How the BGP works

BGP (Border Gateway Protocol) is a routing protocol mainly used by Internet Service Providers (ISPs) to provide Internet access services. A key feature of BGP is its ability to scale to handle vast amounts of Internet route information, allowing for flexible control over optimal route decisions.

This section describes how BGP works and how to configure it on network devices such as Cisco routers.

BGP Basic Configuration and Verification Commands

The basic configuration process for BGP consists of the following three steps
1. Enable the BGP routing process
2. Configuring the Neighbor
3. Generating BGP routes

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BGP Neighbor Status

The states of BGP neighbors are Idle/Conect/Active/OpenSent/OpenConfirm/Established. This section describes the states of these BGP neighbors.

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BGP Neighbor Authentication

BGP neighbor authentication allows you to establish a neighbor only with a legitimate BGP router.
This section describes the configuration commands for neighbor authentication and a simple configuration example.

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BGP Well Known Mandatory Attributes

There are various path attributes added to the BGP route. Well Known Mandatory attributes are always added to a BGP route.

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BGP KEEPALIVE timer/Hold time Configuration

BGP neighbors are basically maintained by exchanging KEEPALIVE messages periodically; this section describes the timer settings related to KEEPALIVE messages.

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BGP Route Filter Overview

It is possible to filter when advertising and receiving route information to and from a specified neighbor with BGP. "How to identify the route information" is important in route filtering with BGP.

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BGP Route Filter : distribute-list

This section describes the commands for configuring and verifying the BGP route filter using distribute-list. The distribute-list identifies the BGP route information with ACL to be allowed or denied.

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BGP Route Filter : distribute-list Configuration Example

This is an example of BGP route filter configuration using distribute-list. BGP routes to be filtered are identified with both standard and extended ACLs.

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BGP Route Filter : prefix-list

This section describes the configuration and verification commands for the BGP route filter using prefix-list. The prefix-list identifies BGP routes to be filtered by network address and subnet mask.

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BGP Route Filter : prefix-list Configuration Example

The following is an example of configuring BGP route filter with prefix-list. Configure the same conditions with prefix-list as with distribut-list filter.

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BGP Route Filter : filter-list(AS_PATH ACL)-

This section describes the commands for configuring and verifying the filter-list.
The filter-list identifies the BGP routes to be filtered by AS_PATH ACL.

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BGP Route Filter : filter-list(AS_PATH ACL) Configuration Example

This is an example of BGP route filter configuration by filter list (AS_PATH ACL). It is important to know how to represent the AS_PATH attribute in a regular expression.

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BGP Route Filter : Route-map

This section describes the configuration and verification commands for BGP route filtering using route-map. The route map allows you to identify BGP routes to filter by reference to standard/extended ACLs, prefix lists, AS_PATH ACLs, etc. In addition to filters, additional path attributes can be set for BGP routes.

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BGP Route Filter : route-map Configuration Example

This is an example of BGP route filter configuration with route-map. Route-maps allow you to flexibly identify BGP routes to filter by referencing standard/extended ACLs, prefix-lists, and AS_PATH ACLs in the match condition.

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BGP neighbor allowas-in command

The neighbor allowas-in command disables AS_PATH loop prevention for BGP routes received from a specific neighbor.

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BGP neighbor as-override command

The neighbor as-override command rewrites the AS_PATH attribute of a BGP route to advertise to a specific neighbor. This section describes the neighbor as-override command.

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BGP Route RIB Failure

If the best-path BGP route is not registered in the routing table, the route is placed in RIB Failure status.This section describes RIB Failure of BGP route.

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BGP local-as

local-as allows you to establish a BGP neighbor for a specific EBGP neighbor with an AS other than the original AS number and exchange routes.
This section describes the operation of BGP local-as and the configuration commands.

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BGP neighbor remove-private-AS

Use the neighbor remove-private-AS command to remove private AS number from the AS_PATH of BGP route. This section describes the neighbor remove-private-AS command.

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bgp fast-external-fallover

bgp fast-external-fallover is a feature that immediately drops the neighbor if the interface used for the BGP connection goes down.

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BGP Prefix Limitation

If a large amount of route information (prefix) is sent due to a neighbor's misconfiguration, for example, the load on the router may increase and have an adverse effect on the router. Therefore, the neighbor maximum-prefix command is used to limit the number of route information received from BGP neighbors.

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