IP Routing: BGP Configuration Guide, Cisco IOS Release 15M&T
BGP Link Bandwidth
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BGP Link Bandwidth

The BGP (Border Gateway Protocol) Link Bandwidth feature is used to advertise the bandwidth of an autonomous system exit link as an extended community. This feature is configured for links between directly connected external BGP (eBGP) neighbors. The link bandwidth extended community attribute is propagated to iBGP peers when extended community exchange is enabled. This feature is used with BGP multipath features to configure load balancing over links with unequal bandwidth.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.

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.

Prerequisites for BGP Link Bandwidth

  • BGP load balancing or multipath load balancing must be configured before BGP Link Bandwidth feature is enabled.
  • BGP extended community exchange must be enabled between iBGP neighbors to which the link bandwidth attribute is to be advertised.
  • Cisco Express Forwarding or distributed Cisco Express Forwarding must be enabled on all participating routers.

Restrictions for BGP Link Bandwidth

  • The BGP Link Bandwidth feature can be configured only under IPv4 and VPNv4 address family sessions.
  • BGP can originate the link bandwidth community only for directly connected links to eBGP neighbors.
  • Both iBGP and eBGP load balancing are supported in IPv4 and VPNv4 address families. However, eiBGP load balancing is supported only in VPNv4 address families.

Information About BGP Link Bandwidth

BGP Link Bandwidth Overview

The BGP Link Bandwidth feature is used to enable multipath load balancing for external links with unequal bandwidth capacity. This feature is enabled under an IPv4 or VPNv4 address family session by entering the bgp dmzlink-bw command. This feature supports iBGP, eBGP multipath load balancing, and eiBGP multipath load balancing in Multiprotocol Label Switching (MPLS) VPNs. When this feature is enabled, routes learned from directly connected external neighbor are propagated through the internal BGP (iBGP) network with the bandwidth of the source external link.

The link bandwidth extended community indicates the preference of an autonomous system exit link in terms of bandwidth. This extended community is applied to external links between directly connected eBGP peers by entering the neighbor dmzlink-bw command. The link bandwidth extended community attribute is propagated to iBGP peers when extended community exchange is enabled with the neighbor send-community command.

Link Bandwidth Extended Community Attribute

The link bandwidth extended community attribute is a 4-byte value that is configured for a link on the demilitarized zone (DMZ) interface that connects two single hop eBGP peers. The link bandwidth extended community attribute is used as a traffic sharing value relative to other paths while traffic is being forwarded. Two paths are designated as equal for load balancing if the weight, local-pref, as-path length, Multi Exit Discriminator (MED), and Interior Gateway Protocol (IGP) costs are the same.

Benefits of the BGP Link Bandwidth Feature

The BGP Link Bandwidth feature allows BGP to be configured to send traffic over multiple iBGP or eBGP learned paths where the traffic that is sent is proportional to the bandwidth of the links that are used to exit the autonomous system. The configuration of this feature can be used with eBGP and iBGP multipath features to enable unequal cost load balancing over multiple links. Unequal cost load balancing over links with unequal bandwidth was not possible in BGP before the BGP Link Bandwidth feature was introduced.

How to Configure BGP Link Bandwidth

Configuring and Verifying BGP Link Bandwidth

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    router bgp autonomous-system-number

    4.    address-family ipv4

    5.    address-family ipv4 [mdt | multicast | unicast [vrf vrf-name] | vrf vrf-name]

    6.    bgp dmzlink-bw

    7.    neighbor ip-address dmzlink-bw

    8.    neighbor ip-address send-community [both | extended | standard]

    9.    end

    10.    show ip bgp ip-address [longer-prefixes [injected] | shorter-prefixes [mask-length]]

    11.    show ip route ip-address [mask] [longer-prefixes] | protocol [process-id] | [list access-list-number | access-list-name] | static download]


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example:
    Device> enable
     

    Enables higher privilege levels, such as privileged EXEC mode.

    • Enter your password if prompted.
     
    Step 2 configure terminal


    Example:
    Device# configure terminal
     

    Enters global configuration mode.

     
    Step 3 router bgp autonomous-system-number


    Example:
    Device(config)# router bgp 50000
     

    Enters router configuration mode to create or configure a BGP routing process.

     
    Step 4 address-family ipv4


    Example:
    Device(config-router-af)# address-family ipv4
     

    Enters address family configuration mode.

     
    Step 5 address-family ipv4 [mdt | multicast | unicast [vrf vrf-name] | vrf vrf-name]


    Example:
    Device(config-router)# address-family ipv4
     

    The BGP Link Bandwidth feature is supported only under the IPv4 and VPNv4 address families.

     
    Step 6 bgp dmzlink-bw


    Example:
    Router(config-router-af)# bgp dmzlink-bw
     

    Configures BGP to distribute traffic proportionally to the bandwidth of the link.

    • This command must be entered on each router that contains an external interface that is to be used for multipath load balancing.
     
    Step 7 neighbor ip-address dmzlink-bw


    Example:
    Device(config-router-af)# neighbor 172.16.1.1 dmzlink-bw
     

    Configures BGP to include the link bandwidth attribute for routes learned from the external interface specified IP address.

    • This command must be configured for each eBGP link that is to be configured as a multipath. Enabling this command allows the bandwidth of the external link to be propagated through the link bandwidth extended community.
     
    Step 8 neighbor ip-address send-community [both | extended | standard]


    Example:
    Device(config-router-af)# neighbor 10.10.10.1 send-community extended
     

    (Optional) Enables community or extended community exchange with the specified neighbor.

    • This command must be configured for iBGP peers to which the link bandwidth extended community attribute is to be propagated.
     
    Step 9 end


    Example:
    Device(config-router-af)# end
     

    Exits address family configuration mode, and enters privileged EXEC mode.

     
    Step 10 show ip bgp ip-address [longer-prefixes [injected] | shorter-prefixes [mask-length]]


    Example:
    Device# show ip bgp 10.0.0.0
     

    (Optional) Displays information about the TCP and BGP connections to neighbors.

    • The output displays the status of the link bandwidth configuration. The bandwidth of the link is shown in kilobytes.
     
    Step 11 show ip route ip-address [mask] [longer-prefixes] | protocol [process-id] | [list access-list-number | access-list-name] | static download]


    Example:
    Device# show ip route 10.0.0.0
     

    (Optional) Displays the current state of the routing table.

    • The output displays traffic share values, including the weights of the links that are used to direct traffic proportionally to the bandwidth of each link.
     

    Configuration Examples for BGP Link Bandwidth

    Example: BGP Link Bandwidth Configuration

    In the following examples, the BGP Link Bandwidth feature is configured so BGP will distribute traffic proportionally to the bandwidth of each external link. The figure below shows two external autonomous systems connected by three links that each carry a different amount of bandwidth (unequal cost links). Multipath load balancing is enabled and traffic is balanced proportionally.


    Note

    The BGP Link Bandwidth feature functions for simple topologies that have a single path toward the exit points.


    Caution

    The BGP Link Bandwidth feature might not function properly if load balancing is required toward the exit points.

    Figure 1. BGP Link Bandwidth Configuration

    Router A Configuration

    In the following example, Router A is configured to support iBGP multipath load balancing and to exchange the BGP extended community attribute with iBGP neighbors: 

    RouterA(config)# router bgp 100
 
RouterA(config-router)# neighbor 10.10.10.2 remote-as 100
 
RouterA(config-router)# neighbor 10.10.10.2 update-source Loopback 0
 
RouterA(config-router)# neighbor 10.10.10.3 remote-as 100
 
RouterA(config-router)# neighbor 10.10.10.3 update-source Loopback 0
 
RouterA(config-router)# address-family ipv4
 
RouterA(config-router-af)# bgp dmzlink-bw 
RouterA(config-router-af)# neighbor 10.10.10.2 activate
 
RouterA(config-router-af)# neighbor 10.10.10.2 send-community both
 
RouterA(config-router-af)# neighbor 10.10.10.3 activate
 
RouterA(config-router-af)# neighbor 10.10.10.3 send-community both
 
RouterA(config-router-af)# maximum-paths ibgp 6

    Router B Configuration

    In the following example Router B is configured to support multipath load balancing, to distribute Router D and Router E link traffic proportionally to the bandwidth of each link, and to advertise the bandwidth of these links to iBGP neighbors as an extended community: 

    RouterB(config)# router bgp 100
 
RouterB(config-router)# neighbor 10.10.10.1 remote-as 100
 
RouterB(config-router)# neighbor 10.10.10.1 update-source Loopback 0
 
RouterB(config-router)# neighbor 10.10.10.3 remote-as 100
 
RouterB(config-router)# neighbor 10.10.10.3 update-source Loopback 0 
RouterB(config-router)# neighbor 172.16.1.1 remote-as 200
 
RouterB(config-router)# neighbor 172.16.1.1 ebgp-multihop 1 
RouterB(config-router)# neighbor 172.16.2.2 remote-as 200
 
RouterB(config-router)# neighbor 172.16.2.2 ebgp-multihop 1 
RouterB(config-router)# address-family ipv4
 
RouterB(config-router-af)# bgp dmzlink-bw
 
RouterB(config-router-af)# neighbor 10.10.10.1 activate
 
RouterB(config-router-af)# neighbor 10.10.10.1 next-hop-self
 
RouterB(config-router-af)# neighbor 10.10.10.1 send-community both
 
RouterB(config-router-af)# neighbor 10.10.10.3 activate
 
RouterB(config-router-af)# neighbor 10.10.10.3 next-hop-self
 
RouterB(config-router-af)# neighbor 10.10.10.3 send-community both
 
RouterB(config-router-af)# neighbor 172.16.1.1
 activate 
RouterB(config-router-af)# neighbor 172.16.1.1 dmzlink-bw
 
RouterB(config-router-af)# neighbor 172.16.2.2 activate 
RouterB(config-router-af)# neighbor 172.16.2.2 dmzlink-bw
RouterB(config-router-af)# maximum-paths ibgp 6
RouterB(config-router-af)# maximum-paths 6

    Router C Configuration

    In the following example Router C is configured to support multipath load balancing and to advertise the bandwidth of the link with Router E to iBGP neighbors as an extended community: 

    RouterC(config)# router bgp 100
RouterC(config-router)# neighbor 10.10.10.1 remote-as 100
RouterC(config-router)# neighbor 10.10.10.1 update-source Loopback 0
RouterC(config-router)# neighbor 10.10.10.2 remote-as 100
RouterC(config-router)# neighbor 10.10.10.2 update-source Loopback 0
RouterC(config-router)# neighbor 172.16.3.30 remote-as 200
RouterC(config-router)# neighbor 172.16.3.30 ebgp-multihop 1
RouterC(config-router)# address-family ipv4 
RouterC(config-router-af)# bgp dmzlink-bw
 
RouterC(config-router-af)# neighbor 10.10.10.1 activate
RouterC(config-router-af)# neighbor 10.10.10.1 send-community both
RouterC(config-router-af)# neighbor 10.10.10.1 next-hop-self
RouterC(config-router-af)# neighbor 10.10.10.2 activate 
RouterC(config-router-af)# neighbor 10.10.10.2 send-community both
RouterC(config-router-af)# neighbor 10.10.10.2 next-hop-self 
RouterC(config-router-af)# neighbor 172.16.3.3 activate 
RouterC(config-router-af)# neighbor 172.16.3.3 dmzlink-bw
 
RouterC(config-router-af)# maximum-paths ibgp 6
RouterC(config-router-af)# maximum-paths 6

    Example: Verifying BGP Link Bandwidth

    The examples in this section show the verification of this feature on Router A, Router B, and Router C.

    Router B

    In the following example, the show ip bgp command is entered on Router B to verify that two unequal cost best paths have been installed into the BGP routing table. The bandwidth for each link is displayed with each route. 

    RouterB# show ip bgp 192.168.1.0

BGP routing table entry for 192.168.1.0/24, version 48
Paths: (2 available, best #2)
Multipath: eBGP
  Advertised to update-groups:
     1          2
  200
    172.16.1.1 from 172.16.1.2 (192.168.1.1)
      Origin incomplete, metric 0, localpref 100, valid, external, multipath, best
      Extended Community: 0x0:0:0
      DMZ-Link Bw 278 kbytes
  200
    172.16.2.2 from 172.16.2.2 (192.168.1.1)
      Origin incomplete, metric 0, localpref 100, valid, external, multipath, best
      Extended Community: 0x0:0:0
      DMZ-Link Bw 625 kbytes

    Router A

    In the following example, the show ip bgp command is entered on Router A to verify that the link bandwidth extended community has been propagated through the iBGP network to Router A. Exit links are located on Router B and Router C. The output shows that a route for each exit link to autonomous system 200 has been installed as a best path in the BGP routing table. 

    RouterA# show ip bgp 192.168.1.0

BGP routing table entry for 192.168.1.0/24, version 48
Paths: (3 available, best #3)
Multipath: eBGP
  Advertised to update-groups:
     1          2
  200
    172.16.1.1 from 172.16.1.2 (192.168.1.1)
      Origin incomplete, metric 0, localpref 100, valid, external, multipath
      Extended Community: 0x0:0:0
      DMZ-Link Bw 278 kbytes
  200
    172.16.2.2 from 172.16.2.2 (192.168.1.1)
      Origin incomplete, metric 0, localpref 100, valid, external, multipath, best
      Extended Community: 0x0:0:0
      DMZ-Link Bw 625 kbytes
  200
    172.16.3.3 from 172.16.3.3 (192.168.1.1)
      Origin incomplete, metric 0, localpref 100, valid, external, multipath, best
      Extended Community: 0x0:0:0
      DMZ-Link Bw 2500 kbytes

    Router A

    In the following example, the show ip route command is entered on Router A to verify the multipath routes that are advertised and the associated traffic share values: 

    RouterA# show ip route 192.168.1.0 

Routing entry for 192.168.1.0/24 
  Known via "bgp 100", distance 200, metric 0 
  Tag 200, type internal 
  Last update from 172.168.1.1 00:01:43 ago 
  Routing Descriptor Blocks: 
  * 172.168.1.1, from 172.168.1.1, 00:01:43 ago 
      Route metric is 0, traffic share count is 13 
      AS Hops 1, BGP network version 0 
      Route tag 200 
    172.168.2.2, from 172.168.2.2, 00:01:43 ago 
      Route metric is 0, traffic share count is 30 
      AS Hops 1, BGP network version 0 
      Route tag 200 
    172.168.3.3, from 172.168.3.3, 00:01:43 ago 
      Route metric is 0, traffic share count is 120 
      AS Hops 1, BGP network version 0 
      Route tag 200

    Where to Go Next

    For information about the BGP Multipath Load Sharing for Both eBGP and iBGP in an MPLS-VPN feature, see the “BGP Multipath Load Sharing for Both eBGP and iBGP in an MPLS-VPN” module in the IP Routing: BGP Configuration Guide.

    For information about the iBGP Multipath Load Sharing feature, see the “iBGP Multipath Load Sharing” module in the IP Routing: BGP Configuration Guide .

    Additional References

    Related Documents

    Related Topic

    Document Title

    BGP commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples

    Cisco IOS IP Routing: BGP Command Reference

    CEF configuration tasks

    IP Switching Cisco Express Forwarding Configuration Guide

    Standards

    Standard

    Title

    No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.

    —

    MIBs

    MIB

    MIBs Link

    No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature.

    To obtain lists of supported MIBs by platform and Cisco IOS release, and to download MIB modules, go to the Cisco MIB website on Cisco.com at the following URL:

    http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml

    RFCs

    RFC

    Title

    draft-ramachandra-bgp-ext-communities-09.txt

    BGP Extended Communities Attribute

    Technical Assistance

    Description

    Link

    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.com user ID and password.

    http:/​/​www.cisco.com/​cisco/​web/​support/​index.html

    Feature Information for BGP Link Bandwidth

    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 BGP Link Bandwidth

    Feature Name

    Releases

    Feature Information

    BGP Link Bandwidth

    12.2(2)T

    12.2(14)S

    This feature advertises the bandwidth of an autonomous system exit link as an extended community. The link bandwidth extended community attribute is propagated to iBGP peers when extended community exchange is enabled.

    The following commands were introduced or modified: router bgp, address-family ipv4, address-family ipv4, bgp dmzlink-bw, neighbor, show ip bgp, show ip route.