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Example: Configuring BGP Local Preference

Understanding the BGP Local Preference

Internal BGP (IBGP) sessions use a metric called the local preference, which is carried in IBGP update packets in the path attribute LOCAL_PREF. When an autonomous system (AS) has multiple routes to another AS, the local preference indicates the degree of preference for one route over the other routes. The route with the highest local preference value is preferred.

The LOCAL_PREF path attribute is always advertised to IBGP peers and to neighboring confederations. It is never advertised to external BGP (EBGP) peers. The default behavior is to not modify the LOCAL_PREF path attribute if it is present.

The LOCAL_PREF path attribute applies at export time only, when the routes are exported from the routing table into BGP.

If a BGP route is received without a LOCAL_PREF attribute, the route is stored in the routing table and advertised by BGP as if it were received with a LOCAL_PREF value of 100. A non-BGP route that is advertised by BGP is advertised with a LOCAL_PREF value of 100 by default.

Example: Configuring the Local Preference Value for BGP Routes

This example shows how to configure local preference in internal BGP (IBGP) peer sessions.

• Requirements
• Overview
• Configuration
• Verification

Requirements

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

Overview

To change the local preference metric advertised in the path attribute, you must include the local-preference statement, specifying a value from 0 through 4,294,967,295 (2³² – 1).

There are several reasons you might want to prefer one path over another. For example, compared to other paths, one path might be less expensive to use, might have higher bandwidth, or might be more stable.

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

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

To reach Device R4, Device R1 can take a path through either Device R2 or Device R3. By default, the local preference is 100 for either route. When the local preferences are equal, Junos OS has rules for breaking the tie and choosing a path. (See Understanding BGP Path Selection.) In this example, the active route is through Device R2 because the router ID of Device R2 is lower than the router ID of Device R3. The following example shows how to override the default behavior with an explicit setting for the local preference. The example configures a local preference of 300 on Device R3, thereby making Device R3 the preferred path to reach Device R4.

Configuration

• Configuring Device R1
• Configuring Device R2
• Configuring Device R3
• Configuring Device R4

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.

Configuring Device R1

Step-by-Step Procedure

1. Configure the interfaces.
   [edit interfaces fe-1/2/0 unit 1]user@R1# set family inet address 12.12.12.1/24
   [edit interfaces fe-1/2/1 unit 2]user@R1# set family inet address 13.13.13.1/24
   [edit interfaces lo0 unit 1]user@R1# set family inet address 192.168.1.1/32
2. Configure BGP.
   [edit protocols bgp group internal]user@R1# set type internaluser@R1# set local-address 192.168.1.1user@R1# set export send-directuser@R1# set neighbor 192.168.2.1user@R1# set neighbor 192.168.3.1
3. Configure OSPF.
   [edit protocols ospf area 0.0.0.0]user@R1# set interface lo0.1 passiveuser@R1# set interface fe-1/2/0.1user@R1# set interface fe-1/2/1.2
4. Configure a policy that accepts direct routes.

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

   [edit policy-options policy-statement send-direct term 1]user@R1# set from protocol directuser@R1# set then accept
5. Configure the router ID and autonomous system (AS) number.
   [edit routing-options]user@R1# set autonomous-system 123user@R1# set router-id 192.168.1.1

Results

From configuration mode, confirm your configuration by entering the show interfaces, show policy-options, show protocols, 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

1. Configure the interfaces.
   [edit interfaces fe-1/2/0 unit 3]user@R2# set family inet address 12.12.12.21/24
   [edit interfaces fe-1/2/1 unit 4]user@R2# set family inet address 24.24.24.2/24
   [edit interfaces lo0 unit 2]user@R2# set family inet address 192.168.2.1/32
2. Configure BGP.
   [edit protocols bgp group internal]user@R2# set type internaluser@R2# set local-address 192.168.2.1user@R2# set export send-directuser@R2# set neighbor 192.168.1.1user@R2# set neighbor 192.168.3.1
   [edit protocols bgp group external]user@R2# set type externaluser@R2# set export send-directuser@R2# set peer-as 4user@R2# set neighbor 24.24.24.4
3. Configure OSPF.
   [edit protocols ospf area 0.0.0.0]user@R2# set interface lo0.2 passiveuser@R2# set interface fe-1/2/0.3user@R2# set interface fe-1/2/1.4
4. Configure a policy that accepts direct routes.

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

   [edit policy-options policy-statement send-direct term 1]user@R2# set from protocol directuser@R2# set then accept
5. Configure the router ID and autonomous system (AS) number.
   [edit routing-options]user@R2# set autonomous-system 123user@R2# set router-id 192.168.2.1

Results

From configuration mode, confirm your configuration by entering the show interfaces, show policy-options, show protocols, 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

1. Configure the interfaces.
   [edit interfaces fe-1/2/0 unit 5]user@R3# set family inet address 13.13.13.3/24
   [edit interfaces fe-1/2/1 unit 6]user@R3# set family inet address 34.34.34.3/24
   [edit interfaces lo0 unit 3]user@R3# set family inet address 192.168.3.1/32
2. Configure BGP.
   [edit protocols bgp group internal]user@R3# set type internaluser@R3# set local-address 192.168.3.1user@R3# set export send-directuser@R3# set neighbor 192.168.1.1user@R3# set neighbor 192.168.2.1
   [edit protocols bgp group external]user@R3# set type externaluser@R3# set export send-directuser@R3# set peer-as 4user@R3# set neighbor 34.34.34.4
3. Configure OSPF.
   [edit protocols ospf area 0.0.0.0]user@R3# set interface lo0.3 passiveuser@R3# set interface fe-1/2/0.5user@R3# set interface fe-1/2/1.6
4. Configure a policy that accepts direct routes.

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

   [edit policy-options policy-statement send-direct term 1]user@R3# set from protocol directuser@R3# set then accept
5. Configure the router ID and autonomous system (AS) number.
   [edit routing-options]user@R3# set autonomous-system 123user@R3# set router-id 192.168.3.1

Results

From configuration mode, confirm your configuration by entering the show interfaces, show policy-options, show protocols, 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

1. Configure the interfaces.
   [edit interfaces fe-1/2/0 unit 7]user@R4# set family inet address 24.24.24.4/24
   [edit interfaces fe-1/2/1 unit 8]user@R4# set family inet address 34.34.34.4/24
   [edit interfaces lo0 unit 4]user@R4# set family inet address 192.168.4.1/32
2. Configure BGP.
   [edit protocols bgp group external]user@R4# set type externaluser@R4# set export send-directuser@R4# set peer-as 123user@R4# set neighbor 34.34.34.3user@R4# set neighbor 24.24.24.2
3. Configure a policy that accepts direct routes.

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

   [edit policy-options policy-statement send-direct term 1]user@R4# set from protocol directuser@R4# set then accept
4. Configure the router ID and autonomous system (AS) number.
   [edit routing-options]user@R4# set autonomous-system 4user@R4# set router-id 192.168.4.1

Results

From configuration mode, confirm your configuration by entering the show interfaces, show policy-options, show protocols, 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
• Altering the Local Preference to Change the Path Selection
• Rechecking the Active Path From Device R1 to Device R4

Checking the Active Path From Device R1 to Device R4

Purpose

Verify that the active path from Device R1 to Device R4 goes through Device R2.

Action

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

inet.0: 11 destinations, 18 routes (11 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

12.12.12.0/24       [BGP/170] 00:11:48, localpref 100, from 192.168.2.1
                      AS path: I
                    > to 12.12.12.2 via fe-1/2/0.1
13.13.13.0/24       [BGP/170] 00:11:48, localpref 100, from 192.168.3.1
                      AS path: I
                    > to 13.13.13.3 via fe-1/2/1.2
24.24.24.0/24       [BGP/170] 00:11:48, localpref 100, from 192.168.2.1
                      AS path: I
                    > to 12.12.12.2 via fe-1/2/0.1
34.34.34.0/24       [BGP/170] 00:11:48, localpref 100, from 192.168.3.1
                      AS path: I
                    > to 13.13.13.3 via fe-1/2/1.2
192.168.2.1/32      [BGP/170] 00:11:48, localpref 100, from 192.168.2.1
                      AS path: I
                    > to 12.12.12.2 via fe-1/2/0.1
192.168.3.1/32      [BGP/170] 00:11:48, localpref 100, from 192.168.3.1
                      AS path: I
                    > to 13.13.13.3 via fe-1/2/1.2
192.168.4.1/32     *[BGP/170] 00:05:14, localpref 100, from 192.168.2.1
                      AS path: 4 I
                    > to 12.12.12.2 via fe-1/2/0.1
                    [BGP/170] 00:05:14, localpref 100, from 192.168.3.1
                      AS path: 4 I
                    > to 13.13.13.3 via fe-1/2/1.2

Meaning

The asterisk (*) shows that the preferred path is through Device R2. In the default configuration, Device R2 has a lower router ID than Device R3. The router ID is controlling the path selection.

Altering the Local Preference to Change the Path Selection

Purpose

Change the path so that it goes through Device R3.

Action

From configuration mode, enter the set local-preference 300 command.

Rechecking the Active Path From Device R1 to Device R4

Purpose

Verify that the active path from Device R1 to Device R4 goes through Device R3.

Action

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

inet.0: 11 destinations, 17 routes (11 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

12.12.12.0/24       [BGP/170] 00:16:48, localpref 100, from 192.168.2.1
                      AS path: I
                    > to 12.12.12.2 via fe-1/2/0.1
13.13.13.0/24       [BGP/170] 00:00:22, localpref 300, from 192.168.3.1
                      AS path: I
                    > to 13.13.13.3 via fe-1/2/1.2
24.24.24.0/24       [BGP/170] 00:16:48, localpref 100, from 192.168.2.1
                      AS path: I
                    > to 12.12.12.2 via fe-1/2/0.1
34.34.34.0/24       [BGP/170] 00:00:22, localpref 300, from 192.168.3.1
                      AS path: I
                    > to 13.13.13.3 via fe-1/2/1.2
192.168.2.1/32      [BGP/170] 00:16:48, localpref 100, from 192.168.2.1
                      AS path: I
                    > to 12.12.12.2 via fe-1/2/0.1
192.168.3.1/32      [BGP/170] 00:00:22, localpref 300, from 192.168.3.1
                      AS path: I
                    > to 13.13.13.3 via fe-1/2/1.2
192.168.4.1/32     *[BGP/170] 00:00:21, localpref 300, from 192.168.3.1
                      AS path: 4 I
                    > to 13.13.13.3 via fe-1/2/1.2 

Meaning

The asterisk (*) shows that the preferred path is through Device R3. In the altered configuration, Device R3 has a higher local preference than Device R2. The local preference is controlling the path selection.