I’ve always been a bit fuzzy on the difference between routing policy and policy-based routing. Drawing from my own experience and some reference materials, here’s a summary of how routing policy and policy-based routing differ!
1. Routing Policy
A routing policy governs the rules for advertising and receiving routes. In fact, choosing a routing protocol itself is a form of routing policy, because with the same network topology, different routing protocols can produce different routing tables due to varying mechanisms, cost calculation rules, and priority definitions—these are the fundamentals.
What we typically refer to as a routing policy, though, goes a step further: on top of the normal routing protocol, we apply certain rules—by changing specific parameters or setting control mechanisms—to alter the outcome of route generation, advertisement, and selection. Note: it changes the result (i.e., the routing table). The underlying rules themselves don’t change; we are simply applying these policies.
2. Policy-Based Routing
Policy-based routing comes into play after the routing table already exists. Instead of forwarding traffic according to the existing table, it selects alternative paths for certain traffic flows based on specific requirements.
3. Connection and Difference
Connection:
Both are path-selection strategies for forwarding data packets. Each uses some set of rules to change parameters or control methods in order to set different forwarding paths.
Difference:
A routing policy uses strategies and rules to change the parameters that influence route advertisement, reception, or selection, thereby altering the outcome of route discovery. Ultimately, it changes what is in the routing table. It takes effect during the route discovery process.
Policy-based routing, on the other hand, ignores the current optimal route in the routing table for certain specific hosts (or applications, protocols) and uses a different forwarding path instead. It takes effect during actual packet forwarding and does not alter the routing table in any way.
Policy-based routing has a higher priority than routing policy. When a router receives a data packet and needs to forward it, it first tries to match it against policy-based routing rules. If a match is found, it forwards according to the policy-based routing; otherwise, it uses the forwarding path from the routing table.
To summarize: a routing policy is a route discovery rule, while policy-based routing is a packet forwarding rule. It actually makes more sense to think of “policy-based routing” as a “forwarding policy”—that way, it’s easier to understand and distinguish between the two. Since forwarding operates at a lower level and routing at a higher level, it follows logically that forwarding has a higher priority than routing.
In fact, routers maintain two types and levels of tables: a routing table and a forwarding table. The forwarding table is mapped from the routing table. Policy-based routing acts directly upon the forwarding table, whereas a routing policy acts directly upon the routing table.
4. Pros and Cons
The rule