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Table of Contents Expand all

play_arrow Understanding and Configuring Junos Routing Policies
- play_arrow Overview
- play_arrow Evaluating Routing Policies Using Match Conditions, Actions, Terms, and Expressions
- play_arrow Evaluating Complex Cases Using Policy Chains and Subroutines
- play_arrow Configuring Route Filters and Prefix Lists as Match Conditions
- play_arrow Configuring AS Paths as Match Conditions
- play_arrow Configuring Communities as Match Conditions
- play_arrow Increasing Network Stability with BGP Route Flapping Actions
- play_arrow Tracking Traffic Usage with Source Class Usage and Destination Class Usage Actions
- play_arrow Avoiding Traffic Routing Threats with Conditional Routing Policies
- play_arrow Protecting Against DoS Attacks by Forwarding Traffic to the Discard Interface
- play_arrow Improving Commit Times with Dynamic Routing Policies
  - Understanding Dynamic Routing Policies
  - Example: Configuring Dynamic Routing Policies
- play_arrow Testing Before Applying Routing Policies
  - Understanding Routing Policy Tests
  - Example: Testing a Routing Policy with Complex Regular Expressions
play_arrow Configuring Firewall Filters
play_arrow Configuring Traffic Policers
play_arrow Configuration Statements and Operational Commands
play_arrow Troubleshooting
- play_arrow Knowledge Base
  - https://kb.juniper.net/

list Table of Contents

English

Example: Using Routing Policy to Set a Preference Value for BGP Routes

date_range 24-Nov-23

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This example shows how to use routing policy to set the preference for routes learned from BGP. Routing information can be learned from multiple sources. To break ties among equally specific routes learned from multiple sources, each source has a preference value. Routes that are learned through explicit administrative action, such as static routes, are preferred over routes learned from a routing protocol, such as BGP or OSPF. This concept is called administrative distance by some vendors.

Requirements

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

Overview

Routing information can be learned from multiple sources, such as through static configuration, BGP, or an interior gateway protocol (IGP). When Junos OS determines a route’s preference to become the active route, it selects the route with the lowest preference as the active route and installs this route into the forwarding table. By default, the routing software assigns a preference of 170 to routes that originated from BGP. Of all the routing protocols, BGP has the highest default preference value, which means that routes learned by BGP are the least likely to become the active route.

Some vendors have a preference (distance) of 20 for external BGP (EBGP) and a distance of 200 for internal BGP (IGBP). Junos OS uses the same value (170) for both EBGP and IBGP. However, this difference between vendors has no operational impact because Junos OS always prefers EBGP routes over IBGP routes.

Another area in which vendors differ is in regard to IGP distance compared to BGP distance. For example, some vendors assign a distance of 110 to OSPF routes. This is higher than the EBGP distance of 20, and results in the selection of an EBGP route over an equivalent OSPF route. In the same scenario, Junos OS chooses the OSPF route, because of the default preference 10 for an internal OSPF route and 150 for an external OSPF route, which are both lower than the 170 preference assigned to all BGP routes.

This example shows a routing policy that matches routes from specific next hops and sets a preference. If a route does not match the first term, it is evaluated by the second term.

Topology

In the sample network, Device R1 and Device R3 have EBGP sessions with Device R2.

On Device R2, an import policy takes the following actions:

For routes received through BGP from next-hop 10.0.0.1 (Device R1), set the route preference to 10.
For routes received through BGP from next-hop 10.1.0.2 (Device R3), set the route preference to 15.

Figure 1 shows the sample network.

Figure 1: BGP Preference Value Topology

CLI Quick Configuration shows the configuration for all of the devices in Figure 1.

The section #configuration178__bgp-preference-policy-st describes the steps on Device R2.

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

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set interfaces fe-1/2/0 unit 0 family inet address 10.0.0.1/30
set interfaces lo0 unit 0 family inet address 192.168.0.1/32
set protocols bgp group ext type external
set protocols bgp group ext export send-direct
set protocols bgp group ext peer-as 200
set protocols bgp group ext neighbor 10.0.0.2
set policy-options policy-statement send-direct term 1 from protocol direct
set policy-options policy-statement send-direct term 1 then accept
set routing-options autonomous-system 100

Device R2

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set interfaces fe-1/2/0 unit 0 family inet address 10.0.0.2/30
set interfaces fe-1/2/1 unit 0 family inet address 10.1.0.1/30
set interfaces lo0 unit 0 family inet address 192.168.0.2/32
set protocols bgp group ext type external
set protocols bgp group ext import set-preference
set protocols bgp group ext export send-direct
set protocols bgp group ext neighbor 10.0.0.1 peer-as 100
set protocols bgp group ext neighbor 10.1.0.2 peer-as 300
set policy-options policy-statement send-direct term 1 from protocol direct
set policy-options policy-statement send-direct term 1 then accept
set policy-options policy-statement set-preference term term1 from protocol bgp
set policy-options policy-statement set-preference term term1 from next-hop 10.0.0.1
set policy-options policy-statement set-preference term term1 then preference 10
set policy-options policy-statement set-preference term term2 from protocol bgp
set policy-options policy-statement set-preference term term2 from next-hop 10.1.0.2
set policy-options policy-statement set-preference term term2 then preference 15
set routing-options autonomous-system 200

Device R3

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set interfaces fe-1/2/1 unit 0 family inet address 10.1.0.2/30
set interfaces lo0 unit 0 family inet address 192.168.0.3/32
set protocols bgp group ext type external
set protocols bgp group ext export send-direct
set protocols bgp group ext peer-as 200
set protocols bgp group ext neighbor 10.1.0.1
set policy-options policy-statement send-direct term 1 from protocol direct
set policy-options policy-statement send-direct term 1 then accept
set routing-options autonomous-system 300

Procedure

Step-by-Step Procedure
Results

Step-by-Step Procedure

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

To configure Device R2:

Configure the device interfaces.

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[edit interfaces]
user@R2# set fe-1/2/0 unit 0 family inet address 10.0.0.2/30
user@R2# set fe-1/2/1 unit 0 family inet address 10.1.0.1/30
user@R2# set lo0 unit 0 family inet address 192.168.0.2/32

Configure the local autonomous system.
```
[edit routing-options]
user@R2# set autonomous-system 200
```

Configure the routing policy that sends direct routes.

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[edit policy-options policy-statement send-direct term 1]
user@R2# set from protocol direct
user@R2# set then accept

Configure the routing policy that changes the preference of received routes.

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[edit policy-options policy-statement set-preference]
user@R2# set term term1 from protocol bgp
user@R2# set term term1 from next-hop 10.0.0.1
user@R2# set term term1 then preference 10
user@R2# set term term2 from protocol bgp
user@R2# set term term2 from next-hop 10.1.0.2
user@R2# set term term2 then preference 15

Configure the external peering with Device R2.

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[edit protocols bgp group ext]
user@R2# set type external
user@R2# set export send-direct
user@R2# set neighbor 10.0.0.1 peer-as 100
user@R2# set neighbor 10.1.0.2 peer-as 300

Apply the set-preference policy as an import policy.

This affects Device R2’s routing table and has no impact on Device R1 and Device R3.
```
[edit protocols bgp group ext]
user@R2# set import set-preference
```

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.

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user@R2# show interfaces
fe-1/2/0 {
    unit 0 {
        family inet {
            address 10.0.0.2/30;
        }
    }
}
fe-1/2/1 {
    unit 0 {
        family inet {
            address 10.1.0.1/30;
        }
    }
}
lo0 {
    unit 0{
        family inet {
            address 192.168.0.2/32;
        }
    }
}

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user@R2# show protocols
bgp {
    group ext {
        type external;
        import set-preference;
        export send-direct;
        neighbor 10.0.0.1 {
            peer-as 100;
        }
        neighbor 10.1.0.2 {
            peer-as 300;
        }
    }
}

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user@R2# show policy-options
policy-statement send-direct {
    term 1 {
        from protocol direct;
        then accept;
    }
}
policy-statement set-preference {
    term term1 {
        from {
            protocol bgp;
            next-hop 10.0.0.1;
        }
        then {
            preference 10;
        }
    }
    term term2 {
        from {
            protocol bgp;
            next-hop 10.1.0.2;
        }
        then {
            preference 15;
        }
    }
}

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user@R2# show routing-options
autonomous-system 200;

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

Verification

Confirm that the configuration is working properly.

Verifying the Preference

Purpose
Action
Meaning

Purpose

Make sure that the routing tables on Device R1 and Device R2 reflect the fact that Device R1 is using the configured EBGP preference of 8, and Device R2 is using the default EBGP preference of 170.

Action

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

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user@R2> show route protocols bgp
inet.0: 7 destinations, 9 routes (7 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.0.0.0/30         [BGP/10] 04:42:23, localpref 100
                      AS path: 100 I, validation-state: unverified
                    > to 10.0.0.1 via fe-1/2/0.0
10.1.0.0/30         [BGP/15] 04:42:23, localpref 100
                      AS path: 300 I, validation-state: unverified
                    > to 10.1.0.2 via fe-1/2/1.0
192.168.0.1/32     *[BGP/10] 04:42:23, localpref 100
                      AS path: 100 I, validation-state: unverified
                    > to 10.0.0.1 via fe-1/2/0.0
192.168.0.3/32     *[BGP/15] 04:42:23, localpref 100
                      AS path: 300 I, validation-state: unverified
                    > to 10.1.0.2 via fe-1/2/1.0

Meaning

The output shows that on Device R2, the preference values have been changed to 15 for routes learned from Device R3, and the preference values have been changed to 10 for routes learned from Device R1.

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Routing Policies, Firewall Filters, and Traffic Policers User Guide

ON THIS PAGE

Example: Using Routing Policy to Set a Preference Value for BGP Routes

Requirements

Overview

Topology

Configuration

CLI Quick Configuration

Procedure

Step-by-Step Procedure

Results

Verification

Verifying the Preference

Purpose

Action

Meaning

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Routing Policies, Firewall Filters, and Traffic Policers User Guide

ON THIS PAGE

Example: Using Routing Policy to Set a Preference Value for BGP Routes

Requirements

Overview

Topology

Configuration

CLI Quick Configuration

Procedure

Step-by-Step Procedure

Results

Verification

Verifying the Preference

Purpose

Action

Meaning

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