Network Protocol Reference
Network protocol explanations with standards references and practical lab observations.
Ethernet
How Ethernet carries frames and how switches learn where to send them.
Layer 2
802.1Q VLANs
How switches assign frames to VLANs and carry multiple VLANs across one link.
Layer 2
Q-in-Q (VLAN stacking)
How a provider carries a customer’s VLAN tags inside a service VLAN.
Layer 2
Private VLANs (PVLANs)
How hosts can share a subnet and gateway while the switch restricts direct traffic between them.
Layer 2
VXLAN
How VXLAN carries Ethernet frames between tunnel endpoints over a routed IP network.
Network overlays
Geneve
How Geneve carries virtual network traffic and optional metadata in a UDP tunnel.
Network overlays
NVGRE
How NVGRE uses the GRE Key field to identify a virtual Ethernet subnet.
Network overlays
Spanning Tree Protocol (STP)
How Ethernet switches elect a root and keep redundant links from forming a forwarding loop.
Layer 2
Rapid Spanning Tree Protocol (RSTP)
How RSTP uses port roles and link handshakes to restore a loop-free path quickly.
Layer 2
Multiple Spanning Tree Protocol (MSTP)
How a region maps several VLANs to a shared tree while allowing different groups to take different paths.
Layer 2
PVST+ and Rapid PVST+
How Cisco’s per-VLAN spanning tree modes give each VLAN its own root and port roles.
Layer 2
Unidirectional Link Detection (UDLD)
How UDLD detects a link that still reports carrier even though frames do not travel in both directions.
Layer 2
Link Aggregation Control Protocol (LACP)
How switches negotiate which physical links can work together as one logical connection.
Layer 2
Port Aggregation Protocol (PAgP)
How Cisco switches use auto and desirable modes to negotiate an EtherChannel.
Layer 2
Static link aggregation
How a manually configured bundle forwards traffic without exchanging aggregation messages.
Layer 2
Multichassis link aggregation and stacking
How vPC, MLAG and switch stacks let one bundle span two physical switches.
Layer 2
Address Resolution Protocol (ARP)
How an IPv4 host finds the Ethernet address for a next hop on its local link.
Address resolution and discovery
IPv6 Neighbor Discovery (ND)
How IPv6 nodes find neighbors and routers with ICMPv6 messages on the local link.
Address resolution and discovery
Link Layer Discovery Protocol (LLDP)
How adjacent devices advertise their identity and port details across vendor boundaries.
Address resolution and discovery
Cisco Discovery Protocol (CDP)
How Cisco devices discover directly connected neighbors and identify the ports between them.
Address resolution and discovery
Switch port security
How a switch limits the source MAC addresses that can send traffic on a port.
Layer 2 access security
DHCP snooping
How a switch filters DHCP server traffic and learns where clients received their addresses.
Layer 2 access security
Dynamic ARP Inspection (DAI)
How a switch checks whether ARP messages claim an authorized IPv4-to-MAC mapping.
Layer 2 access security
IP Source Guard
How a switch filters IPv4 traffic based on the source binding allowed on its ingress port.
Layer 2 access security
802.1X port access control
How a network authenticates an endpoint before giving it normal access to a port.
Layer 2 access security
MAC Security (MACsec)
How Ethernet peers verify protected frames and can encrypt traffic across a link.
Layer 2 access security
Point-to-Point Protocol (PPP)
How two peers bring up a data link, authenticate if needed, and negotiate network protocols.
WAN and access
PPP over Ethernet (PPPoE)
How a client finds an access concentrator and runs a point-to-point PPP session over Ethernet.
WAN and access
High-Level Data Link Control (HDLC)
How older synchronous serial links delimit and check frames, and how Cisco HDLC differs.
WAN and access
Frame Relay
How a legacy WAN forwards variable-length frames across virtual circuits identified by local labels.
WAN and access
Asynchronous Transfer Mode (ATM)
How legacy ATM networks switch fixed-size cells and reassemble higher-layer data.
WAN and access
Carrier Ethernet service models
How E-Line, E-LAN and E-Tree define which subscriber endpoints can exchange frames.
Layer 2
Ethernet Connectivity Fault Management (CFM)
How maintenance endpoints detect a broken Ethernet service even when access links remain up.
Layer 2
ITU-T Y.1731 Ethernet OAM
How Ethernet service endpoints measure frame loss, delay and delay variation.
Layer 2
Ethernet in the First Mile (EFM) link OAM
How directly connected Ethernet peers discover one another and monitor their link.
Layer 2
Routing Information Protocol (RIPv1 and RIPv2)
How IPv4 RIP exchanges routes, chooses a path by hop count and carries subnet information in version 2.
Layer 3
RIP next generation (RIPng)
How IPv6 RIP advertises prefixes over a local link and uses hop count to select a path.
Layer 3
Interior Gateway Routing Protocol (IGRP)
How Cisco’s legacy IPv4 distance-vector protocol weighs link bandwidth and path delay.
Layer 3
Enhanced Interior Gateway Routing Protocol (EIGRP)
How EIGRP exchanges route changes and identifies loop-free backup paths with DUAL.
Layer 3
Open Shortest Path First version 2 (OSPFv2)
How OSPFv2 builds an IPv4 topology inside each area and calculates routes from link costs.
Layer 3
Open Shortest Path First version 3 (OSPFv3)
How OSPFv3 forms adjacencies over IPv6 links and advertises topology and prefixes in separate LSAs.
Layer 3
Intermediate System to Intermediate System (IS-IS)
How Integrated IS-IS builds separate topologies by level and advertises IP reachability in TLVs.
Layer 3
Border Gateway Protocol 4 (BGP-4)
How BGP exchanges reachability, chooses paths using policy and distributes them inside an autonomous system.
Layer 3
Multiprotocol BGP (MP-BGP)
How one BGP session carries several route families identified by AFI and SAFI.
Layer 3
Protocol Independent Multicast (PIM)
How multicast routers build forwarding trees for groups and sources.
Layer 3
Internet Group Management Protocol (IGMP)
How IPv4 receivers report multicast group and source interest on a local network.
Layer 3
Multicast Listener Discovery (MLD)
How IPv6 routers learn which multicast groups and sources have listeners on an attached link.
Layer 3
Multicast Source Discovery Protocol (MSDP)
How IPv4 PIM sparse-mode rendezvous points learn about active sources in other domains.
Layer 3
Hot Standby Router Protocol (HSRP)
How Cisco routers share a default gateway address and transfer its forwarding role after a failure.
Layer 3
Virtual Router Redundancy Protocol (VRRP)
How an IETF standard elects an active IPv4 or IPv6 virtual router on a LAN.
Layer 3
Gateway Load Balancing Protocol (GLBP)
How Cisco GLBP shares traffic across multiple forwarders using one virtual IPv4 gateway.
Layer 3
Multiprotocol Label Switching (MPLS)
How a provider classifies packets, switches labels and applies QoS across an MPLS path.
Layer 3
Label Distribution Protocol (LDP and targeted LDP)
How MPLS routers discover peers and distribute bindings for forwarding equivalence classes.
Layer 3
RSVP Traffic Engineering (RSVP-TE)
How RSVP-TE signals an MPLS label-switched path with explicit routing and optional bandwidth reservation.
Layer 3
BGP labeled unicast (BGP-LU)
How BGP advertises an IP prefix and an MPLS label together using SAFI 4.
Layer 3
BGP/MPLS Layer 3 VPN (L3VPN)
How providers route customer prefixes in separate VRFs and exchange VPN routes across one or more autonomous systems.
Layer 3
Virtual Private Wire Service (VPWS)
How a provider connects two customer Ethernet attachment circuits as one point-to-point service.
Layer 2
Virtual Private LAN Service (VPLS)
How a provider extends one multipoint Ethernet broadcast domain across customer sites.
Layer 2
Ethernet VPN (EVPN)
How EVPN advertises Ethernet reachability and multihoming state in BGP over MPLS or an IP overlay.
Layer 2
Segment Routing with MPLS (SR-MPLS)
How an ingress router steers packets with a stack of MPLS segment identifiers.
Layer 3
Segment Routing over IPv6 (SRv6)
How IPv6 segment endpoints process instructions carried as IPv6 addresses.
Layer 3
MPLS traffic engineering and fast reroute
How RSVP-TE protects an engineered label-switched path with a local detour or bypass.
Layer 3
Topology Independent Loop-Free Alternate (TI-LFA)
How a segment-routing node precomputes a loop-free local repair around a protected resource.
Layer 3