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Border Gateway Protocol (English: Border Gateway Protocol, abbreviation: BGP) is a core decentralized autonomous routing protocol on the Internet. It achieves reachability between autonomous systems (AS) by maintaining IP routing table or 'prefix' table. It belongs to vector routing protocol. First, understand the BGP protocol

BGP is a path vector routing protocol, which is used to transmit routing information between autonomous systems. BGP propagates the entire routing table at startup, and then only propagates the partial trigger update that changes in the network. It uses TCP connection to transmit information. The port number is 179. on Internet, BGP needs to advertise a large number of routes. because TCP provides a reliable transmission mechanism, and TCP uses sliding window mechanism, BGP can continuously send packets. There is no need to stop sending and wait for confirmation like OSPF or EIGRP.

1. Generally speaking, BGP is used in the following situations:

An AS allows packets to traverse it and reach other AS

One AS connects multiple AS

Data flows into and out of AS must be controlled

two。 If you do not use BGP, you usually have the following situations:

AS has only one exit.

All exits of AS are 1 ASP

Low routing performance, small memory, slow CPU, and small bandwidth

3. Get to know BGP AS-Path

BGP is a path vector protocol, it uses a list of AS numbers, packets must pass through these AS to reach their destination, and at the same time make a certain strategy for the resulting AS-path. AS-Path is very easy to detect for routing loops, and if the router receives an AS-path with a local AS number, there is a loop. BGP does not give the topology in each AS domain, so BGP can only see the AS tree, while IGP can only see the topology in the AS domain.

When a special purpose network has juxtaposed, equal cost paths, Cisco executes EBGP by default to select only one path, but you can use maximum-paths to change the default maximum number of parallel paths, but it is only valid for EBGP

2. BGP message type 1. Open

After the TCP conversation is established, both neighbors send an Open message that each neighbor uses to identify himself and specify his or her own BGP operation parameters.

BGP version: it specifies the BGP version number that the initiator is running (2meme 3pr 4), which can be modified through neighbor version. The default version number is 4. If the version number is different, the router will automatically lower the version number to resend the Open message until the version is consistent.

AS number: the AS number of the initiating session router, which is used to confirm the EBGP or IBGP session

Hold time: the maximum number of seconds that a router must pass before receiving a keepalive or update message. Holdtime must be 0 (in this case, no Keepalive must be sent) or at least 3s Cisco default holdtime is 180s. If the holdtime is not consistent between the two neighbors, choose the shorter one as the acceptable time for both.

BGP router-ID: in the same way as OSPF, the loopback port address with the highest value is used. Without loopback, the address with the highest value on the physical interface is used.

Optional parameters: support for some optional features. Such as authentication, multi-protocol support and routing refresh, etc.

2. Keepalive

If the router accepts its parameters in the neighbor's Open message, it sends a reply Keepalive message.

By default, the Keepalive interval is 60s, or the agreed hold time is 1 to 3.

3. Update

A byte group used by Update message to advertise available routes, revoked routes, or both Network Layer Reachability Information (NLRI) to publish IP address prefixes and prefix lengths, such as Path Attributes. This attribute provides BGP with the shortest path choice. The information that examines the routing loop and determines the routing policy Withdrawn Routes is used to describe the destination address byte group (length and prefix) that has become unreachable and is being revoked from the service. Although the NLRI field can contain multiple prefixes, each update message describes only one BGP route (because the path attribute describes only one path, but the path may reach multiple destinations)

4. Notification

When an error is detected, an Notification message is sent, which usually causes the BGP connection to be terminated.

Original address: https://www.linuxprobe.com/operator-bgp.html

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