Dynamic Routing

📘 Dynamic Routing Protocols & Route Selection

A Complete Technical Guide for Learners, Interviewees, and Network Engineers

🔹 Part 1: Dynamic Routing Basics

What is Dynamic Routing?

Dynamic routing allows routers to automatically exchange routing information using protocols, keeping their routing tables updated without manual configuration.

🔄 Dynamic Routing vs. Static Routing

Feature	Static Routing	Dynamic Routing
Configured by	Manually by admin	Automatically by protocol
Reaction to network changes	No automatic response	Adapts dynamically
Scalability	Limited and error-prone	Scales to large networks
Use case	Small or stable networks	Large, complex, or changing topologies

✅ A router can use both static and dynamic routing simultaneously.

🔹 Part 2: Types of Dynamic Routing Protocols

1️⃣ Interior Gateway Protocols (IGPs)

Used within a single Autonomous System (AS).

▶ By Algorithm:

Algorithm	Description	Protocols
Distance-Vector	Routers exchange known routes and hop counts without a complete network map.	RIP, EIGRP
Link-State	Routers share link info, build a full network map (LSDB), and compute best paths.	OSPF, IS-IS

2️⃣ Exterior Gateway Protocols (EGPs)

Used between Autonomous Systems (e.g., enterprise to ISP).

Algorithm	Description	Protocol
Path-Vector	Selects routes based on AS-paths (AS-to-AS logic).	BGP

🔹 Part 3: Understanding Route Selection

2 Core Meanings of "Route Selection":

1. Routing Table Population: Choosing which route(s) to insert into the routing table (only the best per destination is kept).

2. Packet Forwarding Decision: Using the routing table to decide how to forward individual packets.

🔹 Part 4: Routing Table Population Logic

When multiple routes to the same destination exist, routers use:

1️⃣ Administrative Distance (AD)

Determines the trustworthiness of a source protocol.

Route Source	AD (Lower = Better)
Directly Connected	0
Static Route	1
External BGP	20
EIGRP	90
OSPF	110
IS-IS	115
RIP	120
Internal BGP	200
Unusable	255

✅ AD is only compared when multiple protocols provide routes to the same destination.

2️⃣ Metric

Used when multiple routes from the same protocol exist.

Protocol	Metric Type	Description
RIP	Hop Count	Fewer hops = better
OSPF	Cost	Lower bandwidth = higher cost
EIGRP	Composite	Uses bandwidth, delay (and others if configured)

👉 Lower metric is always preferred.

3️⃣ Equal-Cost Multi-Path (ECMP)

If multiple routes to the same destination from the same protocol have equal metrics, they are all installed in the routing table for load balancing.

4️⃣ Floating Static Routes

A static route with higher AD than a dynamic route (e.g., AD 111 vs OSPF AD 110) is used as a backup, inserted only if the preferred route fails.

🔹 Part 5: Packet Forwarding Logic

After routing table population:

• 🧭 Forwarding Rule: Choose the most specific route (longest prefix match) for the destination IP.
• ❌ AD and Metric are NOT used during packet forwarding.
• ✅ Example:
  • Destination: 10.0.1.5
  • Match:
    • 10.0.0.0/8 (Generic)
    • 10.0.1.0/24 ✅ (More specific → Used)

🔹 Part 6: Activating Routing Protocols

📥 Enabling Routing on Interfaces (OSPF Example)

router ospf 1
 network 192.168.1.0 0.0.0.255 area 0

🔍 Understanding Wildcard Masks

Wildcard mask ≠ Subnet mask Wildcard mask inverts the subnet mask.

Subnet Mask	Wildcard Mask
255.255.255.0 (/24)	0.0.0.255
255.255.255.252 (/30)	0.0.0.3

Binary Logic:

• 0 = bit must match
• 1 = bit can vary

🎯 OSPF network Command Tasks:

1. Interface Matching: Finds interfaces with IPs matching the network + wildcard.
2. OSPF Activation: Enables OSPF on those interfaces.
3. Prefix Advertisement: Router advertises the actual interface subnet, not the network command range.

🧠 Pro Tip:

To control precisely which interface runs OSPF:

network 192.168.1.1 0.0.0.0 area 0

This ensures only that interface is affected.

📝 Final Notes for Interview & Exam Readiness

✅ Know All AD Values ✅ Compare Metric Logic of RIP, OSPF, and EIGRP ✅ Understand ECMP & Floating Static Routes ✅ Be clear on the difference between Routing Table Population and Packet Forwarding ✅ Master OSPF network command & wildcard mask behavior

• In enterprise networks, OSPF and EIGRP dominate for internal routing.
• BGP is essential for Internet routing and multi-ISP connections.
• Floating static routes are key for backup WAN paths.
• Always test routing configurations using tools like:
  • show ip route
  • show ip protocols
  • show ip protocols

✅ Summary Checklist

Concept	Must Know?
Difference: Static vs Dynamic Routing	✅
Types of Routing Protocols (IGP, EGP)	✅
AD & Metric Logic	✅
ECMP	✅
Floating Static Routes	✅
Longest Prefix Match (Forwarding)	✅
OSPF Network Command & Wildcard	✅

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