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CLI-based Configuration Guide - IP Unicast Routing

AR100, AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R009

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Basic Concepts of BGP

Basic Concepts of BGP

Autonomous System

An Autonomous System (AS) is a group of Internet Protocol (IP) networks that are controlled by one entity, typically an Internet service provider (ISP), and that have the same routing policy. Each AS is assigned a unique AS number, which identifies an AS on a BGP network. Two types of AS numbers are available: 2-byte AS numbers and 4-byte AS numbers. A 2-byte AS number ranges from 1 to 65535, and a 4-byte AS number ranges from 1 to 4294967295. Devices supporting 4-byte AS numbers are compatible with devices supporting 2-byte AS numbers.

BGP Classification

As shown in Figure 9-1, BGP is classified into two types according to where it runs: Internal BGP (IBGP) and External BGP (EBGP).

Figure 9-1 BGP operating mode

  • EBGP: runs between ASs. To prevent routing loops between ASs, a BGP device discards the routes with the local AS number when receiving the routes from EBGP peers.

  • IBGP: runs within an AS. To prevent routing loops within an AS, a BGP device does not advertise the routes learned from an IBGP peer to the other IBGP peers and establishes full-mesh connections with all the IBGP peers. To address the problem of too many IBGP connections between IBGP peers, BGP uses Route Reflector and BGP Confederation.

    NOTE:
    If a BGP device needs to advertise the route received from an EBGP peer outside an AS through another BGP device, IBGP is recommended.

Device Roles in BGP Message Exchange

There are two device roles in BGP message exchange:

  • Speaker: The device that sends BGP messages is called a BGP speaker. The speaker receives and generates new routes, and advertises the routes to other BGP speakers.

  • Peer: The speakers that exchange messages with each other are called BGP peers. A group of peers sharing the same policies can form a peer group.

BGP Router ID

The BGP router ID is a 32-bit value that is often represented by an IPv4 address to identify a BGP device. It is carried in the Open message sent during the establishment of a BGP session. When two BGP peers need to establish a BGP session, they each require a unique router ID. Otherwise, the two peers cannot establish a BGP session.

The BGP router ID of a device must be unique on a BGP network. It can be manually configured or selected from IPv4 addresses on the device. By default, an IPv4 address of a loopback interface on a device is used as the BGP router ID. If no loopback interface is configured on the device, the system selects the largest IPv4 address from all IPv4 addresses of interfaces as the BGP router ID. Once the BGP router ID is selected, the system retains this router ID even if a larger IPv4 address is configured on the device later. The system changes the BGP router ID only when the corresponding IPv4 address is deleted.

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Updated: 2019-05-17

Document ID: EDOC1000174069

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