Example: Configuring a Conditional Default Route Policy

 

This example shows how to configure a conditional default route on one routing device and redistribute the default route into OSPF.

Requirements

No special configuration beyond device initialization is required before configuring this example.

Overview

In this example, OSPF area 0 contains three routing devices. Device R3 has a BGP session with an external peer, for example, an Internet Service Provider (ISP).

To propagate a static route into BGP, this example includes the discard statement when defining the route. The ISP injects a default static route into BGP, which provides the customer network with a default static route to reach external networks. The static route has a discard next hop. This means that if a packet does not match a more specific route, the packet is rejected and a reject route for this destination is installed in the routing table, but Internet Control Message Protocol (ICMP) unreachable messages are not sent. The discard next hop allows you to originate a summary route, which can be advertised through dynamic routing protocols.

Device R3 exports the default route into OSPF. The route policy on Device R3 is conditional such that if the connection to the ISP goes down, the default route is no longer exported into OSPF because it is no longer active in the routing table. This policy prevents packets from being silently dropped without notification (also known as blackholing).

This example shows the configuration for all of the devices and the step-by-step configuration on Device R3.

Figure 1 shows the sample network.

Figure 1: OSPF with a Conditional Default Route to an ISP
OSPF with a Conditional Default
Route to an ISP

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Device R1

Device R2

Device R3

Device ISP

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure Device R3:

  1. Configure the interfaces.
  2. Configure the autonomous system (AS) number.

  3. Configure the BGP session with the ISP device.
  4. Configure OSPF.

  5. Configure the routing policy.

  6. Apply the export policy to OSPF.

  7. If you are done configuring the device, commit the configuration.

Results

Confirm your configuration by issuing the show command. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Verification

Confirm that the configuration is working properly.

Verifying That the Route to the ISP Is Working

Purpose

Make sure connectivity is established between Device R3 and the ISP’s router.

Action

user@R3> ping 10.0.45.1

Meaning

The ping command confirms reachability.

Verifying That the Static Route Is Redistributed

Purpose

Make sure that the BGP policy is redistributing the static route into Device R3’s routing table. Also make sure that the OSPF policy is redistributing the static route into the routing tables of Device R1 and Device R2.

Action

user@R3> show route protocol bgp
user@R1> show route protocol ospf
user@R2> show route protocol ospf

Meaning

The routing tables contain the default 0.0.0.0/0 route. If Device R1 and Device R2 receive packets destined for networks not specified in their routing tables, those packets will be sent to Device R3 for further processing. If Device R3 receives packets destined for networks not specified in its routing table, those packets will be sent to the ISP for further processing.

Testing the Policy Condition

Purpose

Deactivate the interface to make sure that the route is removed from the routing tables if the external network becomes unreachable.

Action

user@R3> deactivate interfaces ge-0/0/2 unit 0 family inet address 10.0.45.2/30


user@R3> commit
user@R1> show route protocol ospf


user@R2> show route protocol ospf

Meaning

The routing tables on Device R1 and Device R2 do not contain the default 0.0.0.0/0 route. This verifies that the default route is no longer present in the OSPF domain. To reactivate the ge-0/0/2.6 interface, issue the activate interfaces ge-0/0/2 unit 0 family inet address 10.0.45.2/30 configuration mode command.