Advanced routing techniques, such as policy-based routing (PBR) and route redistribution, are used to manipulate how network traffic is forwarded between different networks or subnets. They are commonly used in complex enterprise networks, where more granular control is required for traffic flow.
## Policy-based Routing (PBR)
In traditional routing, traffic is forwarded based on the destination IP address, subnet mask, and a routing table lookup. However, with policy-based routing, traffic forwarding decisions are based on additional criteria, such as the source IP address, the protocol used, or the application-layer data. This allows for more granular control over which path traffic takes through the network.
PBR works by using a set of policies that define the conditions under which traffic is forwarded through a specific interface or next-hop router. These policies can be based on a range of attributes, such as source address, packet size, or application type. Once a policy is matched, PBR applies an alternative forwarding path, overriding the default routing table.
For example, a company may have multiple internet service providers (ISPs) with different bandwidths, latencies, and costs. PBR can be used to route traffic from specific subnets or applications through a particular ISP link based on policies such as cost, application type, or quality of service (QoS) requirements.
## Route Redistribution
Route redistribution is another technique used to control traffic flow between different networks in a more granular way. It involves redistributing routes learned from one routing protocol into another. For example, a network may have routers running different routing protocols, such as OSPF and BGP. In such a scenario, route redistribution can be used to enable communication between networks running different protocols.
Route redistribution works by allowing one routing protocol to import network routes learned from another protocol. For example, the BGP protocol can be configured to import routes learned from OSPF, which can then be propagated throughout the BGP network. This enables more efficient use of network resources and allows for better traffic management.
For example, a company may have an e-commerce application running on a server connected to a LAN running OSPF, while their corporate website runs on a different server connected to a WAN running BGP. With route redistribution, the OSPF routes learned by the e-commerce application can be redistributed into BGP, allowing traffic to flow between the two networks.
## When to Use Advanced Routing Techniques
Advanced routing techniques like PBR and route redistribution should be used in situations where more granular control over traffic flow is required. The decision to use these techniques depends on the complexity of the network, the requirements for traffic management, and the availability of resources.
For example, PBR can be used in networks where there are multiple WAN links and different QoS requirements for different applications. It can be used to ensure that specific applications are routed through the most appropriate link based on their QoS requirements.
Similarly, route redistribution can be used in networks where there are multiple routing protocols and the need to propagate routes between them. It can be used to ensure that routes learned from one protocol are available to other protocols, enabling more efficient use of network resources.
In summary, advanced routing techniques like PBR and route redistribution can provide more granular control over traffic flow in complex networks. They should be used when it is necessary to manage traffic based on specific criteria and when more efficient use of network resources is required.