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Example: Configuring the MED Using Route Filters

 

This example shows how to configure a policy that uses route filters to modify the multiple exit discriminator (MED) metric to advertise in BGP update messages.

Requirements

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

Overview

To configure a route-filter policy that modifies the advertised MED metric in BGP update messages, include the metric statement in the policy action.

Figure 1 shows a typical network with internal peer sessions and multiple exit points to a neighboring autonomous system (AS).

Figure 1: Typical Network with IBGP Sessions and Multiple Exit Points
Typical Network with IBGP
Sessions and Multiple Exit Points

Device R4 has multiple loopback interfaces configured to simulate advertised prefixes. The extra loopback interface addresses are 172.16.44.0/32 and 172.16.144.0/32. This example shows how to configure Device R4 to advertise a MED value of 30 to Device R3 for all routes except 172.16.144.0. For 172.16.144.0, a MED value of 10 is advertised to Device 3. A MED value of 20 is advertised to Device R2, regardless of the route prefix.

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 R4

Configuring Device R1

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 R1:

  1. Configure the device interfaces.
  2. Configure BGP.
  3. Configure OSPF.
  4. Configure a policy that accepts direct routes.

    Other useful options for this scenario might be to accept routes learned through OSPF or local routes.

  5. Configure the router ID and autonomous system (AS) number.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols, show policy-options, and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

If you are done configuring the device, enter commit from configuration mode.

Configuring Device R2

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 R2:

  1. Configure the device interfaces.
  2. Configure BGP.
  3. Configure OSPF.
  4. Configure a policy that accepts direct routes.

    Other useful options for this scenario might be to accept routes learned through OSPF or local routes.

  5. Configure the router ID and autonomous system (AS) number.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols, show policy-options, and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

If you are done configuring the device, enter commit from configuration mode.

Configuring Device R3

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 device interfaces.
  2. Configure BGP.
  3. Configure OSPF.
  4. Configure a policy that accepts direct routes.

    Other useful options for this scenario might be to accept routes learned through OSPF or local routes.

  5. Configure the router ID and autonomous system (AS) number.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols, show policy-options, and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

If you are done configuring the device, enter commit from configuration mode.

Configuring Device R4

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 R4:

  1. Configure the device interfaces.

    Device R4 has multiple loopback interface addresses to simulate advertised prefixes.

  2. Configure a policy that accepts direct routes.

    Other useful options for this scenario might be to accept routes learned through OSPF or local routes.

  3. Configure BGP.
  4. Configure the two MED policies.
  5. Configure the two EBGP neighbors, applying the two MED policies to Device R3, and a MED value of 20 to Device R2.
  6. Configure the router ID and autonomous system (AS) number.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols, show policy-options, and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

If you are done configuring the device, enter commit from configuration mode.

Verification

Confirm that the configuration is working properly.

Checking the Active Path from Device R1 to Device R4

Purpose

Verify that the active path goes through Device R2.

Action

From operational mode, enter the show route protocol bgp command.

user@R1> show route protocol bgp

Meaning

The output shows that the preferred path to the routes advertised by Device R4 is through Device R2 for all routes except 172.16.144.0/32. For 172.16.144.0/32, the preferred path is through Device R3.

Verifying That Device R4 Is Sending Its Routes Correctly

Purpose

Make sure that Device R4 is sending update messages with a value of 20 to Device R2 and a value of 30 to Device R3.

Action

From operational mode, enter the show route advertising-protocol bgp command.

user@R4> show route advertising-protocol bgp 172.16.24.2
user@R4> show route advertising-protocol bgp 172.16.34.3

Meaning

The MED column shows that Device R4 is sending the correct MED values to its two EBGP neighbors.