BGP routing protocol "excellent" and "suffering"

Source: Internet
Author: User

As mentioned above, the RIP Protocol is the original routing protocol. As the network is becoming increasingly complex, this Protocol cannot meet the needs, so the new generation of BGP routing protocol has become the new controller of each network. So why is this agreement so favored? Next, let's analyze it.

BGP Route Protocol features

◆ BGP is an external routing protocol. Unlike internal routing protocols such as OSPF and RIP, BGP focuses not on discovering and calculating routes, but on controlling route propagation and selecting the best routes 。

◆ Carry AS path information in BGP routes to completely solve the routing loop problem 。

◆ Using TCP as its transport layer protocol improves the reliability of the Protocol 。

BGP-4 supports classless Inter-Domain Routing CIDR. This is an important improvement over BGP-3, no longer distinguishes A-class network, B-class network, and C-class network. For example, an illegal C-class network address ( uses CIDR notation to become A legal Super network, "/16" indicates that the subnet mask consists of 16 bits starting from the left end of the address. The introduction of CIDR simplifies Routes Aggregation ), route aggregation is actually a process of combining several different routes, so that the route entry is changed from advertised to advertised, reducing the route table size 。

When a route is updated, the BGP routing protocol only sends updated routes, which greatly reduces the bandwidth occupied by BGP transmission routes and is suitable for transmitting a large amount of route information on the Internet 。

For the sake of management and security, every Autonomous System wants to control the routes in and out of the autonomous system, BGP-4 provides a variety of routing policies, it can implement flexible filtering and selection of routes, and is easy to expand to support the new development of the network 。

Problems with the scalability of BGP routing protocols

(1) The Full-Mesh problem of I-BGP

The BGP routing protocol is divided into two parts: I-BGP and E-BGP. I-BGP is used between the routers in the autonomous domain, the E-BGP is used between the routers in the autonomous domain, BGP protocol requires a router to learn through the I-BGP routing, no longer broadcast to other I-BGP neighbors, so a self-governing domain all participate in the I-BGP protocol router to establish a session with other routers, so as to ensure that the routing information can be correctly broadcast to each router. According to this principle, the total number of I-BGP sessions in an autonomous domain is N × (N-1)/2 (N is the number of routers running the I-BGP ), when N increases, this number is astonishing: for example, if 100 routers, the number of sessions is 4950. this is a huge burden on network devices, and it will also make the management and configuration of the network become abnormal and complex. The backbone network is usually composed of a large number of routers running I-BGP, therefore, whether the problem can be solved directly affects the size of the network 。

(2) routing oscillation during route Policy Change

The BGP routing protocol is an incremental Update routing protocol. When a new route is to be published, the router sends an Update message to the neighbor. If you want to delete a route, the Withdraw message is sent. The Flap of the BGP Route is defined as: when a route is reclaimed (Withdraw) and then broadcast (Update), it is regarded as a Flap. because the recovery and update of any route will lead to the re-calculation of the entire route table of a router, when there are many Flap cases, the load on the router equipment will generate a huge amount of pressure. Based on my experience in practical work, a high-end router is usually used to calculate BGP routing, the CPU load is basically 80% ~ About 90%, or even 100%, occupies almost all of the CPU resources. Although most high-end routers currently distribute routing computing modules and forwarding modules on different hardware, to reduce the performance degradation caused by the busy master CPU. However, frequent changes and updates to the route table still have a certain impact on the operation of the entire device, in addition, such computing will continue to expand to the autonomous domain as the route is withdrawn or broadcast, so that the internal router has the same problem 。

(3) Other issues to be considered

In addition to the above two problems that may cause excessive consumption of router resources, there are other factors, such as the number of routes, the size of the BGP Route table, and the route calculation method, it also affects the performance of the router 。

In addition, the larger the network, the more route entries, the more complicated the configuration and management work. Therefore, you need to simplify the configuration during network design to reduce the work intensity of management personnel, avoid manual faults 。

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