CCNA
Cisco Certified Network Associate certification resources
What Is a Protocol-Dependent Module (PDM) in EIGRP?
EIGRP can route more than one network-layer protocol at once, and the mechanism that makes this possible is the Protocol-Dependent Module (PDM). Each PDM handles everything specific to one particular routed protocol — building its own neighbor table, its own topology table, and interfacing with the shared DUAL algorithm — while DUAL itself remains protocol-agnostic....
Introduction to EIGRP
Enhanced Interior Gateway Routing Protocol (EIGRP) was introduced by Cisco in 1992 as an advanced distance-vector routing protocol, originally running only on Cisco devices. It remained Cisco-proprietary for over two decades before becoming an open, multi-vendor standard — a milestone worth getting the date right on, since it’s commonly misstated. From Proprietary Protocol to Open...
How to Verify Multiarea OSPF
Most multiarea OSPF verification uses the exact same commands already familiar from single-area OSPF — show ip ospf neighbor, show ip ospf, and show ip ospf interface all work identically regardless of how many areas are involved. A handful of additional commands specifically surface multiarea information: which areas a router participates in, how routes crossed...
Interarea Route Summarization: Calculating the Summary Address by Hand
Interarea route summarization must be manually configured on an ABR — OSPF has no auto-summarization feature. Once configured, the ABR condenses multiple specific routes from one area into a single Type 3 Summary LSA before forwarding it into the backbone. This guide focuses specifically on the part most guides skip: how to actually calculate the...
OSPF Route Summarization: Inter-Area and External, with Both Commands
Route summarization converts multiple specific routes into a single, condensed route, shrinking routing tables and reducing the volume of routing updates that need to flood through a network. This guide covers both types of OSPF summarization — inter-area (at an ABR) and external (at an ASBR) — including the specific command for each, since one...
How to Configure Multiarea OSPF (OSPFv2 and OSPFv3)
OSPF can be deployed as a single flat area or split across multiple areas — the multiarea design covered in Types of Routers in OSPF and OSPF LSA Types. This guide walks through actually configuring a four-router, three-area OSPF topology, in both OSPFv2 (IPv4) and OSPFv3 (IPv6), with a fully consistent addressing scheme you can...
OSPF LSA Types: A Complete Guide from Type 1 to Type 11
OSPF is a link-state routing protocol that uses the SPF algorithm to calculate the best path between source and destination. Routers running OSPF exchange Link-State Advertisements (LSAs) to build and maintain their Link-State Databases (LSDBs) — LSAs function as the individual database records that describe specific pieces of network topology. Eleven distinct LSA types are...
Types of Routers in OSPF: Internal, Backbone, ABR, and ASBR
OSPF classifies every router in a multi-area domain into one or more of four functional categories, based on where its interfaces sit within the area hierarchy. These categories aren’t just naming conventions — they determine what kind of Link-State Advertisements (LSAs) a router generates and how many separate link-state databases it has to maintain. This...
How to Configure OSPF MD5 Authentication
MD5 authentication is OSPF’s long-standing cryptographic option for verifying that routing updates come from a trusted neighbor rather than an attacker or a misconfigured device. This guide covers exactly how MD5 authentication works, the two ways to enable it (area-wide and per-interface), a full worked example including the adjacency drop you’ll see mid-migration, and how...
OSPF Authentication: Configuration, Migration, and Best Practices
OSPF packets are not encrypted or authenticated by default. Any device on a shared broadcast segment can listen to, replay, or forge Hello and Link-State Advertisement (LSA) packets unless authentication is explicitly configured. This guide covers how OSPF authentication actually works — MD5, the modern HMAC-SHA replacement, vendor configuration differences, common failure modes, and how...