Policy-Based Routing (PBR) Explained

Key Concepts

• Policy-Based Routing (PBR)
• Match Criteria
• Set Actions
• Route Maps
• Access Control Lists (ACLs)

Policy-Based Routing (PBR)

Policy-Based Routing (PBR) is a method of controlling the path that packets take through a network based on policies defined by network administrators. Unlike traditional routing, which uses the destination IP address to determine the path, PBR allows for more granular control by considering additional factors such as source IP address, protocol type, and port number.

Match Criteria

Match criteria are the conditions that determine whether a packet should be subject to PBR. These criteria can include source IP address, destination IP address, protocol type, and port number. For example, an administrator might define a match criterion that applies PBR to all packets originating from a specific subnet.

Set Actions

Set actions are the instructions that specify what should happen to packets that match the defined criteria. These actions can include changing the next-hop address, modifying the interface through which the packet is sent, or applying a specific QoS policy. For instance, a set action might redirect all packets from a particular source to a specific next-hop router.

Route Maps

Route maps are a configuration tool used to define and apply PBR policies. They consist of a series of match criteria and set actions that are evaluated in sequence. If a packet matches a criterion in a route map, the corresponding set action is applied. Route maps provide a flexible and powerful way to implement complex PBR policies.

Access Control Lists (ACLs)

Access Control Lists (ACLs) are used in conjunction with route maps to define the match criteria for PBR. ACLs allow administrators to specify which packets should be subject to PBR based on various attributes such as source IP address, destination IP address, and protocol type. For example, an ACL might be used to identify all packets from a specific subnet, which are then processed by a route map to apply PBR.

Examples and Analogies

Consider a large enterprise with multiple departments, each requiring different network paths for optimal performance. Using PBR, the network administrator can define policies that route traffic from the Sales department through a high-speed link, while routing traffic from the Marketing department through a different, more cost-effective link. This is similar to having a traffic management system that directs different types of vehicles to specific roads based on their needs.

In another scenario, imagine a company that needs to prioritize VoIP traffic over other types of traffic. Using PBR, the network administrator can define a policy that matches all VoIP packets and sets them to use a higher-priority path with lower latency. This is akin to having a dedicated lane on a highway for emergency vehicles, ensuring they reach their destination quickly and efficiently.

For route maps, think of a series of filters in a water treatment plant. Each filter removes specific contaminants based on predefined criteria. Similarly, route maps filter packets based on match criteria and apply specific actions to ensure they follow the desired path.

ACLs can be compared to security guards at a building entrance. They check each person's credentials (source IP address, protocol type) before allowing them to enter. In the same way, ACLs check packet attributes to determine if they should be subject to PBR.