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
BGP Neighbor Status
The states of BGP neighbors are Idle/Conect/Active/OpenSent/OpenConfirm/Established. This section describes the states of these BGP neighbors.Read more
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.
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.Read more
Illustration: BGP Best Path Selection
When multiple BGP routes are received for the same network address, the best path (optimal route) is determined and registered in the routing table.
In what order the path attributes of the BGP routes are evaluated to determine the best path is illustrated in detail.
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.Read more
BGP Route Minimum Advertisement Interval Configuration
BGP applies triggered updates that send routes when there is some kind of change. The minimum advertisement interval slightly delays the advertisement of BGP routes when changes occur.
This section describes the commands for configuring the minimum advertisement interval for BGP routes.
BGP Route Dampening
Route dumping is the feature that reduces the impact of BGP route flap. This section explains how BGP route dampening works and the configuration commands in Cisco IOS.Read more
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.Read more
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.Read more
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.Read more
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.Read more
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.Read more
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.
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.Read more
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.Read more
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.Read more
BGP neighbor allowas-in command
The neighbor allowas-in command disables AS_PATH loop prevention for BGP routes received from a specific neighbor.Read more
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.Read more
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.Read more
BGP Route Administrative Distance Adjustment
Adjusting the Administrative Distance of BGP routes allows BGP routes to be preferred over routes by other routing processes. This section describes how to adjust the administrative distance for BGP routes.Read more
BGP Route Load Balancing
Packets destined to a network learned by BGP (BGP route) can also be load balanced.This section describes two methods for load balancing BGP routes.Read more
BGP Auto Summary
This section describes BGP auto-summary, which is different from auto-summary with RIP/EIGRP.Read more
BGP Route Summary : network command
The network command can be used to generate BGP routes for known network addresses in the routing table.
This section describes the generation of BGP aggregate route by the network command.
BGP Route Summarization : network command configuration example
This is an example of BGP route summarization configuration using the network command. network command generates known network addresses in the routing table as BGP routes.Read more
BGP Route Summary aggregate-address command
The aggregate-address command can be used to generate BGP aggregate routes.
This section describes the aggregate-address command and the conditions for the command to work.
aggregte-address command : summary-only opiton
The summary-only option is an option regarding "how to handle routes pre-aggregation". The option summary-only advertises only aggregate route. This section describes the summary-only option in more detail.Read more
aggregte-address command : attribute-map opiton
The attribute-map is an option for "what to do with the attributes of the aggregate route". The attribute-map allows attributes to be added to the aggregate route.Read more
aggregte-address command : as-set opiton
The as-set is an option for “what to do with the attributes of the aggregate route”. The as-set option allows the aggregate route to inherit attributes of the pre-aggregate routes.Read more
aggregte-address command : advertise-map opiton
This section describes advertise-map, an option of the aggregate-address command that generates BGP aggregate routes.Read more
BGP Selective Aggregation Overview
When BGP aggregates routes, not only the aggregated route is advertised, but also the individual routes before the aggregation. Selective Aggregation allows control over which pre-aggregation routes are advertised along with the aggregate route.Read more
BGP Selective Aggregation : suppress-map
This section describes the suppress-map option of the aggregate-address command for selective aggregation. suppress-map option allows you to determine the pre-aggregation routes that are not advertised.Read more
BGP Selective Aggregation : unsuppress-map
The unsuppress-map allows you to determine which pre-aggregation routes to advertise along with the aggregation routes for each neighbor.Read more
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.
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.Read more
bgp fast-external-fallover is a feature that immediately drops the neighbor if the interface used for the BGP connection goes down.Read more
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.Read more