How to Configure BGP Local Preference: A Step-by-Step Guide

When working with Border Gateway Protocol (BGP), setting the local preference is crucial for controlling the path selection process within an Autonomous System (AS). This value determines the preferred exit gateway for outgoing traffic, making it a powerful tool for traffic engineering and network optimization. In this guide, we'll walk you through the process of configuring BGP local preference step-by-step, ensuring you can tailor your network's routing behavior effectively.

Understanding BGP Local Preference

The BGP local preference (LOCAL_PREF) is an attribute used within an AS to rank routes. Routes with a higher local preference value are preferred over those with a lower value. Unlike other BGP attributes like AS Path, which are considered in inter-AS routing, local preference is assessed in intra-AS route selection. It's transmitted between routers in the same AS but does not leave the AS, making it an internal tool for influencing route decisions.

Setting the local preference is particularly vital for organizations with multiple connections to the internet or between large internal networks. It's an integral part of BGP that can dictate how traffic is directed in complex network scenarios, ensuring efficiency and resilience.

Importance of Local Preference in Traffic Flow

Local preference gives network administrators the leverage to control the flow of outbound traffic. By manipulating this attribute, administrators can steer traffic through less congested, more cost-effective, or more reliable pathways, enhancing the overall performance and reliability of the network.

Especially in scenarios involving backup links or multiple ISPs, adjusting the LOCAL_PREF attribute allows for a customized approach in handling network traffic, ensuring critical processes maintain optimal connectivity and performance.

Typical Use Cases for Adjusting BGP Local Preference

Understanding typical applications of BGP local preference can help you better grasp its functionality and practical benefits. Common use cases include: - **Load balancing**: Distributing traffic load evenly across multiple routes to prevent any single link from becoming a bottleneck. - **Redundancy**: Prioritizing primary over backup routes and vice versa can ensure failover capabilities, boosting the network's resilience. - **Cost-efficiency**: Routing traffic through more cost-effective ISP links when multiple upstream connections are available.

Knowing when and how to fine-tune the local preference is a key skill in network administration, capable of optimizing both performance and expenditure.

How to Configure BGP Local Preference on Different Devices

Configuring BGP local preference can vary significantly depending on the device and operating system. However, the fundamental concepts remain constant. Below, we delve into the specifics of setting up LOCAL_PREF on key network appliances and OSes, ensuring you have a comprehensive grasp of the procedure.

If you want to dive deeply into the general understanding of BGP configurations and the theories backing them, consider checking out our detailed course on BGP fundamentals here. This resource will provide an extensive backdrop that complements practical, hands-on configuration skills.

In the following sections, we'll cover the configuration steps for commonly used devices in the networking world, catering to a wide range of environments and user expertise.

Configuring BGP Local Preference on Cisco IOS

When configuring BGP local preference on Cisco devices, you need to access the router's global configuration mode. Cisco IOS uses route maps along with policy-based routing to set the LOCAL_PREF attribute. Here are the steps to handle this configuration:

1. Access your Cisco router and enter global configuration mode by typing enable followed by configure terminal.
2. Create a route map and specify the local preference value. For example, to set the local preference to 150, you would enter:

   route-map PREFER_PATH permit 10
   match ip address 101
   set local-preference 150

3. Apply this route map to BGP neighbors from where you want to receive the routes with the altered local preference. Apply it using:

   router bgp 65001
   neighbor 192.168.1.1 route-map PREFER_PATH in

4. Save your configuration with write memory.

This procedure tailors the BGP local preference, influencing the preferred path for traffic exiting the AS, all operational from your Cisco device. Remember, using route-maps provides a flexible method to implement granular routing policies beyond just setting LOCAL_PREF.

Adjusting Local Preference on Juniper Devices

Juniper routers handle BGP preferences somewhat differently. Juniper uses policies to dictate routing behaviors. To set LOCAL_PREF on Juniper equipment:

1. Enter configuration mode using edit.
2. Set a policy statement that defines the local preference. For example:

   policy-options {
   policy-statement set-local-preference {
       from protocol bgp;
       then {
           local-preference 200;
       }
   }
   }

3. Apply this policy to the relevant BGP group or neighbor under the BGP protocol section:

   protocols {
           bgp {
               group external-peers {
                   import set-local-preference;
               }
           }
   }

4. Commit the configuration changes with commit.

By using Junos policy rules, you fine-tune the routing decisions made by your network, ensuring that the strategic goals regarding traffic flow and reliability are achieved efficiently.

Setting Local Preference on Arista Devices

Configuring BGP local preference on Arista devices is also relatively straightforward. Follow these steps for an Arista configuration:

1. Log into the Arista device and switch to configuration mode with configure.
2. Define a route map with the desired LOCAL_PREF value:

   route-map SET_PREF permit 10
   set local-preference 200

3. Attach this route map to the BGP configuration:

   router bgp 65001
   neighbor 10.20.30.40 route-map SET_PREF in

4. Execute write to save the settings.

This approach ensures that the specified local preferences are implemented across your Arista network, guiding traffic according to your strategic requirements.

Verifying BGP Local Preference Configuration

After configuring the BGP local preference on your devices, it's crucial to verify that the settings are correctly implemented and actively influencing the BGP route selection as intended. Each vendor offers specific commands to audit the current BGP configurations and the state of the routes received by BGP neighbors.

Verification on Cisco Devices

To check the local preference on Cisco routers, use the following commands:

show ip bgp

This command displays the BGP table with all paths and their attributes. By examining the 'local-preference' attribute, you can confirm if your local preference modifications are effective.

show ip bgp neighbors [neighbor_IP_address] routes

Specially useful, this shows the routes learned from a specific BGP neighbor, including their respective local preference settings.

Checking Configurations on Juniper Equipment

In Juniper systems, you can verify the BGP local preference with these commands:

show route receive-protocol bgp [neighbor_IP_address]

This command lists the routes received from a given BGP neighbor and their properties such as local preference, helping ensure your policy settings are active.

show route advertising-protocol bgp [neighbor_IP_address]

It’s also useful to confirm that the expected routes are being advertised to your peers with the correct attributes.

Arista Verification Commands

For Arista devices, use the following commands to verify your configuration:

show ip bgp

Like Cisco, this command in Arista’s EOS software reveals detailed information about the BGP routing table, including local preference values for each route.

show ip bgp neighbors [neighbor_IP_address] received-routes

This specific command is helpful for focusing on what is being received from a particular neighbor, ensuring your settings have been applied properly.

Proper verification not only confirms that your configuration is working as expected but also provides an opportunity to troubleshoot any issues that may arise. By regularly checking the state of BGP local preferences and other routing attributes, network administrators can ensure optimal performance and stability of their networks.

Conclusion

Configuring BGP local preference is a vital administrative task for optimizing the routing decisions in a network. By carefully setting and verifying these preferences on devices from Cisco, Juniper, and Arista, administrators can control the path that outbound traffic takes through their networks. This control is crucial for achieving desired traffic flow patterns, improving network performance, and reducing costs. Regularly evaluating these configurations ensures that the network continues to meet business and technical requirements effectively.