Lab 9.4.2 Multihoming to a Single Provider

Objective:

Configure a multihomed connection to a single provider using default only routing, one primary, and one backup link.

Scenario:

You are going to setup a multihomed connection to your ISP (AS 100). One link will be used as the primary link while the other will be used as a backup link. The following policies must be applied:

  • Outbound traffic from AS 200 should always go on the primary link unless that link fails.
  • Inbound traffic must also come in on the primary link unless that link fails.
  • No BGP updates should be permitted into AS 200.

Tasks:

  1. Cable the lab as shown in the diagram above.
  2. Fully configure routers B and C in AS 100 to support the above network. Include all necessary BGP statements, IGP routing, and advertise the network between B and C in BGP. AS 100 is going to be running IBGP as well as EBGP.
  3. Now configure Router A for BGP connectivity. Advertise network 10.0.0.0.
  4. Verify that BGP is running correctly. Do not factor in the 3 requirements for this network. Simply make sure you have connectivity.
  5. Outbound traffic should always go on the primary link. This issue can be addressed by configuring static routes with different metrics for each link. Since the primary link is preferred, we will assign it a lower metric. The commands are as follows:
  • RouterA(config)#ip route 0.0.0.0 0.0.0.0 2.2.2.1 30 (Primary Link)
  • RouterA(config)#ip route 0.0.0.0 0.0.0.0 1.1.1.1 50 (Backup Link)

Use the trace command to get to 192.68.11.1 from Router A. What route was taken? Are you surprised? Why did it choose that route?

  1. The next requirement is to be sure that all inbound traffic comes in on the primary link. This can be accomplished by sending different metrics into AS 100 on both links. This is easily accomplished using a route map. For simplicity, call the route maps PrimaryMetric and BackupMetric. The configuration of the route map is quite simply:
  • RouterA(config)#route-map PrimaryMetric permit 10
  • RouterA(config-Route-map)#set metric 50
  • RouterA(config)#route-map BackupMetric permit 10
  • RouterA(config-route-map)#set metric 100

Why did we set the metric lower on the primary link?

  1. Don't forget to apply each route map to the appropriate neighbor. PrimaryMetric should be applied to 2.2.2.1. BackupMetric should be applied to 1.1.1.1.
  2. The last requirement is to prevent AS 100 from sending updates into AS 200. This can be done with another route map statement. The route map will be as follows:

RouterA(config)#route-map BlockUpdates deny 10

Which neighbor(s) should this route map be applied to?

Should it be applied in or out?

  1. Verify everything is functioning as outlined.

Can you ping the 192.68.11.0 network?

  1. Use the trace command to get to the 192.68.11.0 network.

What the primary link taken?

Does the backup link work?

Unplug the primary link cable.

Can you still access network 192.68.11.0?

  1. Reconnect and save your configuration. Note any issues you have encountered with this lab.