Policy-based routing (PBR) is a versatile feature in Google Cloud Platform (GCP) that enables users to control traffic flow beyond the standard destination-based routing. This article explains the fundamentals, specifications, limitations, and advanced configurations of policy-based routing, helping you understand its role in modern network architectures.

What Is Policy-Based Routing in GCP?

Policy-based routing allows traffic to be routed based on criteria beyond the destination IP address. It provides enhanced control by enabling traffic redirection based on specific attributes like source IP address and protocol. This capability is particularly useful for inserting network appliances like firewalls into traffic paths, enhancing security and traffic management.

How Does Policy-Based Routing Work?

Policy-based routes match specific traffic characteristics and redirect the traffic to a designated next hop, which must be an internal passthrough Network Load Balancer. This approach supports scenarios like:

1. Selective Traffic Management: Apply rules to traffic originating from specific virtual machine (VM) instances in a Virtual Private Cloud (VPC) network.
2. Interconnect Traffic Control: Route all traffic entering a VPC via VLAN attachments in a region or an entire network.
3. Network-Wide Application: Enforce routing rules for all VMs, VLAN attachments, and Cloud VPN tunnels within the VPC.

By configuring these routes, organizations can ensure that traffic is processed through specialized infrastructure before reaching its destination.

What Are the Key Specifications for Policy-Based Routing?

Supported Next Hops

The next hop for a policy-based route must be an internal passthrough Network Load Balancer, residing either in:

• The same VPC as the route, or
• A peered VPC network (Preview feature).

Traffic Types and Prioritization

• Policy-based routes can handle both IPv4 and IPv6 traffic (IPv6 support is currently in Preview).
• These routes have higher precedence over other route types, except special routing paths.
• For routes with identical priority, GCP uses an internal deterministic algorithm to select one.

Bidirectional Traffic Management

Policy-based routes can manage unidirectional traffic or be configured with multiple rules to handle bidirectional flows.

IP Forwarding Requirement

VM instances receiving traffic from policy-based routes must have IP forwarding enabled.

Interconnect Traffic Application

When applied to Cloud Interconnect, routes must encompass all VLAN attachments within a region or a network.

What Are the Limitations of Policy-Based Routing?

Despite its capabilities, policy-based routing has certain limitations:

1. No Route Exchange: Policy-based routes are not shared between VPC networks connected through VPC Network Peering or Network Connectivity Center hubs and spokes.
2. Port Matching Unavailable: Routes cannot match traffic based on specific ports.
3. Immutability: Once created, policy-based routes cannot be updated. Instead, you must delete and recreate them.
4. Load Balancer Restrictions: The internal passthrough Network Load Balancer must have a dedicated IP address, not a shared one.
5. GKE Communication Impact: Routes may interfere with communication between Google Kubernetes Engine (GKE) control planes and nodes.
6. Private Service Connect Incompatibility: Policy-based routes do not support Private Service Connect endpoints or backends.
7. Dataplane Requirement: VLAN attachments must use Dataplane v2.

These constraints necessitate careful planning when implementing policy-based routes.

How Do You Skip Other Policy-Based Routes?

GCP allows users to configure a policy-based route to bypass evaluation of other policy-based routes by setting the --next-hop-other-routes=DEFAULT_ROUTING flag using the Google Cloud CLI. Alternatively, include "nextHopOtherRoutes": "DEFAULT_ROUTING" in an API request.

For example:

• A policy-based route with a source range of 0.0.0.0/0 and a network tag compute-vm might need to skip certain packets.
• To achieve this, create a higher-priority route with a specific source IP range to bypass the evaluation of the original route.

This feature ensures granular control over traffic flows, especially in complex network environments.

How Can Policy-Based Routes Enhance Security and Performance?

By routing traffic through internal passthrough Network Load Balancers, policy-based routes enable the insertion of critical appliances such as:

1. Firewalls: Enforce security policies by inspecting and filtering traffic.
2. Intrusion Detection Systems (IDS): Monitor traffic for suspicious activity.
3. Traffic Shaping Tools: Optimize network performance by managing bandwidth usage.

This capability transforms the network into a flexible, software-defined environment, accommodating modern security and performance needs.

What Are the Use Cases for Policy-Based Routing?

Policy-based routing is applicable in scenarios like:

1. Hybrid Cloud Deployments: Route traffic between on-premises systems and GCP resources via specific paths.
2. Advanced Traffic Inspection: Use passthrough load balancers to integrate deep packet inspection tools.
3. Traffic Segmentation: Separate traffic flows based on source attributes to meet compliance or performance requirements.
4. Failover Mechanisms: Configure backup routes for improved reliability.

Conclusion

Policy-based routing in GCP offers robust traffic control by allowing routing decisions based on advanced criteria. With support for IPv4 and IPv6, compatibility with internal passthrough Network Load Balancers, and the ability to manage traffic across various network scenarios, it provides unparalleled flexibility. However, users must be aware of its limitations and configure routes thoughtfully to maximize benefits while minimizing risks.

Whether enhancing security, optimizing performance, or enabling hybrid cloud strategies, policy-based routing is an invaluable tool for modern network architects.