iBGP Multipath Load Sharing

Last Updated: November 2, 2011

This feature module describes the iBGP Multipath Load Sharing feature. This feature enables the BGP speaking router to select multiple iBGP paths as the best paths to a destination. The best paths or multipaths are then installed in the IP routing table of the router.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see 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 document.

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.

iBGP Multipath Load Sharing Overview

When a Border Gateway Protocol (BGP) speaking router with no local policy configured receives multiple network layer reachability information (NLRI) from the internal BGP (iBGP) for the same destination, the router will choose one iBGP path as the best path. The best path is then installed in the IP routing table of the router. For example, in the figure below, although there are three paths to autonomous system 200, Router 2 determines that one of the paths to autonomous system 200 is the best path and uses this path only to reach autonomous system 200.

Figure 1 Non-MPLS Topology with One Best Path


The iBGP Multipath Load Sharing feature enables the BGP speaking router to select multiple iBGP paths as the best paths to a destination. The best paths or multipaths are then installed in the IP routing table of the router. For example, on router 2 in the figure below, the paths to routers 3, 4, and 5 are configured as multipaths and can be used to reach autonomous system 200, thereby equally sharing the load to autonomous system 200.

Figure 2 Non-MPLS Topology with Three Multipaths


The iBGP Multipath Load Sharing feature functions similarly in a Multiprotocol Label Switching (MPLS) Virtual Private Network (VPN) with a service provider backbone. For example, on router PE1 in the figure below, the paths to routers PE2, PE3, and PE4 can be selected as multipaths and can be used to equally share the load to site 2.

Figure 3 MPLS VPN with Three Multipaths


For multiple paths to the same destination to be considered as multipaths, the following criteria must be met:

  • All attributes must be the same. The attributes include weight, local preference, autonomous system path (entire attribute and not just length), origin code, Multi Exit Discriminator (MED), and Interior Gateway Protocol (IGP) distance.
  • The next hop router for each multipath must be different.

Even if the criteria are met and multiple paths are considered multipaths, the BGP speaking router will still designate one of the multipaths as the best path and advertise this best path to its neighbors.

Benefits of iBGP Multipath Load Sharing

Configuring multiple iBGP best paths enables a router to evenly share the traffic destined for a particular site.

Restrictions on iBGP Multipath Load Sharing

Route Reflector Limitation

With multiple iBGP paths installed in a routing table, a route reflector will advertise only one of the paths (one next hop).

Memory Consumption Restriction

Each IP routing table entry for a BGP prefix that has multiple iBGP paths uses approximately 350 bytes of additional memory. We recommend not using this feature on a router with a low amount of available memory and especially when the router is carrying a full Internet routing table.

How To Configure iBGP Multipath Load Sharing

Configuring iBGP Multipath Load Sharing

To configure the iBGP Multipath Load Sharing feature, use the following command in router configuration mode:

Command

Purpose

 
Router(config-router)# maximum-paths ibgp maximum-number

Controls the maximum number of parallel iBGP routes that can be installed in a routing table.

Verifying iBGP Multipath Load Sharing

To verify that the iBGP Multipath Load Sharing feature is configured correctly, perform the following steps:

SUMMARY STEPS

1.    Enter the show ip bgp network-number EXEC command to display attributes for a network in a non-MPLS topology, or the show ip bgp vpnv4 all ip-prefixEXEC commandtodisplay attributes for a network in an MPLS VPN:

2.    In the display resulting from the show ip bgp network-number EXEC command or the show ip bgp vpnv4 all ip-prefixEXEC command, verify that the intended multipaths are marked as "multipaths." Notice that one of the multipaths is marked as "best."

3.    Enter the show ip route ip-address EXEC command to display routing information for a network in a non-MPLS topology or the show ip route vrf vrf-name ip-prefix EXEC command to display routing information for a network in an MPLS VPN:

4.    Verify that the paths marked as "multipath" in the display resulting from the show ip bgp ip-prefixEXEC command or the show ip bgp vpnv4 all ip-prefix EXEC command are included in the routing information. (The routing information is displayed after performing Step 3.)


DETAILED STEPS
Step 1   Enter the show ip bgp network-number EXEC command to display attributes for a network in a non-MPLS topology, or the show ip bgp vpnv4 all ip-prefixEXEC commandtodisplay attributes for a network in an MPLS VPN:

Example: 
Router# show ip bgp 10.22.22.0
BGP routing table entry for 10.22.22.0/24, version 119
Paths:(6 available, best #1)
Multipath:iBGP
Flag:0x820
  Advertised to non peer-group peers:
  10.1.12.12 
  22
    10.2.3.8 (metric 11) from 10.1.3.4 (100.0.0.5)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath, best
      Originator:100.0.0.5, Cluster list:100.0.0.4
  22
    10.2.1.9 (metric 11) from 10.1.1.2 (100.0.0.9)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath
      Originator:100.0.0.9, Cluster list:100.0.0.2
  22
    10.2.5.10 (metric 11) from 10.1.5.6 (100.0.0.10)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath
      Originator:100.0.0.10, Cluster list:100.0.0.6
  22
    10.2.4.10 (metric 11) from 10.1.4.5 (100.0.0.10)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath
      Originator:100.0.0.10, Cluster list:100.0.0.5
  22
    10.2.6.10 (metric 11) from 10.1.6.7 (100.0.0.10)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath
      Originator:100.0.0.10, Cluster list:100.0.0.7
Router# show ip bgp vpnv4 all 10.22.22.0
BGP routing table entry for 100:1:10.22.22.0/24, version 50
Paths:(6 available, best #1)
Multipath:iBGP
  Advertised to non peer-group peers:
  200.1.12.12 
  22
    10.22.7.8 (metric 11) from 10.11.3.4 (100.0.0.8)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath, best
      Extended Community:RT:100:1
      Originator:100.0.0.8, Cluster list:100.1.1.44
  22
    10.22.1.9 (metric 11) from 10.11.1.2 (100.0.0.9)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath
      Extended Community:RT:100:1
      Originator:100.0.0.9, Cluster list:100.1.1.22
  22
    10.22.6.10 (metric 11) from 10.11.6.7 (100.0.0.10)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath
      Extended Community:RT:100:1
      Originator:100.0.0.10, Cluster list:100.0.0.7
  22
    10.22.4.10 (metric 11) from 10.11.4.5 (100.0.0.10)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath
      Extended Community:RT:100:1
      Originator:100.0.0.10, Cluster list:100.0.0.5
  22
    10.22.5.10 (metric 11) from 10.11.5.6 (100.0.0.10)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath
      Extended Community:RT:100:1
      Originator:100.0.0.10, Cluster list:100.0.0.6
Step 2   In the display resulting from the show ip bgp network-number EXEC command or the show ip bgp vpnv4 all ip-prefixEXEC command, verify that the intended multipaths are marked as "multipaths." Notice that one of the multipaths is marked as "best."
Step 3   Enter the show ip route ip-address EXEC command to display routing information for a network in a non-MPLS topology or the show ip route vrf vrf-name ip-prefix EXEC command to display routing information for a network in an MPLS VPN:

Example: 
Router# show ip route 10.22.22.0
Routing entry for 10.22.22.0/24
  Known via "bgp 1", distance 200, metric 0
  Tag 22, type internal
  Last update from 10.2.6.10 00:00:03 ago
  Routing Descriptor Blocks:
  * 10.2.3.8, from 10.1.3.4, 00:00:03 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1
    10.2.1.9, from 10.1.1.2, 00:00:03 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1
    10.2.5.10, from 10.1.5.6, 00:00:03 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1
    10.2.4.10, from 10.1.4.5, 00:00:03 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1
    10.2.6.10, from 10.1.6.7, 00:00:03 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1
Router# show ip route vrf PATH 10.22.22.0
Routing entry for 10.22.22.0/24
  Known via "bgp 1", distance 200, metric 0
  Tag 22, type internal
  Last update from 10.22.5.10 00:01:07 ago
  Routing Descriptor Blocks:
  * 10.22.7.8 (Default-IP-Routing-Table), from 10.11.3.4, 00:01:07 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1
    10.22.1.9 (Default-IP-Routing-Table), from 10.11.1.2, 00:01:07 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1
    10.22.6.10 (Default-IP-Routing-Table), from 10.11.6.7, 00:01:07 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1
    10.22.4.10 (Default-IP-Routing-Table), from 10.11.4.5, 00:01:07 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1
    10.22.5.10 (Default-IP-Routing-Table), from 10.11.5.6, 00:01:07 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1
Step 4   Verify that the paths marked as "multipath" in the display resulting from the show ip bgp ip-prefixEXEC command or the show ip bgp vpnv4 all ip-prefix EXEC command are included in the routing information. (The routing information is displayed after performing Step 3.)

Monitoring and Maintaining iBGP Multipath Load Sharing

To display iBGP Multipath Load Sharing information, use the following commands in EXEC mode, as needed:

Command

Purpose

 
Router# show ip bgp ip-prefix

Displays attributes and multipaths for a network in a non-MPLS topology. 

Router# show ip bgp vpnv4 all  ip-prefix

Displays attributes and multipaths for a network in an MPLS VPN.

 
Router# show ip route ip-prefix

Displays routing information for a network in a non-MPLS topology.

 
Router# show ip route vrf  vrf-name ip-prefix

Displays routing information for a network in an MPLS VPN.

Configuration Examples

Both examples assume that the appropriate attributes for each path are equal and that the next hop router for each multipath is different.

Example iBGP Multipath Load Sharing in a Non-MPLS Topology

The following example shows how to set up the iBGP Multipath Load Sharing feature in a non-MPLS topology (see the figure below).

Figure 4 Non-MPLS Topology Example


Router 2 Configuration

router bgp 100
maximum-paths ibgp 3

Example iBGP Multipath Load Sharing in an MPLS VPN Topology

The following example shows how to set up the iBGP Multipath Load Sharing feature in an MPLS VPN topology (see the figure below).

Figure 5 MPLS VPN Topology Example


Router PE1 Configuration

router bgp 100
address-family ipv4 unicast vrf site2
 maximum-paths ibgp 3

Additional References

The following sections provide references related to the iBGP Multipath Load Sharing feature.

Related Documents

Related Topic

Document Title

BGP multipath load sharing for both eBGP and iBGP in an MPLS-VPN

"BGP Multipath Load Sharing for Both eBGP and iBGP in an MPLS-VPN"

Advertising the bandwidth of an autonomous system exit link as an extended community

" BGP Link Bandwidth"

BGP commands

Cisco IOS IP Routing: BGP Command Reference

Cisco IOS master command list, all releases

Cisco IOS Master Command List, All Releases

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

MIBs

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 locate and download MIBs for selected platforms, Cisco IOS XE software releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs

RFCs

RFC

Title

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

--

Technical Assistance

Description

Link

The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies.

To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds.

Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.

http://www.cisco.com/techsupport

Feature Information for iBGP Multipath Load Sharing

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 iBGP Multipath Load Sharing

Feature Name

Releases

Feature Information

iBGP multipath load sharing

Cisco IOS XE Release 2.1

This feature was introduced on the Cisco ASR 1000 Series Routers.

The following commands were modified by this feature: maximum paths ibgp, show ip bgp, show ip bgp vpnv4, show ip route, show ip route vrf.

Cisco and the Cisco Logo are trademarks of Cisco Systems, Inc. and/or its affiliates in the U.S. and other countries. A listing of Cisco's trademarks can be found at www.cisco.com/go/trademarks. Third party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1005R)

Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.

© 2011 Cisco Systems, Inc. All rights reserved.