CompTIA Network+ Preparation · lesson 13 of 30 · 25 min
Routing Fundamentals
A router forwards packets between networks based on its routing table. The routing table contains network prefixes and the next-hop or exit interface to reach them.
Route sources: Directly connected (C), Static routes (S), Dynamic routing protocols (R=RIP, O=OSPF, D=EIGRP, B=BGP).
Administrative Distance (AD) — trustworthiness of a route source. Lower AD = more trusted. When multiple sources advertise the same network, the route with the lowest AD wins.
Metric — cost of a route within a routing protocol. Lower metric = preferred path. Each protocol uses a different metric (hop count, bandwidth, composite).
Dynamic Routing Protocols
RIP (Routing Information Protocol) — distance-vector protocol. Metric: hop count (max 15 — 16 = unreachable). Sends full routing table every 30 seconds. RIPv2 supports CIDR and authentication. Slow convergence. Used only in small/legacy networks.
OSPF (Open Shortest Path First) — link-state protocol. Metric: cost (based on bandwidth — higher bandwidth = lower cost). Builds a complete topology map (LSDB). Uses Dijkstra’s SPF algorithm. Fast convergence. Scales well. Uses areas (Area 0 = backbone). Open standard — supported by all vendors.
EIGRP (Enhanced Interior Gateway Routing Protocol) — Cisco proprietary (now open). Advanced distance-vector (hybrid). Metric: composite (bandwidth + delay by default). Very fast convergence using DUAL algorithm. Supports unequal-cost load balancing.
BGP (Border Gateway Protocol) — the routing protocol of the internet. Path-vector protocol. Used between autonomous systems (ISPs, large enterprises). Metric: path attributes (AS path, local preference, MED). Extremely scalable but slow convergence. eBGP (between ASes) vs iBGP (within an AS).
Exam Tip
OSPF AD=110, EIGRP AD=90, RIP AD=120. Lower AD wins. OSPF uses cost (bandwidth-based). RIP uses hop count with max 15. BGP is the internet routing protocol. OSPF requires Area 0 (backbone area).
Key Terms
Key Terms
Knowledge Check — 10 Questions
Network+ — Routing Protocols
Which routing protocol has the lowest Administrative Distance (most trusted)?
- OSPF (110)
- RIP (120)
- EIGRP (90)
- Static route (1)
Answer: Static route (1)
Static routes have AD=1 (only directly connected routes at AD=0 are more trusted). EIGRP=90, OSPF=110, RIP=120.
RIP uses which metric?
- Cost (bandwidth)
- Hop count (max 15)
- Delay
- Path attributes
Answer: Hop count (max 15)
RIP metric = hop count. Maximum is 15 — a hop count of 16 means unreachable. This limits RIP to small networks.
OSPF calculates the best path using:
- Bellman-Ford algorithm
- Dijkstra’s SPF algorithm
- DUAL algorithm
- Distance-vector
- Answer: Dijkstra’s SPF algorithm
OSPF uses Dijkstra’s Shortest Path First (SPF) algorithm on its Link-State Database to calculate the best routes. BGP is used for:
- Routing within a single company network
- Routing between autonomous systems on the internet
- Small office networks
- Wireless routing
- Answer: Routing between autonomous systems on the internet
BGP is the internet’s routing protocol — it routes between autonomous systems (ISPs, large enterprises). It carries the global routing table.
OSPF requires which area to exist?
- Area 1
- Area 0 (backbone)
- Area 255
- No areas required
Answer: Area 0 (backbone)
OSPF Area 0 is the backbone area — all other areas must connect to Area 0 directly or via a virtual link.
EIGRP is primarily associated with which vendor?
- Juniper
- Cisco
- HP
- Aruba
Answer: Cisco
EIGRP was originally Cisco proprietary (now open standard). It is most commonly deployed in Cisco environments.
Which protocol has the fastest convergence?
- RIP
- BGP
- EIGRP
- They are all equal
Answer: EIGRP
EIGRP uses the DUAL algorithm with pre-computed backup routes — convergence is nearly instant. OSPF is also fast. RIP is slow (30-second updates).
OSPF metric (cost) is based on:
- Hop count
- Bandwidth — higher bandwidth = lower cost
- Delay
- Reliability
- Answer: Bandwidth — higher bandwidth = lower cost
OSPF cost = reference bandwidth / interface bandwidth. Higher bandwidth interfaces have lower cost and are preferred.
If OSPF and RIP both advertise the same network, which route is installed?
- RIP (120)
- OSPF (110) — lower AD wins
- Both are installed
- Neither — conflict
Answer: OSPF (110) — lower AD wins
Administrative Distance determines which route wins when multiple protocols advertise the same network. OSPF AD=110 beats RIP AD=120.
BGP routes between _____ systems (ASes).
Answer: autonomous
BGP = Border Gateway Protocol — routes between Autonomous Systems. Each AS has a unique AS number (ASN).
Previous lesson: Network devices: switches, routers, firewalls, IDS/IPS, load balancers, VPN concentrators, NAS/SAN
Back to CompTIA Network+ Preparation
Next lesson: Spanning Tree Protocol: STP, RSTP, port roles, port states, BPDU