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CCNP ENCOR lab 14 of 36

BGP best-path selection and dual-homed path control

Fernhill adds a second provider. Work through the BGP best-path algorithm on real tables, stop the site becoming a transit AS, and steer outbound traffic with weight and local preference and inbound traffic with AS-path prepending.

Run this lab

It needs Lite: its 4 nodes are set to 4 vCPU and 2,048 MB in total. That fits the free demo too.

In your workspace: Shared, then CCNP ENCOR 350-401 guided labs, then 14 - BGP best-path selection and dual-homed path control

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Level
Advanced
Time
About 75 minutes
Nodes need
4 vCPU, 2,048 MB
Smallest plan
Lite

The scenario

Fernhill Freight now has a second provider, ISP2 (AS 64501), on a cheaper 500 Mbit/s circuit. Both eBGP sessions are up and R1 advertises 192.0.2.0/24 to both providers, but nobody has set any policy. The network manager's requirements:

  1. Fernhill must never carry traffic between the two providers.
  2. Outbound: use ISP1 for everything, except the video platform 198.51.100.128/25, which performs better through ISP2.
  3. Inbound: the internet should reach 192.0.2.0/24 through ISP1, keeping ISP2 as a backup.
DeviceInterfaceAddressConnects to
R1 (AS 65010)Gi0/0203.0.113.2/30ISP1 Gi0/0
R1Gi0/1203.0.113.6/30ISP2 Gi0/0
R1Lo100192.0.2.1/32 (public service address)–
ISP1 (AS 64500, RID 11.11.11.11)Gi0/0 / Gi0/1203.0.113.1/30 / 203.0.113.9/30R1 / INET
ISP2 (AS 64501, RID 12.12.12.12)Gi0/0 / Gi0/1203.0.113.5/30 / 203.0.113.13/30R1 / INET
INET (AS 64510)Gi0/0 / Gi0/1203.0.113.10/30 / 203.0.113.14/30ISP1 / ISP2
INETLo1 / Lo2198.51.100.1/25 / 198.51.100.129/25–

You manage R1 only. ISP1, ISP2 and INET are there for you to observe.

Topology: R1 is dual-homed to ISP1 and ISP2; both providers peer with INET.

What you’ll practise

  • Walk the BGP best-path algorithm and explain why IOS picked each best path
  • Stop a dual-homed site from becoming a transit AS with outbound route maps
  • Compare weight (local to one router) with local preference (shared with iBGP peers)
  • Prefer different providers per prefix with route maps, prefix lists and local preference
  • Influence how the internet reaches you with AS-path prepending, and say why MED wouldn't help here

The tasks

  1. Explain today's best paths
  2. Stop being a transit AS
  3. Experiment with weight
  4. Set the outbound policy with local preference
  5. Steer inbound traffic with AS-path prepending
  6. Prove the traffic flows

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 BGP best-path selection and dual-homed path control4 nodes, 4 links. Every link is listed as text below the diagram.R1ISP1ISP2INETGi0/0Gi0/0Gi0/1Gi0/0Gi0/1Gi0/0Gi0/1Gi0/1
4 nodes, with the interface names each link uses. On a narrow screen, scroll the diagram sideways.
Every link, as text
  • R1 Gi0/0 to ISP1 Gi0/0
  • R1 Gi0/1 to ISP2 Gi0/0
  • ISP1 Gi0/1 to INET Gi0/0
  • ISP2 Gi0/1 to INET Gi0/1
Nodes in BGP best-path selection and dual-homed path control, 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.m91 vCPU, 512 MB
ISP1Cisco IOSvvios-adventerprisek9-m.spa.159-3.m91 vCPU, 512 MB
ISP2Cisco IOSvvios-adventerprisek9-m.spa.159-3.m91 vCPU, 512 MB
INETCisco IOSvvios-adventerprisek9-m.spa.159-3.m91 vCPU, 512 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 CCNP ENCOR 350-401 guided labs.

  3. Open 14 - BGP best-path selection and dual-homed path control 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.