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CCNP ENARSI lab 18 of 35

BGP policy: prefix lists, AS-path filters and communities

Protect an edge router from bad Internet routes with prefix lists and AS-path filters, act on a partner's communities, and use the no-export community so that more-specific routes help your provider without leaking into the global table.

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 ENARSI 300-410 guided labs, then 18 - BGP policy - prefix lists AS-path filters and communities

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

The scenario

Merlin Bank (AS 64500, R1) has one Internet provider (R2, AS 64501) and a payments partner (R3, AS 64520) connected directly. Today R1 accepts everything and advertises everything, and an audit found three problems: R1 accepts routes that should never be trusted, the partner's internal network is visible on the Internet through Merlin, and Merlin's two /25 traffic-engineering routes are visible far beyond the provider.

The new BGP policy:

  1. From the ISP: accept the default route and prefixes from /8 to /24 only, and nothing whose AS path contains AS 64666 (a sanctioned network).
  2. From the partner: accept only the partner's block 198.18.0.0/15, at /24 or shorter.
  3. To the ISP: advertise Merlin's 203.0.113.0/24; advertise the two /25s only to the ISP itself (tag them no-export); pass on partner routes only if the partner tagged them 64520:100 (production). Nothing else.
DeviceInterfaceAddressPeer
R1Gi0/0172.16.12.1/30ISP R2 172.16.12.2 (AS 64501)
R1Gi0/1172.16.13.1/30partner R3 172.16.13.2 (AS 64520)
R1Lo1 / Lo2203.0.113.1/25 / 203.0.113.129/25public block halves
R3Lo1 / Lo2198.18.1.1/24 / 198.18.9.1/24partner production / internal
R4 (AS 64510)Lo1 / Lo2 / Lo3192.0.2.1/24 / 198.51.100.1/24 / 198.19.10.1/28Internet routes

R4 sends 198.51.100.0/24 with AS 64666 in its path, and a /28. R3 tags its routes with communities 64520:100 (production) and 64520:999 (internal, do not pass on).

What you’ll practise

  • Filter inbound BGP routes by prefix and length with prefix lists
  • Write AS-path access lists with regular expressions and apply them as filter lists
  • Match standard communities with community lists and set well-known communities
  • Build an outbound policy that advertises only what the business allows
  • Explain the order in which IOS applies prefix lists, filter lists and route maps

The tasks

  1. See what crosses the edge today
  2. Filter the ISP's routes by prefix
  3. Filter by AS path
  4. Accept only the partner's block
  5. Build the outbound policy to the ISP with communities
  6. Test

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 policy - prefix lists AS-path filters and communities4 nodes, 3 links. Every link is listed as text below the diagram.R1R2R3R4Gi0/1Gi0/0Gi0/1Gi0/0Gi0/0Gi0/0
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 R2 Gi0/0
  • R1 Gi0/1 to R3 Gi0/0
  • R2 Gi0/1 to R4 Gi0/0
Nodes in BGP policy: prefix lists, AS-path filters and communities, 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
R2Cisco IOSvvios-adventerprisek9-m.spa.159-3.m91 vCPU, 512 MB
R3Cisco IOSvvios-adventerprisek9-m.spa.159-3.m91 vCPU, 512 MB
R4Cisco 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 ENARSI 300-410 guided labs.

  3. Open 18 - BGP policy - prefix lists AS-path filters and communities 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.