BGP basic knowledge: Message type and state machine

BGP profound, in the study time we will discover and the tradition IGP to have the very big difference, so many things in the study time we are subverting our traditional concept, but lets the study become more difficult.

Learning is a gradual process. So for learning BGP This routing protocol must be well prepared.

Before beginning to learn the BGP technology itself, it is best to check the background of the BGP routing protocol on the Internet, why do people need BGP this large-scale routing technology, he can help mankind to solve the communication problem?

Why traditional IGP are not qualified for these jobs.

Before using the principle of experimental verification, we will first put all the principles of BGP more important to be sorted out. As the saying goes, the knife does not mistake wood.

Any technology behind it has his background, only after understanding the background, can do extrapolate.

I have prepared myself to be ready to learn the BGP system again within 3 months.

BGP Basics:

Considering that BGP is used between as, as the role of interface between large networks, there is a high requirement for the stability of message transmission, BGP chooses TCP as the hosting protocol and uses port number 179. Because TCP provides stable and reliable transmission, BGP does not need a special mechanism to deal with the details of complex packet fragmentation, retransmission and acknowledgement.

BGP building Neighbors is the way to use the common IGP dynamic to discover neighbors? Or do you use the static manual configuration method?

BGP explicitly chooses the latter, as long as the two sides specify the address route to reach. Then you can build a neighbor. There are at least 2 advantages to doing this:

0 The neighbor can be built with any IP address on the device, not limited to a fixed interface IP. In this way, when two devices adopt a loopback address instead of a direct-attached address to establish a BGP neighbor, you can switch to the backup link, even if the primary link is interrupted, to keep the neighbors constantly. This kind of stability is the necessary characteristic of BGP as a large network routing load.

0 You can build a neighbor across multiple devices. When a as has more than one device running BGP to establish a full connection within the domain, do not need each device physical direct attached, as long as the IGP to ensure the establishment of a neighbor's address can be reached, you can establish a full network connection, reduce unnecessary link construction.

In the same as (self-contained system), the neighbors between devices are called IBGP neighbors, and between different as, the neighbors of the devices are called EBGP neighbors, which become BGP peers (BGP peer).

BGP has four types of messages:

Here online cattle people with a more powerful life language to explain the role of these four categories of messages:

OPEN: "Hello, make a friend with me!" ”

KEEPALIVE: I'm still alive, don't ignore me

UPDATE: There is news ...

NOTIFICATION: I'm not playing with you!

Here is a look at the detailed message content:

0OPEN message:

Version: This field is a BGP version number, and if a peer is older than the other, it rejects the new version of the connection, and the other side lowers the version number and negotiates again until the two versions agree.

My as: The as number of the peer, used to determine whether both sides are IBGP neighbors or EBGP neighbors.

Hole Time: The peer is still in the end by sending regular keepalive message notifications to keep the neighbors. The general keepalive is sent every 60 seconds, hold time is 180 seconds. When negotiating, using the open message is relatively small that hold time as both sides of the holdtime.

BGP identifier: An IP address that is used to represent a neighbor and is also an identifier.

Optional Parameter: Announces support for some optional features, such as authentication, multi-protocol support, and more.