layer 2 header and trailer.

2. Path Determination: R1 identifies the destination IPv4 address and finds the best matching route in its table, designating R2 as the next hop.

3. Forwarding: R1 re-encapsulates the packet into a new Ethernet frame and sends it out the appropriate interface toward the next router.

The Network Layer

Provides services to allow end devices to exchange data.

The Network Layer

IP version 4 (IPv4) and IP version 6 (IPv6) are the principal network layer communication protocols.

1. Addressing end devices

2. Encapsulation

3. Routing

4. De-encapsulation

The network layer performs four basic operations:

IP address does not change from source to destination, IP is encapsulated into a segment from the transport layer, IP packet will be examined by all layer 3 devices as it is transported.

Characteristics of IP encapsulation

Connectionless

There is no connection with the destination established before sending data packets

Connectionless Communication

is conceptually similar to sending a letter to someone without
notifying the recipient in advance

Connectionless

IP requires no initial exchange of control information to establish an end-to-end connection before packets are forwarded

Best Effort

IP is inherently unreliable because packet delivery is not guaranteed.

Best Effort

With no pre-established end-to- end connection, senders are unaware whether destination devices

Best Effort

Are present and functional when sending packets,

Best Effort

What network layer characteristic is this

• Receives the packet
• Are able to access and read the packet

Best Effort

The IP protocol was not designed to track and manage the flow of packets. These functions, if required, are performed by other protocols at other layers, primarily TCP at Layer 4.

Media Independent

Operation is independent of the medium (i.e., copper, fiber-optic, or wireless) carrying the data

Media Independent

IP does not concern itself with the type of frame required at the data link layer or the media type at the physical layer.

Media Independent

As shown in the figure, IP packets can be sent over any media type: copper, fiber, or wireless.

IPv4 Packet Header

Diagram is read from left to right, 4 bytes per line

IPv4 Packet Header

The two most commonly referenced fields are the source and destination IP addresses

IPv4 Packet Header

These fields identify where the packet is coming from and where it is going

IPv4 Packet Header

Typically, these addresses do not change while travelling from the source to the destination.

Version

This will be for v4, as opposed to v6, a 4 bit field= 0100

Differentiated Services

Used for QoS: DiffServ – DS field or the older IntServ – ToS or Type of Service

Header Checksum

Detect corruption in the IPv4 header

Time to Live (TTL)

Layer 3 hop count. When it becomes zero the router will discard the packet.

Protocol

I.D.s next level protocol: ICMP, TCP, UDP, etc.

Source IPv4 Address

32 bit source address

Destination IPv4 Address

32 bit destination address

IPv4 address depletion, Lack of end-to-end connectivity, Increased network complexity

What are the limitations of IPv4

IPv6

Developed to overcomes the limitations of IPv4.

Increased address space, Improved packet handling, Eliminates the need for NAT

What are the IPv6 Improvements

NAT

causes latency and troubleshooting issues.

Network Address Translation

What does NAT stand for

Increased address space

based on 128 bit address, not 32 bits

Improved packet handling

simplified header with fewer fields

Eliminates the need for NAT

since there is a huge amount of addressing, there is no need to use private addressing internally and be mapped to a shared public address

The IPv6 Header

is simplified, but not smaller.

IPv6

What header is this

IPv4 Header

What header is this

IPv6

The header is fixed at 40 Bytes long.

Flag, Fragment Offset, Header Checksum

Some IPv4 fields were removed to improve performance:

Version

This will be for v6, as opposed to v4, a 4 bit field= 0110

Traffic Class

Used for QoS: Equivalent to DiffServ – DS field

Flow Label

Informs device to handle identical flow labels the same way, 20 bit field

Payload Length

This 16-bit field indicates the length of the data portion or payload of the IPv6 packet.

Next Header

I.D.s next level protocol: ICMP, TCP, UDP, etc

Hop Limit

Replaces TTL field Layer 3 hop count

Source IPv6 Address

128 bit source address

Destination IPV6 Address

128 bit destination address

Host Routing

Each host device creates its own routing table.

Itself, Local Hosts, Remote Hosts

A host can send packets to the following destinations:

Itself

127.0.0.1 (IPv4) – the special address of the loopback interface

Local Hosts

on the same LAN

Remote Hosts

on the different LANs

Default Gateway

Remote traffic is forwarded directly to the _________ on the LAN.

Host Forwarding Decision

The source device determines whether the destination is local or remote by comparing its own IP address with the destination IP address.

Default Gateway

A router or layer 3 switch can be a _______.

Default Gateway

It must have an IP address in the same range as the rest of the LAN.

Default Gateway

It can accept data from the LAN and forward traffic to other networks.

False

True or False

If a device has no default gateway or a bad default gateway, its traffic will be able to leave the LAN.

statically or through DHCP in IPv4

The host will know the default gateway either ________________.

Routing Table

Use route print or netstat -r to display the _______.

Interface List and MAC addressing, IPv4 Routing Table, IPv6 Routing Table

What are the three sections of the routing table

Routing table

contain information used by the router to forward packets.

Directly Connected Networks, Remote Networks, Default Route

What are the three types of routes in a routing table

Directly Connected networks

Are automatically added by the router, provided the interface is active
and has IP addressing

Remote networks

Routes that the router does not have a direct connection and may be learned manually or dynamically

Manually

With a static route

Dynamically

by using a dynamic routing protocol

Default Route

a gateway of last resort, used when there is not a match in the routing table.

Static Route

Must be configured manually

Static Routing

Must be adjusted manually by the administrator when there is a change in the topology

Static Routing

Good for small non-redundant networks

Static Routing

Often used in conjunction with a dynamic routing protocol for configuring a default route

Dynamic Routing

Discover remote networks

Dynamic Routing

Maintain up-to-date information

Dynamic Routing

Choose the best path to the destination

Dynamic Routing

Find new best paths when there is a topology change

Open Shortest Path First

what does OSPF stand for

L, C, S, O, D

What are the route sources?

L

Directly connected local interface IP address

C

Directly connected network

S

Static route, manually configured

O

OSPF

D

EIGRP

Enhanced Interior Gateway Routing Protocol

EIGRP

Directly Connected, Remote Routes, Default Routes

What are the types of routes

Directly connected

C and L

Remote routes

O, D etc.

Remote Routes

S*

no shutdown

Activates the interface

description Link to LAN

Adds information about the network connected to the interface

show ip interface brief and show ipv6 interface brief

To verify interface configuration or to view status of all interfaces use the
______________ commands shown

Default Gateway

is used when a host sends a packet to a device on another network

Default Gateway Address

is generally the router interface address attached to the local network of the host.

Default Gateway

To reach PC3, PC1 addresses a packet with the IPv4 address of PC3, but forwards the packet to its ________, the G0/0/0 interface of R1.

isp

Password

False

True or False

A switch must not have a default gateway address configured to remotely managed the switch from another network.

ip default-gateway

Configured by using the __________ command.