BGP—Selective Route Download

The BGP—Selective Route Download feature allows a network administrator to selectively download some or none of the BGP routes into the Routing Information Base (RIB). The primary application for this feature is to suppress the unnecessary downloading of certain BGP routes to the RIB or Forwarding Information Base (FIB) on a dedicated route reflector, which propagates BGP updates without carrying transit traffic. The feature thereby helps to maximize resources available and to improve routing scalability and convergence on the dedicated route reflector.

Information About BGP—Selective Route Download

Dedicated Route Reflector Does Not Need All Routes

The role of a dedicated route reflector (RR) is to propagate BGP updates without participating in the actual forwarding of transit traffic. That means the RR does not need to have all BGP routes downloaded into its RIB or FIB. It is beneficial for the RR to preserve its resources by not processing and storing those routes.

By default, BGP routes are downloaded to the RIB. To save resources on a dedicated route reflector, such downloading can be reduced or prevented by configuring a table map. A table map is so named because it controls what is put into the BGP routing table.

A table map references a route map, in this context to control the downloading of routes. A table map can be used in other features, such as the BGP Policy Accounting Output Interface Accounting feature.

It is important to understand the use of the filter keyword in the table-map command.

  • When the table-map command is used without the filter keyword, the route map referenced in the table-map command is used to set certain properties (such as the traffic index) of the routes for installation into the RIB. The route is always downloaded, regardless of whether it is permitted or denied by the route map.

  • When the table-map command is used with the filter keyword, the route map referenced is also used to control whether a BGP route is to be downloaded to the RIB (hence the filter). A BGP route is not downloaded to the RIB if it is denied by the route map.

Note that the Selective Route Download feature is not applicable to Multiprotocol Label Switching (MPLS) Layer 3 VPN because the route download is already automatically suppressed on a route reflector.

Benefits of Selective Route Download

The BGP—Selective Route Download feature allows a network administrator to selectively download some or none of the BGP routes into the Routing Information Base (RIB). The primary application for this feature is to suppress the unnecessary downloading of certain BGP routes to the RIB or Forwarding Information Base (FIB) on a dedicated route reflector, which propagates BGP updates without carrying transit traffic. The feature thereby helps to maximize resources available and to improve routing scalability and convergence on the dedicated route reflector.

How to Selectively Download BGP Routes

Suppressing the Downloading of All BGP Routes on a Dedicated RR

Perform this task on a dedicated route reflector (RR) to prevent all BGP routes from being downloaded to the RIB, and thereby save resources.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. route-map route-map-name deny [sequence-number]
  4. exit
  5. router bgp as-number
  6. address-family ipv4 unicast
  7. table-map route-map-name filter
  8. end
  9. clear ip bgp ipv4 unicast table-map

DETAILED STEPS

  Command or Action Purpose

Step 1

enable

Example:


Router> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Router# configure terminal

Enters global configuration mode.

Step 3

route-map route-map-name deny [sequence-number]

Example:


Router(config)# route-map bgp-to-rib deny 10

Enters route map configuration mode to configure a route map.

  • In this example, the route map named bgp-to-rib denies all routes.

Step 4

exit

Example:


Router(config-route-map)# exit

Exits route-map configuration mode and enters global configuration mode.

Step 5

router bgp as-number

Example:


Router(config)# router bgp 100

Enters router configuration mode and creates a BGP routing process.

Step 6

address-family ipv4 unicast

Example:


Router(config-router)# address-family ipv4 unicast

Enters address family configuration mode to configure BGP peers to accept address-family-specific configurations.

Step 7

table-map route-map-name filter

Example:


Router(config-router-af)# table-map bgp-to-rib filter

Specifies a route map that filters what goes into the BGP routing table (the Routing Information Base [RIB]).

  • The routes that are permitted by the route map are downloaded into the RIB.
  • The routes that are denied by the route map are filtered from (not downloaded into) the RIB.

Step 8

end

Example:


Router(config-router-af)# end

Exits address family configuration mode and enters privileged EXEC mode.

Step 9

clear ip bgp ipv4 unicast table-map

Example:


Router# clear ip bgp ipv4 unicast table-map

Reloads the BGP RIB after the table map or the route map is configured or changed in order to put the changes into effect.

Selectively Downloading BGP Routes on a Dedicated RR

Perform this task on a dedicated route reflector (RR) to selectively download BGP routes to the RIB. When the externally connected routes are carried in BGP, it is necessary to download these routes to the RIB for next hop resolution on the RR. One scalable approach to accomplish the selective route download is to use a BGP community to identify the externally connected routes. That is, attach a designated BGP community during the redistribution of the externally connected routes on the ASBRs, and then on the RR, filter the route download based on the BGP community. This task illustrates the configuration of the RR using a route map that matches on a community list to control which routes are downloaded.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. ip community-list standard-list-number permit AA:NN
  4. route-map route-map-name permit [sequence-number]
  5. match community standard-list-number
  6. exit
  7. router bgp as-number
  8. address-family ipv4 unicast
  9. table-map route-map-name filter
  10. end
  11. clear ip bgp ipv4 unicast table-map

DETAILED STEPS

  Command or Action Purpose

Step 1

enable

Example:


Router> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Router# configure terminal

Enters global configuration mode.

Step 3

ip community-list standard-list-number permit AA:NN

Example:


Router(config)# ip community-list 100 permit 65510:100

Creates a standard community list and specifies an autonomous system and network number allowed in the community list.

Step 4

route-map route-map-name permit [sequence-number]

Example:


Router(config)# route-map bgp-to-rib permit 10

Enters route-map configuration mode to configure a route map.

  • The route map named bgp-to-rib permits routes that match the community list identified in the next step.

Step 5

match community standard-list-number

Example:


Router(config-route-map)# match community 100

Matches on routes that are permitted by community list 100.

Step 6

exit

Example:


Router(config-route-map)# exit

Exits route-map configuration mode and enters global configuration mode.

Step 7

router bgp as-number

Example:


Router(config)# router bgp 65510

Enters router configuration mode and creates a BGP routing process.

Step 8

address-family ipv4 unicast

Example:


Router(config-router)# address-family ipv4 unicast

Enters address family configuration mode to configure BGP peers to accept address family-specific configurations.

Step 9

table-map route-map-name filter

Example:


Router(config-router-af)# table-map bgp-to-rib filter

Specifies a route map that filters what goes into the BGP routing table (the Routing Information Base [RIB]).

Step 10

end

Example:


Router(config-router-af)# end

Exits address family configuration mode and enters privileged EXEC mode.

Step 11

clear ip bgp ipv4 unicast table-map

Example:


Router# clear ip bgp ipv4 unicast table-map

Reloads the BGP RIB after the table map or the route map is configured or changed in order to put the changes into effect.

Configuration Examples for BGP—Selective Route Download

Examples: Selective Route Download

The role of a dedicated route reflector (RR) is to propagate BGP updates without participating in the actual forwarding of transit traffic. In some cases, the dedicated RR may need only selected routes downloaded; in some cases it may not need any routes downloaded.

It is likely that the dedicated RR would have the overload bit set if the IS-IS routing protocol is being used, or an OSPF stub router would be configured if OSPF is being used.

Example: Next Hop is Loopback Address—Filter All Routes From Being Downloaded

In this example, the ASBRs are configured with the next-hop-self command for iBGP sessions. (That configuration is not shown). The next hops of the BGP routes advertised to iBGP sessions are the loopback addresses carried in the IGP (either OSPF or IS-IS). There is no need to download any BGP routes to the RIB. The following configuration on the dedicated RR suppresses the downloading of all BGP routes because the table map command includes the filter keyword, and the route map that the table map references denies all routes. 


route-map bgp-to-rib deny 10
!
router bgp 65000
 address-family ipv6 unicast
 table-map bgp-to-rib filter

Example: Redistribution of Connected Routes in IGP—Filter All Routes From Being Downloaded

In this example, the next hops of the BGP routes are resolved on the externally connected routes, which are carried in an IGP, such as OSPF or IS-IS, via a prefix-list-based selective redistribution of the connected routes. The routes are received from iBGP.

Although the scenario is different from the preceding example, the configuration is the same. The following configuration on the dedicated RR suppresses the downloading of all BGP routes because the table map command includes the filter keyword, and the route map that the table map references denies all routes. 


route-map bgp-to-rib deny 10
!
router bgp 65000
 address-family ipv6 unicast
 table-map bgp-to-rib filter

Example: Redistribution of Connected Routes in BGP—Selectively Filter Routes From Being Downloaded

When the externally connected routes are carried in BGP, it is necessary to download these routes to the RIB, where the nexthop resolution on the RR can be calculated. One scalable way to achieve the selective route download is to use a BGP community on the ASBR to identify these externally connected routes. That is, on the border routers, attach a designated BGP community during the redistribution of the externally connected routes, and then on the RR, filter the route download based on the BGP community. The following shows the configuration on the ASBR and the configuration on the RR.

ASBR Configuration


router bgp 65510
 address-family ipv4 unicast
 redistribute connected route-map connected-to-bgp
!
route-map connected-to-bgp permit 10
 match ip address prefix-list extend-connected
 set community 65510:100
!
ip prefix-list extend-connected permit 192.168.1.1/30

RR Configuration


ip community-list 100 permit 65510:100
!
route-map bgp-to-rib permit 10
 match community 100
!
router bgp 65510
 address-family ipv4 unicast
 table-map bgp-to-rib filter

Additional References for Selective Route Download

Related Documents

Related Topic Document Title

Cisco IOS Commands

Cisco IOS Master Commands List, All Releases

BGP Commands

Cisco IOS IP Routing: BGP Command Reference

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Feature Information for Selective Route Download

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.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 1. Feature Information for Selective Route Download

Feature Name

Releases

Feature Information

Selective Route Download

Cisco IOS XE Release 2.3S

The BGP—Selective Route Download feature allows a network administrator to selectively download some or none of the BGP routes into the Routing Information Base (RIB). The primary application for this feature is to suppress the unnecessary downloading of certain BGP routes to the RIB or Forwarding Information Base (FIB) on a dedicated route reflector, which propagates BGP updates without carrying transit traffic. The feature thereby helps to maximize resources available and to improve routing scalability and convergence on the dedicated route reflector.

The following command was modified:

  • table-map