Comprehensive Guide to IP Routing Process, Static vs Dynamic Routing, and Routing Protocols

1. What Is IP Routing?

IP routing is the process of sending data packets from one device to another across different networks. Routers use routing tables to guide packets toward their destination based on network addresses, neighboring routers, possible paths, and route metrics.

2. What Routers Need to Know

• Destination network address
• Neighboring routers for learning remote networks
• Available paths to those networks
• Best route based on efficiency and cost
• How to update and maintain routing information

3. Static vs Dynamic Routing

Routers build routing tables through two methods:

• Static Routing: Manually configured by the network admin
• Dynamic Routing: Routes are learned and updated automatically using routing protocols

4. Routing Table Example

Router_A#show ip route  
Gateway of last resort is not set  
C 10.10.10.0/24 is directly connected, FastEthernet0/0  
C 10.10.20.0/24 is directly connected, FastEthernet0/1  
C 10.10.30.0/24 is directly connected, FastEthernet0/2  
C 10.10.40.0/24 is directly connected, Serial 0/0

5. Step-by-Step IP Routing Process

Example: Host_A (172.16.10.2) pings Host_B (172.16.20.2)

Host_A Creates the Packet

- ICMP Echo Request is formed  
- IP header created with source/destination IP  
- Packet is checked: remote or local network  
- Gateway address is looked up  
- MAC address resolution via ARP  
- ARP command example:
arp -a  
Interface: 172.16.10.2 --- 0x3  
Internet Address  Physical Address      Type  
172.16.10.1        00-15-05-06-31-b0   dynamic

Frame is built with destination MAC and transmitted physically.

Router Receives the Packet

- Collision domain receives frame  
- CRC and MAC are checked  
- IP extracts the packet  
- Routing table is searched:
Lab_A#show ip route  
172.16.0.0/24 is subnetted, 2 subnets  
C 172.16.10.0 is directly connected, Ethernet0  
C 172.16.20.0 is directly connected, Ethernet1

Router Forwards Packet

- Best route via Ethernet1 is selected  
- MAC address of Host_B is resolved:
Lab_A#show ip arp  
Protocol  Address       Age(min)  Hardware Addr   Type  Interface  
Internet  172.16.20.2   3         0030.9492.a5dd  ARPA  Ethernet1

New frame is created and sent out.

Host_B Receives Packet

• Frame CRC and destination MAC are verified
• IP layer recognizes ICMP and responds with Echo Reply

Reply Packet Returns

Packet flows back from Host_B to Host_A, repeating ARP and routing steps in reverse.

6. Static Routing Details

Command Example:  
Router_A(config)#ip route 172.16.30.0 255.255.255.0 172.16.40.2

• Manual configuration
• Precise control over paths
• Ideal for small, fixed networks
• Needs manual updates on changes

7. Dynamic Routing

• Routes are auto-discovered and updated
• More scalable in large environments
• Requires more CPU/RAM on routers

Popular protocols: RIP, OSPF, EIGRP

8. Routing Protocol Types

Dynamic protocols are split into:

Interior Gateway Protocols (IGP)

• Distance-Vector: Based on hop count (e.g., RIP, IGRP)
• Link-State: Builds network map using Dijkstra algorithm (e.g., OSPF, IS-IS)
• Hybrid: Combines features (e.g., EIGRP)

9. IGP Comparison Table

Protocol	Type	Scalability	Complexity	Use Case
RIP	Distance-Vector	Low	Simple	Small networks
OSPF	Link-State	High	Complex	Enterprise networks
EIGRP	Hybrid	High	Moderate	Cisco environments

Exterior Gateway Protocols (EGP)

• BGP (Border Gateway Protocol): The standard protocol used for Internet routing between ISPs and organizations

10. Routing Scenarios

Example 1: Simple Dual-Router Network

Routing Table:  
C 10.10.10.0/24 → FastEthernet0/0  
C 10.10.20.0/24 → FastEthernet0/1  
Destination 10.10.10.14 → sent via FastEthernet0/0

Example 2: Subnetting Scenario

Routing Table:  
C 10.10.10.16/28 → FastEthernet0/0  
C 10.10.10.8/29  → FastEthernet0/1  
C 10.10.10.4/30  → FastEthernet0/2  
Destination 10.10.10.14 → sent via FastEthernet0/1

Example 3: Ping from Host_A to HTTPS Server

• Host_A: 192.168.10.2
• Server: 192.168.20.2
• MAC Destination: Router’s Fa0/0 interface
• IP Destination: Server NIC
• Port: 443 (HTTPS)

Note: Switches operate at Layer 2 and don't affect routing decisions.

11. Exam Tips for Network+

• Packets remain intact across network, frames change at each hop (MAC updates)
• MAC addresses are local; always target next-hop device