Key Routing Protocol Characteristics – CCNA Lab 33

Lab Objective

• Understand how routing protocols exchange routing tables dynamically
• Learn Metric and Administrative Distance concepts
• Compare Classful and Classless routing protocols
• Read and clear the routing table

What Routing Protocols Do

Routing protocols like RIP, IGRP, EIGRP, OSPF, BGP, and IS-IS let routers and Layer 3 switches exchange their routing tables dynamically and keep them updated automatically.

To use dynamic routing, you configure these protocols on the router. Routers then exchange routing tables with their neighbors and learn the paths in each other’s tables — updating their own tables automatically.

Metric: Choosing the Best Path

In an internetwork, more than one path may reach the same destination. Routers use a unit called Metric to identify the best path.

Each routing protocol calculates its metric differently:

Administrative Distance: Ranking Routing Sources

Cisco routers use another unit to select the best path when multiple routing protocols offer routes to the same destination: Administrative Distance (AD).

When a router’s routing table has multiple routes to the same destination — learned from different routing protocols — the router selects the one with the lowest AD.

AD is a Cisco-proprietary method for ranking routing protocols and sources. It’s a number between 0 and 255:

Classful vs. Classless Routing Protocols

All routing protocols fall into one of two categories:

• Classful Routing Protocol
• Classless Routing Protocol

RIP v1 and IGRP are Classful routing protocols.

RIP v2, OSPF, EIGRP, and BGP are Classless protocols.

The Routing Table: show ip route

Each router’s routing table contains all paths the router can use to forward packets. As a Cisco engineer, you need a solid understanding of the information in the routing table.

Use the following command to view the routing table on routers and Layer 3 switches:

Router#show ip route

Gateway of last resort is not set
C    192.168.0.0/24 is directly connected, FastEthernet0/0
C    192.168.1.0/24 is directly connected, FastEthernet1/0
R    192.168.3.0/24 [120/1] via 192.168.5.2, 00:00:25, Serial2/0
R    192.168.4.0/24 [120/1] via 192.168.5.2, 00:00:25, Serial2/0
C    192.168.5.0/24 is directly connected, Serial2/0

As you can see in the show ip route output, the list of networks and routes in the router’s routing table is displayed. This command shows the routing table of a router or Layer 3 switch.

When a packet arrives at a router, the router first looks at the destination. It then checks its routing table to find that destination. If the router finds the destination network, it forwards the packet out the correct interface. If no route exists in the routing table for that destination, the router drops the packet.

Routing Table Codes

The show ip route output displays codes that identify the routing protocol used:

Routing Table Breakdown

Clearing the Routing Table

Sometimes you need to remove all entries from the router’s routing table. Use this command:

Router#clear ip route *

This command clears all entries in the routing table. The routing protocols then repopulate the table automatically.

To remove a specific route, use:

Router#clear ip route [network] [subnet-mask]

For example:

Router#clear ip route 192.168.3.0 255.255.255.0

In this example, only the route for 192.168.3.0 with mask 255.255.255.0 is removed from the router’s routing table.

Routing Protocol Comparison

Routing protocols are compared based on:

• Routing algorithm
• Summarization support
• Convergence speed
• Update timing
• Supported network size

Classful vs. Classless Summary