Network Layer & IP Protocols

Network Layer in OSI Model

• The functionality of network layer protocols dictates the packet structure accompanied with the processing involved in transmitting data between hosts.
• The actual communication data stays encapsulated inside the network layer PDU.
• The network layer is able to carry packets for various types of communication due to its ability to function independently of the packet's data

IP Characteristics

• IP is a connectionless, best effort protocol that doesn't need a dedicated end-to-end connection and doesn't ensure packet delivery
• IP operates without dependence on underlying carrying network media.

Connectionless Protocol Lower Layer Data Handling

• When connectionless protocols operate in a lower OSI model layer, upper-level protocols at sending and receiving devices handle lost data via retransmission.
• This isn't needed for some applications where a degree of data loss is acceptable.
• Upper-layer connection-oriented protocols track data received and request retransmission from upper-level protocols on sending host.

IPv4 Header and Packet Traversal

• The Time-to-Live (TTL) field present in the IPv4 header serves the purpose of preventing packets from endlessly traversing a network
• The sending host assigns the initial TTL value, which then decreases by one with each router hop.
• When the TTL hits zero, the router discards the packet and sends an Internet Control Message Protocol (ICMP) Time Exceeded message to whom it came from

Identifying Upper Layer Protocols

• The Protocol field in the IP header identifies the upper-layer protocol being carried
• The Identification field reorders fragmented packets, while the Version field identifies the IP version, and Differential Services field is for setting packet priority

IPv6 Header Advantage

• IPv6's simplified header brings a better routing efficiency and efficient packet handling
• IPv6 does not need processing checksums
• IPv6 features efficient extension header mechanisms and a Flow Label field for easier traffic identification.

PC1 and PC2 Routing

• PC1 and PC2 exist on the 192.168.10.0 network indicated by the 255.255.255.0 mask, negating any default gateway necessity.
• The 192.168.10.10 entry identifies PC1

Packet Forwarding

• When a route points to a next hop address, the router must perform another lookup to determine the correct exit interface.

IPv4 Routing Table Codes

• C indicates the route is directly connected

Destination Network Routing Table Entry

• Routing table entries intended for remote networks possess a next hop IP address.
• Directly-connected and local routes do not require a next hop IP address because they do not need to go through another router.

Cisco 1941 Router

• A Cisco 1941 router features two types of ports for console access

Router Boot Sequence

• The router finds and loads the startup-config file from NVRAM

Router Interface Types

• LAN interfaces connect cables terminating with LAN devices
• WAN interfaces – Used for connecting routers to external networks

Cisco Router RAM

• Cisco IOS and the IP routing table are stored in RAM during normal operation.

Router Boot Modes

• Routers boot into setup mode because the configuration file is missing from NVRAM.

Startup Configuration File Purpose

• The startup configuration file contains the commands needed to initially configure a router.

Securing Router Console Access

• The console port can be password protected with the following commands: line console 0, password cisco, and login.

IPv6 Header

• The simplifications of IPv6 improve packet handling.

Router Failure

• The failure is occurring because SW1 does not have a default gateway configured

Re-transmission of Data in Lower Layers of OSI Model

• Upper-layer connection-oriented protocols re-transmit missing data

IPv4 Header Field

• The differentiated services field defines the packet's priority

NAT and IPv6

• NAT is not needed because IPv6 has a large number of available IP addresses

IPv6 Header Service

• The Flow Label field enables network devices to maintain the same path for real-time application packets

Network Destination

• Hosts need to keep their own routing table so they can direct traffic correctly

Windows Host Commands

• route print and netstat -r show Windows hosts' routing tables

Forwarding Traffic

• The router will switch the packet to the directly connected interface after matching the destination IP address to a network on a directly connected routing table entry

Router Power Loss

• If not saved, the routing table and ARP cache will be lost

VTY Lines and Router Access

• The WAN and LAN interfaces allow access via the VTY lines

Files Copied into RAM

• Provided they're found, the IOS image file and startup config are copied into RAM

Cisco IOS Image in ROM Boot Use

• The Cisco IOS image held in ROM gets used to boot the router when the full IOS cannot be found

Saving Router Configuration

• The network administrator must issue the "copy running-config startup-config" so it is used at the next reboot

Setting a Password

• line console 0, login, and password cisco set a password for initial configuration of a new router

Router Interface Statement

• A configured and activated router interface must be connected to another device in order to operate

Displaying Router Interface Info

• show ip interface brief displays a summary chart of all router interfaces, their IP addresses, and their current operational status.

Manual Router

• A default gateway needs configured for internet access

PC Packet Sent in Same LAN

• The packet sends directly to its assigned host

PC Traversal for Exhibited Diagram

• "3" hops are required to reach PC-4

Router ROM and Diagnostic

• The diagnostic software, bootup instructions, and a limited IOS are stored in ROM in a router

New IOS Support

• The router has enough RAM and flash memory for the IOS upgrade

Sort Elements

• IP v4 Header (A) -> Flags (A)
• IP v4 Header (B) -> Total Length (B)
• IP v6 Header (C) ->Traffic Class (C)
• IP v6 Header (D) -> Flow Label (D)

Static Route Exit Interface

• Directly connected static route uses only the exit interface