Standards and Functions

Routing Operations

Default Route

A default route is the interface through which the router forwards a packet containing a destination IP network address that is not in the routing table of the router.

Host Determination

Prior to sending a packet a host will evaluate whether the destination address for a packet is within the same network as itself (by using the subnet mask).

Where a host using the subnet mask determines a destination IP in the same network:

Packet Encapsulation includes the source IP Address and Destination IP Address
Host evaluates it’s internal ARP table to determine whether an ARP request is required to determine the destination MAC address
Frame is encapsulated to include the source MAC address and derived MAC address from the ARP table for destination.

Where a host using the subnet mask determines a destination IP in a different network:

Packet encapsulation includes the source IP address and Destination IP Address
Host evaluates it’s configuration to ensure the default gateway is configured
Host evaluates it’s internal ARP table to determine whether an ARP request is required to determine the default gateway MAC address
Frame is encapsulated to include the source MAC address and the default gateway MAC address from the ARP table.

Route Determination (Router Forwarding)

When a router receives a frame:

Decapsulation: The router strips the Layer 2 (MAC) header to access the Layer 3 IP packet.
Layer 3 Immutability: The Source IP and Destination IP addresses remain immutable (unless Network Address Translation (NAT) is involved).
Route Look-up: The router checks its internal Routing Table to find the longest prefix match for the destination IP address. This determines the exit interface and the next-hop IP address.
Next-Hop MAC Resolution: The router checks its ARP cache for the next-hop IP address to find the corresponding next-hop MAC address. If not found, it performs an ARP request.
Re-encapsulation: The router adds a new Layer 2 frame header with:
- Source MAC: The MAC address of the exit interface.
- Destination MAC: The next-hop MAC address.
Forwarding: The router sends the new frame out the designated exit interface.

Routing Table Lookup Priority

When multiple routes match a destination:

Longest prefix match wins (most specific route)
- Example: 192.168.1.0/25 is preferred over 192.168.1.0/24
If prefix lengths are equal, Administrative Distance (AD) determines preference:

Route Source	AD Value
Directly Connected	0
Static Route	1
EIGRP Summary	5
eBGP	20
EIGRP (internal)	90
IGRP	100
OSPF	110
IS-IS	115
RIP	120
EIGRP (external)	170
iBGP	200

If AD is equal, use metric (protocol-specific)

Layer 2 Switch Operations

EtherChannel

EtherChannel is used when you are working with switches which require a aggregate and redundant links which in a normal circumstance would create a loop and in an STP scenario cause an election issue. With EtherChannel you are able to not only have redundancy but also split traffic between two links effectively doubling the bandwidth of traffic between links (assuming both channels are running at the same speed).

PAgP (Port Aggregation Protocol): Cisco proprietary

desirable: Actively negotiates
auto: Passively waits for negotiation

LACP (Link Aggregation Control Protocol): IEEE 802.3ad standard (vendor-neutral)

active: Actively negotiates
passive: Passively waits for negotiation
Static (on mode): No negotiation protocol, both sides must be set to on

EtherChannel Requirements

EtherChannels act as one logical interface
EtherChannels can be
- manually set or alternatively
- Using a protocol like PAgP or LACP to negotiate the requirements.
EtherChannels require both ports to have:
- the same duplex
- speed
- Access ports or Trunk Ports (Usually would be trunk) -
- Access ports need to be on the same vlans.
- Trunk ports need to have the same allowed vlans and same native vlan
- Same STP Interface Settings

Configuration

Configuration is important to avoid switching loops. When configuring remember:

on/on - Static EtherChannel (no negotiation protocol)
desirable/desirable - PAgP: Both actively negotiate
desirable/auto - PAgP: One actively negotiates, one passively responds
active/active - LACP: Both actively negotiate
active/passive - LACP: One actively negotiates, one passively responds
on with ANY protocol mode (auto/desirable/active/passive) - WILL NOT FORM (mismatched configuration)

EtherChannel Load Balancing

Traffic is distributed across member links based on:

src-mac - Source MAC address
dst-mac - Destination MAC address
src-dst-mac - Both source and destination MAC (most common)
src-ip - Source IP address
dst-ip - Destination IP address
src-dst-ip - Both source and destination IP

Note: The hashing algorithm ensures flows stay on the same link to prevent packet reordering.

General

A Layer 2 switch handles the forwarding of Frames based on the Source and Destination MAC Addresses. This process is independent of ARP but helps facilitate it.

MAC Learning: A switch builds a MAC Address Table (CAM table) by inspecting the source MAC address of every frame it receives and recording which port that address came from.
Forwarding:
- If the switch knows the port for the destination MAC address, it unicasts the frame only out that specific port.
- If the switch does not know the port for the destination MAC address (e.g., during the initial ARP Request broadcast), it floods the frame out all ports except the one it was received on. This flooding allows the broadcast to reach all devices on the segment.

Wi-Fi

2.4 GHz Band:

Channels 1-14 available (channel 14 restricted in most countries)
Only 3 non-overlapping channels: 1, 6, 11
More prone to interference (microwaves, Bluetooth, cordless phones)

5 GHz Band:

24+ non-overlapping channels (varies by regulatory domain)
Less interference
Shorter range than 2.4 GHz
Better for high-bandwidth applications

802.11b

up to 11Mbps (typically 5-6Mbps)
100-150 feet range
2.4ghz
interference prone

802.11a

Up to 54Mbps (typically 15-20 Mbps)
50-75 feet range
More expensive
5ghz
not compatible with b

802.11g

54 Mbps speed
compatible with b
100-150 feet range
2.4ghz

802.11n (Wi-Fi 4)

Up to 600 Mbps (with 4 spatial streams)
2.4 GHz and/or 5 GHz (dual-band capable)
230 feet range (70m indoor)
MIMO technology
Backward compatible with a/b/g

802.11ac (Wi-Fi 5)

Up to 1.3 Gbps (Wave 1) or 6.9 Gbps (Wave 2)
5 GHz only
MU-MIMO technology
Beamforming
Wider channels (80 MHz, 160 MHz)

802.11ax (Wi-Fi 6)

Up to 9.6 Gbps theoretical
2.4 GHz and 5 GHz
OFDMA (Orthogonal Frequency Division Multiple Access) for better efficiency
Target Wake Time (TWT) for IoT battery life
Better performance in dense/congested environments
Backward compatible with previous standards

NAT Network Address Translation

Network Address Translation (NAT) translates IP addresses between a private network and a public network (the Internet). This translation is performed by a router which maintains a translation table of private-to-public address mappings.

Outgoing Traffic: When a packet leaves the private network and is destined for the public Internet, the router converts the packet’s private source IP address and port number to a registered public IP address and a new port number, which it records in the table.
Return Traffic: When the reply packet returns to the router, the router looks up the public destination address and port in its table and translates it back to the original private IP address and port number before forwarding it to the internal host.

The most common form, Port Address Translation (PAT), conserves public IP addresses by allowing thousands of internal hosts to share a single public IP address using unique port numbers.

NAT Types

Static NAT: One-to-one mapping between private and public IP addresses. Used for servers that need consistent external addresses.

Dynamic NAT: Maps private addresses to a pool of public addresses on a first-come, first-served basis.

PAT (Port Address Translation / NAT Overload): The most common form. Allows thousands of internal hosts to share a single public IP address using unique port numbers. This is what most home/small office routers use.