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This article mainly introduces "bgp is not an external routing protocol". In daily operation, I believe many people have doubts about whether bgp is an external routing protocol. I have consulted all kinds of information and sorted out simple and easy operation methods. I hope to help you answer the doubts about "bgp is not an external routing protocol"! Next, please follow the small series to learn together!

BGP is an exterior gateway routing protocol, an autonomous system routing protocol that runs over TCP and is used to transfer routing information between ASs. BGP is the only protocol designed to handle networks the size of the Internet, and it is the only protocol that can properly handle multipath connections between unrelated routing domains.

Operating environment of this tutorial: Windows 10 system, Dell G3 computer.

The Exterior Gateway Protocol (EGP) is a routing protocol used between ASs, originally proposed by Eric C.Rosen and David L.Mills of BBN Technologies in 1982. It was first described in RFC827 and formally standardized in RFC904 in 1984. EGP is a simple reachability protocol that differs from modern distance-vector and path-vector protocols in that it is limited to networks with tree topologies.

Due to the limitations of EGP, the IETF Border Gateway Protocol Working Group developed a standard Border Gateway Protocol (BGP), which is currently widely used.

Introduction to BGP

BGP is an external gateway routing protocol used to transfer routing information between ASs;BGP is a TCP protocol with port number 179.

Border Gateway Protocol (BGP) is a routing protocol for autonomous systems running over TCP. BGP is the only protocol designed to handle networks the size of the Internet, and it is the only protocol that can properly handle multipath connections between unrelated routing domains. BGP builds on the experience of EGP. The main function of BGP is to exchange network reachability information with other BGP systems. The network reachability information includes information about the autonomous systems (AS) listed. This information effectively constructs a topology graph of AS interconnections and thus clears routing loops while policy decisions can be enforced at the AS level.

BGP Features

(1) is an enhanced path vector routing protocol with reliable routing update mechanism;

(2) Rich metric methods;

(3) loop-free protocol design;

(4) Attach various attribute information to route entries;

(5) CIDR (category-free inter-domain routing);

(6) Rich routing filtering and routing strategies;

(7) No periodic updates required:

(8) only incremental routes are sent when routing is updated;

(9) Periodically send KeepAlive messages to maintain TCP connectivity.

Extension:

BGP neighbor relations are peer relations;

BGP neighbors are BGP peers.

BGP Message Type

Routers running BGP are called BGP Speakers. BGP Speakers exchange five types of messages, among which OPEN messages, KEEPALIVE messages and NOTIFICATION messages are used to establish and maintain neighbor relationships.

The five types of BGP messages are as follows:

Open: Responsible for establishing neighbor relationship with peers; mainly includes BGP version, AS number and other information; two routers trying to establish BGP neighbor relationship begin to exchange OPEN information after establishing TCP session connection to confirm whether neighbor relationship can be formed.

KeepAlive: This message is periodically sent between peers, used to judge the reachability between peers, and realize the maintenance of BGP neighbor relations;

Update: This message is used to exchange routing information between BGP peers. It includes withdrawal route information, reachability route information and various route attributes.

Notification: BGP error detection mechanism; once any form of error is detected, BGP Speaker will send an NPTIFICATION message, and then the neighbor relationship associated with it will be closed;

Route-refresh: Used to inform BGP peers that they support route refresh capability.

BGP Database Type:

IP Routing Table-----------IGP

BGP Routing Table---------EGP

BGP's 6 State Machines

Idle: Idle is the first state of a BGP connection. In the idle state, BGP waits for an initiation event. After the initiation event occurs, BGP initializes resources, resets the connection retry timer, initiates a TCP connection, and transitions to the Connect state.

Connect: BGP initiates the first TCP connection in the Connect state. If the Connect-Retry timer expires, it reinitiates the TCP connection and remains in the Connect state. If the TCP connection succeeds, it transitions to the OpenSent state. If the TCP connection fails, it transitions to the Active state.

Active: In the Active state, BGP is always trying to establish TCP connections, returns to the Connect state if the connection retry timer expires, and transitions to the OpenSent state if the TCP connection is successful. If TCP connection fails, it remains in Active state and continues to initiate TCP connection.

OpenSent-Open message sent: In the OpenSent state, TCP connection has been established, BGP has sent the first Open message, and the rest of the work, BGP is waiting for its peer to send an Open message. If there is an error, the system will send an error notification message and return to the Idle state. If there is no error, BGP will start sending Keepalive messages and reset the Keepalive timer to start timing. At the same time, switch to OpenConfirm state;

OpenConfirm-Open Message Acknowledgement: In the OpenConfirm state, BGP sends a Keepalive message and resets the hold timer. If a Keepalive message is received, the BGP neighbor relationship is established. If TCP connection is interrupted, return to Idle state;

Established-Connection established: In the Established state, BGP neighbors have been established and BGP exchanges Update messages with its neighbors while resetting the hold-up timer.

At this point, the study of "bgp is not an external routing protocol" is over, hoping to solve everyone's doubts. Theory and practice can better match to help you learn, go and try it! If you want to continue learning more relevant knowledge, please continue to pay attention to the website, Xiaobian will continue to strive to bring more practical articles for everyone!

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