Comprehensive Explanation of the Application of routing policies in practice

At present, routing policies are widely used. Many people may not understand the application of routing policies. It does not matter. After reading this article, you will certainly have a lot of GAINS. I hope this article will teach you more things. Many people confuse these two concepts for many reasons. I hope this is helpful. The specific configuration is not provided for the time being. A routing policy is a route publishing and receiving policy. In fact, selecting a routing protocol is also a routing policy, because the same network structure, different routing protocols may generate different route tables because of different implementation mechanisms, different overhead calculation rules, and different priority definitions. These are the most basic. Generally, what we call a routing policy is that, based on the normal routing protocol, we change the route generation, release, and selection results based on a rule, by changing certain parameters, or by setting a certain control mode. Note that the result is the route table.) The rules are not changed, these rules are applied, and some examples are given below.

Example of changing parameters: for example, vro A and vrob B have the same bandwidth and are running the OSPF routing protocol, but the stability of the two links is different, the company wants to set up AB1 as the main circuit. When the main circuit AB1) fails, the standby circuit AB2 is used. If the default setting is adopted, the two circuits are Server Load balancer, in this case, you can set the COST overhead of the AB1 and AB2 circuits respectively, and change the COST value of the AB1 circuit to a smaller value or set the COST value of the AB2 circuit to a greater value, OSPF generates two routes with different overhead and the COST overhead). The smaller the COST, the lower the routing COST. Therefore, the higher the priority, the router will use the AB1 circuit first. You can also set the BandWidth of the two circuits to be different without changing the COST value. The BandWidth of AB1 is larger than that of AB2. According to the OSPF route generation and discovery rules, AB1 overhead COST) will be lower than AB2, And the router will also use the AB1 circuit first.

An example of changing the control mode is to use a routing filter policy to perform some operations on a route that complies with a certain rule, for example, deny is deny and Permit is allowed ), the second is to adjust some routing parameters, such as the COST value, based on the allowable conditions. The common policy is ACLAcess Control List Access Control List), ip-prefix, AS-PATH, and route-policy. Most of the routing policies are used in combination with the BGP protocol. They are routing receiving and notification principles.

Contact and Difference

A routing policy uses a policy to change the routing discovery result based on the rules that affect the route Publishing, receiving, or routing parameters. The route table content is eventually changed. It is used in route discovery. A policy route exists in the current optimal route, but it is applicable to some special hosts, applications, and Protocols. Other forwarding paths are used independently instead of the forwarding paths in the current route table. When the data packet is forwarded, it does not change any content in the routing table. The priority of a policy route is higher than that of a route policy. When the router receives and forwards data packets, it first matches the rule of the policy route, the forwarding is based on the route policy. Otherwise, the forwarding is based on the forwarding path in the route table.

In summary, a routing policy is a routing discovery rule, and a policy route is a packet forwarding rule. In fact, "Policy Routing" is understood as "forwarding policy", which makes it easier to understand and distinguish. Because the forwarding is at the bottom layer and the routing is at the top layer, the forwarding priority is higher than the routing priority, which is understandable. In fact, there are two types and layers of tables in the vro, one is the routing-table, and the other is the forwarding table ). The forwarding table is mapped from the route table. A policy route directly acts on the forwarding table, and a routing policy acts on the route table.

Advantages and disadvantages

The network communication rule is to have a route before forwarding. The routing policy only works when a route is detected. After the route table is generated and stable, if the network does not change, the route table usually does not change. At this time, without an application, the routing policy does not occupy resources. Policy Routing acts upon forwarding. After a router initially generates a route table, the basic workload is on packet forwarding. If there is no policy routing, the router only needs to analyze the destination address of each data packet, the next hop can be determined by matching the route table. However, if there is a policy route, the policy route is always in the application state. If the policy route is particularly complex, the router should judge the source address, protocol, and application of the data packet according to the rules, so that it will always occupy a large amount of resources. Therefore, unless you have to, try to use the routing policy as much as possible, instead of using policy routing. This should be taken into consideration during network optimization. If the policy routing is particularly complex, you can simply break down the network to eliminate the Policy Routing as much as possible. Otherwise, the router will be heavily burdened.