Use this task to redistribute a static aggregate route into BPG. A static aggregate route is configured and then redistributed into the BGP routing table. The static route must be configured to point to interface null 0 and the prefix should be a superset of known BGP routes. If the route is not found in the BGP routing table, then the packet will be forwarded to null 0 and discarded. Use this task to create an aggregate route entry in the BGP routing table when at least one specific route falls into the specified range.
The aggregate route is advertised as originating from your autonomous system. Use the aggregate-address command with no keywords to create an aggregate entry if any more-specific BGP routes are available that fall in the specified range.
Do not use the as-set keyword when aggregating many paths because this route is withdrawn and updated every time the reachability information for the aggregated route changes.
Only partial syntax is used in this example. Use this task to create an aggregate route, suppress the advertisement of routes using BGP, and subsequently unsuppress the advertisement of routes.
Routes that are suppressed are not advertised to any neighbors, but it is possible to unsuppress routes that were previously suppressed to specific neighbors. Use the optional summary-only keyword to create the aggregate route for example, Use the optional suppress-map keyword to create the aggregate route but suppress advertisement of specified routes. Routes that are suppressed are not advertised to any neighbors.
You can use the match clauses of route maps to selectively suppress some more-specific routes of the aggregate and leave others unsuppressed. IP access lists and autonomous system path access lists match clauses are supported. Optional Selectively advertises routes previously suppressed by the aggregate-address command. In this example, the routes previously suppressed in Step 5 are advertised to neighbor Perform this task to conditionally advertise selected BGP routes.
The routes or prefixes that will be conditionally advertised are defined in two route maps: an advertise map and either an exist map or nonexist map. The route map associated with the exist map or nonexist map specifies the prefix that the BGP speaker will track.
The route map associated with the advertise map specifies the prefix that will be advertised to the specified neighbor when the condition is met. If a prefix is found to be present in the exist map by the BGP speaker, the prefix specified by the advertise map is advertised. If a prefix is found not to be present in the nonexist map by the BGP speaker, the prefix specified by the advertise map is advertised. If the condition is not met, the route is withdrawn and conditional advertisement does not occur.
All routes that may be dynamically advertised or not advertised must exist in the BGP routing table in order for conditional advertisement to occur. These routes are referenced from an access list or an IP prefix list. In this example, the prefix Configures the route map to match a prefix that is permitted by a standard access list, an extended access list, or a prefix list. In this example, the route map is configured to match a prefix permitted by access list 1.
In this example, the route map is configured to match a prefix permitted by access list 2. In this example, access list 1 permits advertising of the In this example, access list 2 permits the Route aggregation is useful to minimize the size of the BGP table, but there are situations when you want to add more specific prefixes to the BGP table. Route aggregation can hide more specific routes. Perform this task to advertise a default route to BGP peers. The default route is locally originated.
A default route can be useful to simplify configuration or to prevent the device from using too many system resources.
If the device is peered with an Internet service provider ISP , the ISP will carry full routing tables, so configuring a default route into the ISP network saves resources at the local device. Use this task to indicate to border devices which networks are reachable using a backdoor route. A backdoor network is treated the same as a local network, except that it is not advertised. Adds the IP address of the neighbor in the specified autonomous system to the multiprotocol BGP neighbor table of the local device.
In this example, the peer is an internal peer as the autonomous system number specified for the peer is the same number specified in Step 3. Indicates a network that is reachable through a backdoor route.
Exits router configuration mode and returns to privileged EXEC mode. This task explains how to configure a BGP peer group. Often, in a BGP speaker, many neighbors are configured with the same update policies that is, the same outbound route maps, distribute lists, filter lists, update source, and so on. Neighbors with the same update policies can be grouped into peer groups to simplify configuration and, more importantly, to make updating more efficient.
The three steps to configure a BGP peer group, described in the following task, are as follows:. You can disable a BGP peer or peer group without removing all the configuration information using the neighbor shutdown router configuration command.
To exchange other address prefix types, such as IPv6 prefixes, neighbors must also be activated using the neighbor activate command in address family configuration mode for the other prefix types. This is the default. The multicast keyword specifies that IPv4 multicast address prefixes will be exchanged. Enables the neighbor to exchange prefixes for the IPv4 address family with the local device.
By default, neighbors that are defined using the neighbor remote-as command in router configuration mode exchange only unicast address prefixes.
To allow BGP to exchange other address prefix types, such as multicast that is configured in this example, neighbors must also be activated using the neighbor activate command. Some BGP configuration commands can affect other CLI commands and this example demonstrates how the removal of one command affects another command. In the first configuration example, a route map is configured to match and set autonomous system numbers.
BGP neighbors in three different autonomous systems are configured and activated. In the second configuration example, both the route-map command and the redistribute command are disabled. If only the route-map command is removed, it does not automatically disable the redistribution. The redistribution will now occur without any matching or filtering. To remove the redistribution configuration, the redistribute command must also be disabled.
The following examples show two ways to reset the connection for BGP peer The following example shows the command used to initiate a dynamic soft reconfiguration in the BGP peer This command requires that the peer support the route refresh capability. The following example shows how to enable inbound soft reconfiguration for the neighbor When inbound soft reconfiguration is performed later, the stored information will be used to generate a new set of inbound updates.
Specific neighbors or all peers in an autonomous system can be cleared by using the neighbor-address and autonomous-system-number arguments. If no argument is specified, this command will clear and reset all BGP neighbor sessions.
The show ip bgp command is used to display all the entries in the BGP routing table. The following example displays BGP routing table information for the The show ip bgp paths command is used to display all the BGP paths in the database.
The show ip bgp summary command is used to display the status of all BGP connections. The following examples show how you can use aggregate routes in BGP either by redistributing an aggregate route into BGP or by using the BGP conditional aggregation routing feature. In the following example, the redistribute static router configuration command is used to redistribute aggregate route The following configuration shows how to create an aggregate entry in the BGP routing table when at least one specific route falls into the specified range.
The aggregate route will be advertised as coming from your autonomous system and has the atomic aggregate attribute set to show that information might be missing. By default, atomic aggregate is set unless you use the as-set keyword in the aggregate-address router configuration command.
The following example shows how to create the aggregate route for The following example shows how to use an address family to configure a peer group so that all members of the peer group are both unicast- and multicast-capable:. Application of the Border Gateway Protocol in the Internet.
Experience with the BGP Protocol. Multiprotocol Extensions for BGP The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies.
Access to most tools on the Cisco Support and Documentation website requires a Cisco. The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train.
Unless noted otherwise, subsequent releases of that software release train also support that feature. Skip to content Skip to search Skip to footer.
Book Contents Book Contents. Find Matches in This Book. PDF - Complete Book Updated: September 12, Chapter: BGP 4. BGP 4 BGP is an interdomain routing protocol designed to provide loop-free routing between separate routing domains that contain independent routing policies autonomous systems.
Note BGP requires more configuration than other routing protocols and the effects of any configuration changes must be fully understood.
BGP Peer Session Establishment When a BGP routing process establishes a peering session with a peer, it goes through the following state changes: Idle—The initial state that the BGP routing process enters when the routing process is enabled or when the device is reset. BGP Session Reset Whenever the routing policy changes due to a configuration change, BGP peering sessions must be reset by using the clear ip bgp command.
Cisco software supports the following three mechanisms to reset BGP peering sessions: Hard reset—A hard reset tears down the specified peering sessions including the TCP connection and deletes routes coming from the specified peer. The following message is displayed in the output when the device supports the route refresh capability: Received route refresh capability from peer. Routing Policy Change Management Routing policies for a peer include all the configurations for elements such as a route map, distribute list, prefix list, and filter list that may impact inbound or outbound routing table updates.
Table 1. Outbound soft reset No configuration, and no storing of routing table updates. Does not reset inbound routing table updates. Dynamic inbound soft reset Does not clear the BGP session and cache.
Does not require storing of routing table updates, and has no memory overhead. Both BGP devices must support the route refresh capability. Note Does not reset outbound routing table updates. Configured inbound soft reset uses the neighbor soft-reconfiguration router configuration command Can be used when both BGP devices do not support the automatic route refresh capability.
Requires preconfiguration. There are two types of soft reset: When soft reset is used to generate inbound updates from a neighbor, it is called dynamic inbound soft reset. BGP Peer Groups Often, in a BGP network, many neighbors are configured with the same update policies that is, the same outbound route maps, distribute lists, filter lists, update source, and so on. Figure 1. Figure 2. Step 2 configure terminal Example: Device configure terminal Enters global configuration mode.
Step 3 router bgp autonomous-system-number Example: Device config router bgp Configures a BGP routing process, and enters router configuration mode for the specified routing process. Step 4 network network-number [ mask network-mask ] [ route-map route-map-name ] Example: Device config-router network Step 5 bgp router-id ip-address Example: Device config-router bgp router-id Step 7 bgp fast-external-fallover Example: Device config-router bgp fast-external-fallover Optional Enables the automatic resetting of BGP sessions.
Step 8 bgp log-neighbor-changes Example: Device config-router bgp log-neighbor-changes Optional Enables logging of BGP neighbor status changes up or down and neighbor resets. Note Only the syntax applicable to this task is used in this example. Examples The following sample output from the show ip bgp command shows the BGP routing table for Router A in the figure above after this task has been configured on Router A.
BGP table version is 12, local router ID is Note By default, neighbors that are defined using the neighbor remote-as command in router configuration mode exchange only IPv4 unicast address prefixes. Step 3 router bgp autonomous-system-number Example: Device config router bgp Enters router configuration mode for the specified routing process.
Step 4 neighbor ip-address remote-as autonomous-system-number Example: Device config-router neighbor Step 5 address-family ipv4 [ unicast multicast vrf vrf-name ] Example: Device config-router address-family ipv4 unicast Specifies the IPv4 address family and enters address family configuration mode. Step 6 neighbor ip-address activate Example: Device config-router-af neighbor Step 9 show ip bgp neighbors [ neighbor-address ] Example: Device config-router-af show ip bgp neighbors Examples The following sample output from the show ip bgp command shows the BGP routing table for Router A in the figure above after this task has been configured on Router A and Router B.
BGP table version is 13, local router ID is Figure 3. Step 3 interface type number Example: Enters interface configuration mode. Step 5 ip address ip-address mask [ secondary [ vrf vrf-name ]] Example: Device config-if ip address Step 6 exit Example: Device config-if exit Exits interface configuration mode and enters global configuration mode.
Step 8 rd route-distinguisher Example: Device config-vrf rd Creates routing and forwarding tables and specifies the default route distinguisher for a VPN. Step 11 router bgp autonomous-system-number Example: Device config router bgp Enters router configuration mode for the specified routing process.
Step 12 address-family ipv4 [ unicast multicast vrf vrf-name ] Example: Device config-router address-family ipv4 vrf vpn1 Specifies the IPv4 address family and enters address family configuration mode. Step 13 neighbor ip-address remote-as autonomous-system-number Example: Device config-router-af neighbor Step 15 neighbor ip-address activate Example: Device config-router-af neighbor Troubleshooting Tips Troubleshooting Tips Use the ping vrf command to verify basic network connectivity between the BGP devices, and use the show ip vrf command to verify that the VRF instance has been created.
Figure 4. BGP Peer Topology Note By default, neighbors that are defined using the neighbor remote-as command in router configuration mode exchange only IPv4 unicast address prefixes.
Step 4 no bgp default ipv4-unicast Example: Device config-router no bgp default ipv4-unicast Disables the IPv4 unicast address family for the BGP routing process. Note Routing information for the IPv4 unicast address family is advertised by default for each BGP routing session configured with the neighbor remote-as router configuration command unless you configure the no bgp default ipv4-unicast router configuration command before configuring the neighbor remote-as command.
Step 7 address-family ipv4 [ unicast multicast vrf vrf-name ] Example: Device config-router address-family ipv4 multicast Specifies the IPv4 address family and enters address family configuration mode. Step 8 network network-number [ mask network-mask ] [ route-map route-map-name ] Example: Device config-router-af network Step 12 exit-address-family Example: Device config-router-af exit-address-family Exits address family configuration mode and enters router configuration mode.
Note If you perform this step you will not be able to run either of the subsequent show command steps because you have disabled the neighbor. Step 15 show ip bgp ipv4 multicast [ command ] Example: Device show ip bgp ipv4 multicast Optional Displays IPv4 multicast database-related information.
Step 16 show ip bgp neighbors [ neighbor-address ] [ received-routes routes advertised-routes paths regexp dampened-routes received prefix-filter ] Example: Device show ip bgp neighbors BGP table version is 3, local router ID is BGP neighbor is Step 3 no route-map map-name Example: Device config no route-map bgp-to-eigrp Removes a route map from the running configuration.
In this example, a route map named bgp-to-eigrp is removed from the configuration. Step 4 router eigrp autonomous-system-number Example: Device config router eigrp Enters router configuration mode for the specified routing process. Step 5 no redistribute protocol [ as-number ] Example: Device config-router no redistribute bgp Disables the redistribution of routes from one routing domain into another routing domain. Note If a route map was included in the original redistribute command configuration, remember to remove the route-map command configuration as in Step 3 in this example task.
Step 7 show running-config Example: Device show running-config Optional Displays the current running configuration on the router. Monitoring and Maintaining Basic BGP The tasks in this section are concerned with the resetting and display of information about basic BGP processes and peer relationships.
It is expected for that peer to have an increasing Table Version. Conduct a hard clear for the BGP session towards R1 on the peer The peer advertises only one prefix The best path is the path from R4. Ensure that you have debug ip bgp internal enabled in order to see what happens to the BGP Table Versions.
You should have debug ip bgp updates enabled in order to see what happens when the update arrives. In the end, on R1 , there were two best path changes. So, the Table Version got bumped by 2. First, the peer The best path changed to the path received from R5. The Table Version increased to the next available number, which was The Prefix Table Version was bumped to 29 as well.
The RIB was updated with this new best path. Then, R1 sent an update to the BGP peer Every other peer was updated as well. Second, once the peer Introduction The purpose of this memo is to document how the requirements for advancing a routing protocol to Draft Standard have been satisfied by Border Gateway Protocol version 4 BGP This report documents experience with BGP.
This is the second of two reports on the BGP protocol. Please send comments to iwg ans. I'd also like to explicitly thank Yakov Rekhter and Tony Li for the review of this document as well as constructive and valuable comments.
Since then BGP Versions 2, 3, and 4 have been developed. Version 2 was documented in RFC Version 3 is documented in RFC The changes between versions 1, 2 and 3 are explained in Appendix 2 of [ 2 ].
All of the functionality that was present in the previous versions is present in version 4. BGP version 2 removed from the protocol the concept of "up", "down", and "horizontal" relations between autonomous systems that were present in version 1.
BGP version 2 introduced the concept of path attributes. In addition, BGP version 2 clarified parts of the protocol that were "under-specified". BGP version 4 redefines the previously class-based network layer reachability portion of the updates to specify prefixes of arbitrary length in order to represent multiple classful networks in a single entry as discussed in [ 5 ]. Possible applications of BGP in the Internet are documented in [ 3 ].
This Working Group has a mailing list, iwg ans. The Received Path Attribute Table contains all attributes received from all peers before local routing policy has been applied. The actual attributes used in determining a route are a subset of the received attribute table.
Security Considerations BGP provides flexible and extendible mechanism for authentication and security. The mechanism allows to support schemes with various degree of complexity. In addition, all BGP sessions are authenticated based on the autonomous system number advertised by a peer. As part of the BGP authentication mechanism, the protocol allows to carry encrypted digital signature in every BGP message. Implementations There are multiple independent interoperable implementations of BGP currently available.
This section gives a brief overview of the implementations that are currently used in the operational Internet.
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