BGP Commands to configure and monitor Border Gateway Protocol (BGP) | Cisco IOS IP and IP Routing Command Reference

BGP Commands Use the commands in this chapter to configure and monitor Border Gateway Protocol (BGP). For BGP configuration information and examples, refer to the “Configuring BGP” chapter of the Cisco IOS IP and IP Routing Configuration Guide. For multiprotocol BGP configuration information and examples, refer to the “Configuring Multiprotocol BGP Extensions for IP Multicast” chapter of the Cisco IOS IP and IP Routing Configuration Guide. For multiprotocol BGP command descriptions, refer to the “Multiprotocol BGP Extensions for IP Multicast Commands” chapter. aggregate-address P1R-584 Cisco IOS IP and IP Routing Command Reference To create an aggregate entry in a Border Gateway Protocol (BGP) or multiprotocol BGP database, use the aggregate-address command in address family or router configuration mode. To disable this function, use the no form of this command. aggregate-address address mask [as-set] [summary-only] [suppress-map map-name] [advertise-map map-name] [attribute-map map-name] no aggregate-address address mask [as-set] [summary-only] [suppress-map map-name] Syntax Description Defaults This command is disabled by default. Command Modes Address family configuration Router configuration Command History address Aggregate address. mask Aggregate mask. as-set (Optional) Generates autonomous system set path information. summary-only (Optional) Filters all more-specific routes from updates. suppress-map map-name (Optional) Name of the route map used to select the routes to be suppressed. advertise-map map-name (Optional) Name of the route map used to select the routes to create AS-SET origin communities. attribute-map map-name (Optional) Name of the route map used to set the attribute of the aggregate route. Release Modification 10.0 This command was introduced. 11.1(20)CC The nlri unicast, nlri multicast, and nlri unicast multicast keywords were added. 12.0(2)S The nlri unicast, nlri multicast, and nlri unicast multicast keywords were 12.0(7)T The nlri unicast, nlri multicast, and nlri unicast multicast keywords were removed. Address family configuration mode was added. P1R-585 Usage Guidelines You can implement aggregate routing in BGP and multiprotocol BGP either by redistributing an aggregate route into BGP or multiprotocol BGP, or by using this conditional aggregate routing feature. Using the aggregate-address command with no keywords will create an aggregate entry in the BGP or multiprotocol BGP routing table if any more-specific BGP or multiprotocol BGP routes are available that fall in the specified range. The aggregate route will be advertised as coming from your autonomous system and will have the atomic aggregate attribute set to show that information might be missing. (By default, the atomic aggregate attribute is set unless you specify the as-set keyword.) Using the as-set keyword creates an aggregate entry using the same rules that the command follows without this keyword, but the path advertised for this route will be an AS_SET consisting of all elements contained in all paths that are being summarized. Do not use this form of the aggregate-address command when aggregating many paths, because this route must be continually withdrawn and reupdated as autonomous system path reachability information for the summarized routes changes. Using the summary-only keyword not only creates the aggregate route (for example, 10.*.*.*) but also suppresses advertisements of more-specific routes to all neighbors. If you want to suppress only advertisements to certain neighbors, you may use the neighbor distribute-list command, with caution. If a more-specific route leaks out, all BGP or multiprotocol BGP routers will prefer that route over the less-specific aggregate you are generating (using longest-match routing). Using the suppress-map keyword creates the aggregate route but suppresses advertisement of specified routes. 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. Using the advertise-map keyword selects specific routes that will be used to build different components of the aggregate route, such as AS_SET or community. This form of the aggregate-address command is useful when the components of an aggregate are in separate autonomous systems and you want to create an aggregate with AS_SET, and advertise it back to some of the same autonomous systems. You must remember to omit the specific autonomous system numbers from the AS_SET to prevent the aggregate from being dropped by the BGP loop detection mechanism at the receiving router. IP access lists and autonomous system path access lists match clauses are supported. Using the attribute-map keyword allows attributes of the aggregate route to be changed. This form of the aggregate-address command is useful when one of the routes forming the AS_SET is configured with an attribute such as the community no-export attribute, which would prevent the aggregate route from being exported. An attribute map route map can be created to change the aggregate attributes. Examples In the following example, a BGP aggregate address is created in router configuration mode. The path advertised for this route will be an AS_SET consisting of all elements contained in all paths that are being summarized. router bgp 65000 aggregate-address 10.0.0.0 255.0.0.0 as-set In the following example, a multiprotocol BGP aggregate address is created in address family configuration mode and applied to the multicast database only using an IP Version 4 address family. More-specific routes are filtered from updates. address-family ipv4 multicast aggregate-address 10.0.0.0 255.0.0.0 summary-only P1R-586 In the following example, a route map called map-one is created matching on an as-path access list. The path advertised for this route will be an AS_SET consisting of elements contained in paths that are matched in the route map. ip as-path access-list 1 deny ^1234_ ip as-path access-list 1 permit .* route-map map-one match ip as-path 1 aggregate-address 10.0.0.0 255.0.0.0 as-set advertise-map map-one Related Commands Command Description address-family ipv4 Places the router in address family configuration mode for configuring routing sessions such as BGP, RIP, or static routing sessions that use standard IPv4 address prefixes. match ip address Distributes any routes that have a destination network number address that is permitted by a standard or extended access list, and performs policy routing on packets. route-map (IP) Defines the conditions for redistributing routes from one routing protocol into another, or enables policy routing. auto-summary (BGP) P1R-587 To restore the default behavior of automatic summarization of subnet routes into network-level routes, use the auto-summary command in address family or router configuration mode. To disable this feature and send subprefix routing information across classful network boundaries, use the no form of this command. auto-summary no auto-summary Syntax Description This command has no arguments or keywords. Defaults The behavior of this command is enabled by default (the software summarizes subprefixes to the classful network boundary when crossing classful network boundaries). Usage Guidelines Route summarization reduces the amount of routing information in the routing tables. By default, BGP does not accept subnets redistributed from Interior Gateway Protocol (IGP). To advertise and carry subnet routes in BGP, use an explicit network command or the no auto-summary command. If you disable auto-summarization and have not entered a network command, you will not advertise network routes for networks with subnet routes unless they contain a summary route. Examples In the following router configuration mode example, network numbers are not summarized automatically: router bgp 6 In the following address family configuration mode example, network numbers are not summarized address-family ipv4 unicast 12.0(7)T Address family configuration mode was added. P1R-588 address-family vpnv4 Places the router in address family configuration mode for sessions that use standard VPNv4 address prefixes. bgp always-compare-med P1R-589 To allow the comparison of the Multi Exit Discriminator (MED) for paths from neighbors in different autonomous systems, use the bgp always-compare-med command in router configuration mode. To disallow the comparison, use the no form of this command. no bgp always-compare-med Defaults The Cisco IOS software does not compare MEDs for paths from neighbors in different autonomous systems. Command Modes Router configuration Usage Guidelines The MED is one of the parameters that is considered when selecting the best path among many alternative paths. The path with a lower MED is preferred over a path with a higher MED. By default, during the best-path selection process, MED comparison is done only among paths from the same autonomous system. This command changes the default behavior by allowing comparison of MEDs among paths regardless of the autonomous system from which the paths are received. Examples The following example configures the BGP speaker in autonomous system 100 to compare MEDs among alternative paths, regardless of the autonomous system from which the paths are received: router bgp 109 nobgp bestpath as-path ignore no bgp bestpath as-path ignore The following example prevents the BGP router from considering as-path as a factor in choosing a route. 11.0 This command was introduced. 12.0 This command was introduced. P1R-590 Related Commandsr Command Description show ip bgp neighbors Displays information about the TCP and BGP connections to neighbors.