IGMP Snooping & the Querier Trap
IGMP snooping is a Layer 2 switch feature that listens to IGMP messages so that multicast is only sent to ports with interested hosts – instead of being flooded like broadcast. Facts / key topics Standard: not an IETF protocol – a switch feature · depends on IGMP (IPv4) · OSI: Layer 2 Default behaviour […]
Layer 2 Multicast
Layer 2 multicast is about how switches forward multicast frames based on MAC addresses, and how flooding is limited with IGMP snooping. Facts / key topics Standard: IEEE 802.3 (Ethernet) · OSI: Layer 2 (Data Link) MAC range (IPv4 multicast): 0100.5E00.0000 – 0100.5E7F.FFFF MAC range (IPv6 multicast): 33:33:xx:xx:xx:xx Default switch behaviour: flooding (like broadcast) · […]
How a Switch Learns (CAM Table)
A switch learns MAC addresses from the source MAC of incoming frames and uses the CAM table to forward known unicast traffic or flood unknown traffic within the same VLAN. CAM learning process Receive – the switch receives a frame on a port Learn – reads the source MAC address and stores source MAC → […]
802.1Q Trunking
Point Trunk Purpose Multiple VLANs over one link Standard 802.1Q Frames Tagged Native VLAN Untagged frames Access port One VLAN only DTP Cisco only Typical failures Native VLAN mismatch, trunk mismatch, allowed-VLAN mismatch Remember: a trunk created without an allow statement carries all VLANs. switchport nonegotiate disables DTP to prevent automatic trunk negotiation and improve […]
VLANs & the VLAN 1 Problem
A VLAN creates separate Layer 2 broadcast domains on the same physical switch. Devices in the same VLAN communicate directly at Layer 2; communication between VLANs requires routing. VLANs improve security, performance and network structure. Key points (CCNA) Point Explanation Purpose Segment the network Layer Layer 2 Broadcast One broadcast domain per VLAN VLAN ID […]
STP Flavors Compared (802.1D to MST)
Different STP flavors solve the same problem – loop prevention – with different trade-offs in convergence, VLAN support, vendor independence and scalability. Overview table STP type Standard What it does Where used Why used PVST Cisco proprietary One STP instance per VLAN Legacy Cisco Load balancing between VLANs PVST+ Cisco One instance per VLAN + […]
Spanning Tree — Root Election & Config
Spanning Tree Protocol prevents Layer 2 loops by electing one active path and blocking redundant links. The Root Bridge is elected based on the lowest Bridge ID (priority first, then MAC). The Root Port is the port with the lowest path cost toward the Root Bridge. Facts Topic Explanation Problem Broadcast storms, MAC flapping Solution […]
EtherChannel & LACP
EtherChannel is Cisco’s implementation of Link Aggregation (LAG), bundling multiple physical links into one logical connection called a Port-Channel interface. Purpose Increased bandwidth Redundancy One STP link (prevents loops) Hardware limitations (ASIC) All member links must have the same speed Typically 16 interfaces supported – but only 8 active Maximum: 8 active links per Port-Channel […]
IPv6 Address Formats — Cheat Sheet
IPv6 address formats define the purpose and scope of an address – routing, local communication, multicast or special use. Address formats (what you must know) Type Prefix / format Scope Use Exam notes Global Unicast (GUA) 2000::/3 Global Normal routable IPv6 address The most used IPv6 address Link-Local FE80::/10 Link Neighbor Discovery, routing protocols Automatically […]
What is IPv6?
IPv6 is the next generation of IP, designed to replace IPv4 because IPv4 ran out of addresses and needs workarounds like NAT. 👉 IPv6 provides enough addresses for the whole planet, better routing and simpler network design. Basic facts Address length: 128 bits Notation: hexadecimal (0–9, a–f) Example: 2001:0db8:abcd:0012:0000:0000:0000:0001 → shortened 2001:db8:abcd:12::1 OSI layer: Layer […]