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CCNA lab 18 of 46

Reading the routing table and how a router chooses a route

A hub router learns overlapping prefixes from OSPF, static routes and a default route. Decode every field of show ip route, predict the next hop for four destinations, and prove how longest match, administrative distance and metric decide.

Run this lab

It needs Lite: its 5 nodes are set to 5 vCPU and 5,120 MB in total. That fits the free demo too.

In your workspace: Shared, then CCNA 200-301 guided labs, then 18 - Reading the routing table and how a router chooses a route

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Level
Intermediate
Time
About 50 minutes
Nodes need
5 vCPU, 5,120 MB
Smallest plan
Lite

The scenario

Everything in Tamar Shipping's network meets at the hub router R1, and three overlapping address blocks point in three directions: the regional offices summarise as 10.18.0.0/16 behind R2, the head office servers are 10.18.20.0/24 behind R3, and a new lab subnet 10.18.20.128/25 has just appeared behind the Internet edge router R4. The network team keeps arguing about where a packet will go. You'll settle it from the routing table.

Link / prefixWhereLearned by R1 via
10.18.1.0/24R1 Gi0/0, PC1 .10connected
10.0.12.0/30, 10.0.13.0/30, 10.0.14.0/30R1 to R2, R3, R4 (R1 is .1)connected
10.0.23.0/30R2–R3 linkOSPF
10.18.0.0/16R2 Loopback1OSPF
10.18.20.0/24R3 Loopback1OSPF
10.18.20.128/25R4 Loopback1static to 10.0.14.2
0.0.0.0/0R4 (Internet, test address 198.51.100.1)static to 10.0.14.2

What you’ll practise

  • Decode the codes, prefix, mask, administrative distance, metric, next hop and gateway of last resort
  • Predict a forwarding decision with longest prefix match
  • Use administrative distance to choose between sources and build a floating static route
  • Recognise equal-cost multipath in the routing table
  • Explain why a loopback is advertised as a /32 and how to change it

The tasks

  1. Decode every field
  2. Explain the loopback mask trick
  3. Predict next hops with longest match
  4. Make two sources compete: administrative distance
  5. Compare metrics and spot equal-cost paths

The Lab Tasks pane has the scenario, these tasks, the checks to run, hints and the full solution.

Topology and devices

The network as it opens in EVE-NG, drawn from the lab’s own canvas, and the image each node runs.

Topology of Reading the routing table and how a router chooses a route5 nodes, 5 links. Every link is listed as text below the diagram.R1R2R3R4PC1Gi0/1Gi0/1Gi0/2Gi0/1Gi0/0eth0Gi0/3Gi0/1Gi0/2Gi0/2
5 nodes, with the interface names each link uses. On a narrow screen, scroll the diagram sideways.
Every link, as text
  • R1 Gi0/0 to PC1 eth0
  • R1 Gi0/1 to R2 Gi0/1
  • R1 Gi0/2 to R3 Gi0/1
  • R1 Gi0/3 to R4 Gi0/1
  • R2 Gi0/2 to R3 Gi0/2
Nodes in Reading the routing table and how a router chooses a route, with the image each runs and the vCPU and RAM it’s set to
NodeDeviceImage in Add NodeSet to
R1Cisco IOSvvios-adventerprisek9-m.SPA.159-3.M61 vCPU, 1,024 MB
R2Cisco IOSvvios-adventerprisek9-m.SPA.159-3.M61 vCPU, 1,024 MB
R3Cisco IOSvvios-adventerprisek9-m.SPA.159-3.M61 vCPU, 1,024 MB
R4Cisco IOSvvios-adventerprisek9-m.SPA.159-3.M61 vCPU, 1,024 MB
PC1VPCS hostBuilt in to EVE-NGCounted as 1 vCPU, 1,024 MB

How to open this lab

  1. Sign in to your workspace at lab.cloudlabbox.com.

  2. In File Manager, open Shared, then the folder CCNA 200-301 guided labs.

  3. Open 18 - Reading the routing table and how a router chooses a route and start its nodes, then open Lab Tasks for the scenario and the steps.

The lab is read-only. You can start the nodes and configure the devices, but changes to the topology can’t be saved. To change it, rebuild it in your own folder.

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CloudLabBox is independent of Cisco. The lab follows exam topics and isn’t vendor training material; vendor names identify the exams and the images in the workspace.