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Routing Protocols for CompTIA Network+ N10-009

Routing protocols enable routers to dynamically share network reachability information, automatically adapting to topology changes. CompTIA Network+ N10-009 tests routing protocol classification, metrics, and use cases across the Implementation and Concepts domains. You must identify which protocol to use in a given scenario, understand how routing protocols select best paths, and recognize protocol characteristics without needing to configure them via CLI.

9 min

4 sections · 7 exam key points

2 practice questions

Interior vs Exterior Gateway Protocols

IGPs (Interior Gateway Protocols) route within a single autonomous system (AS) — a network under one administrative control (a company, campus, or ISP's internal network). IGPs include RIP, OSPF, EIGRP, and IS-IS. EGPs (Exterior Gateway Protocols) route between autonomous systems — BGP is the only widely used EGP and is the routing protocol of the internet.

An autonomous system is identified by an AS Number (ASN), a unique number assigned by IANA/RIRs. Private ASNs (64512–65534) are used internally; public ASNs are globally unique. BGP peers (neighbors) exchange routing information between ASes, enabling internet-wide reachability.

Distance-Vector Routing Protocols

Distance-vector protocols share routing tables (lists of networks and distances) with directly connected neighbors. Each router only knows routes from its neighbors' perspective — they do not have a full network map. The 'Bellman-Ford' algorithm calculates paths.

RIP (Routing Information Protocol): version 1 (classful, no VLSM) and version 2 (classless, VLSM, MD5 auth). Metric: hop count, max 15. Update interval: 30 seconds. Slow convergence — can take minutes after a topology change. Maximum 15 hops limits RIP to very small networks. RIPng supports IPv6.

EIGRP (Enhanced IGRP): Cisco's advanced distance-vector protocol (now open standard). Composite metric using bandwidth and delay (plus optionally load and reliability). Uses DUAL (Diffusing Update Algorithm) for loop-free, fast convergence — backup paths are pre-computed (feasible successors). Supports VLSM, classless, unequal-cost load balancing. No hop count limit.

Link-State Routing Protocols

Link-state protocols flood the entire network with topology information (LSAs — Link State Advertisements). Each router builds an identical map of the entire network, then runs Dijkstra's Shortest Path First (SPF) algorithm independently to compute best paths. Fast convergence, supports large networks.

OSPF (Open Shortest Path First): the dominant enterprise IGP. Metric: cost (reference bandwidth / interface bandwidth). Organizes network into areas — Area 0 is the backbone; all other areas must connect to Area 0. DR/BDR election on broadcast networks reduces LSA flooding. OSPFv3 adds IPv6 support. Authentication supported (MD5 or SHA).

IS-IS (Intermediate System to Intermediate System): another link-state IGP, preferred by ISPs for its scalability and stability. Uses levels instead of areas. Less common in enterprise environments but important to recognize.

BGP: The Internet Routing Protocol

BGP (Border Gateway Protocol) is a path-vector EGP that exchanges routing information between autonomous systems. Uses TCP port 179. BGP neighbors are manually configured (not auto-discovered). Path selection uses multiple attributes: AS path length (shortest preferred), local preference, MED, origin, and more.

eBGP (external BGP): between different ASes (across the internet). iBGP (internal BGP): within the same AS for distributing external routes. BGP carries the full internet routing table (900,000+ routes) and is the reason the internet works as a globally connected network.

Routing Protocol Quick Reference

Protocol	Type	Algorithm	Metric	Best For
RIP v2	Distance-vector	Bellman-Ford	Hop count (max 15)	Legacy, tiny networks
EIGRP	Advanced distance-vector	DUAL	Bandwidth + delay	Cisco enterprise
OSPF	Link-state	Dijkstra SPF	Cost (bandwidth)	Enterprise, multi-vendor
IS-IS	Link-state	Dijkstra SPF	Cost	ISP networks
BGP	Path-vector (EGP)	Best path attrs	AS path + attributes	Internet, multi-homing

Key exam facts — Network+

IGP = within an AS; EGP = between ASes (BGP is the only EGP used today)
RIP = hop count metric, max 15 hops, 30-second updates, slow convergence
OSPF = cost metric (bandwidth-based), link-state, fast convergence, areas
EIGRP = Cisco advanced distance-vector, DUAL algorithm, composite metric
BGP = internet routing protocol, uses TCP 179, path-vector, AS path attribute
OSPF Area 0 = backbone; all areas must connect to Area 0
Administrative distance: EIGRP=90, OSPF=110, RIP=120

Common exam traps

OSPF metric is hop count

OSPF uses cost based on interface bandwidth — higher bandwidth = lower cost = preferred path. Hop count is RIP's metric

BGP is used inside companies for routing

BGP is used between autonomous systems (ISPs and large organizations with multiple ISP connections). Internal enterprise routing uses OSPF or EIGRP

EIGRP is Cisco-proprietary and cannot be used with other vendors

Cisco published EIGRP as an open standard (RFC 7868) in 2016, though most implementations still run on Cisco devices

Practice questions — Routing Protocols

These questions are representative of what you will see on Network+ exams. The correct answer and explanation are shown immediately below each question.

Q1.A network engineer needs to select a routing protocol for a large multi-vendor enterprise network with redundant paths. The protocol must support fast convergence and scale to hundreds of routers. Which protocol is most appropriate?

A.RIP v2

B.EIGRP

C.OSPF

D.BGP

Explanation: OSPF is the standard choice for large, multi-vendor enterprise networks. It supports fast convergence via SPF algorithm, scales to large topologies through area design, and runs on all major router vendors. RIPv2 is limited to 15 hops and converges slowly. EIGRP is Cisco-focused. BGP is for inter-AS internet routing, not enterprise LAN/WAN.

Q2.Which routing protocol uses TCP port 179 to establish neighbor relationships between autonomous systems?

A.OSPF

B.EIGRP

C.RIP

D.BGP

Explanation: BGP (Border Gateway Protocol) uses TCP port 179 to establish and maintain neighbor (peer) sessions between routers in different autonomous systems. BGP is the only widely used EGP and is the routing protocol of the internet. OSPF, EIGRP, and RIP are IGPs that do not use TCP for neighbor communication.

Frequently asked questions — Routing Protocols

Does Network+ require memorizing BGP attributes?

No. Network+ tests BGP at a conceptual level — know that it is the internet routing protocol, routes between ASes, uses TCP 179, and selects paths based on attributes like AS path length. Deep BGP attribute memorization (local preference, MED, weight) is CCNP-level content.

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