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iBGP Multipath Load Sharing
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iBGP Multipath Load Sharing

Table Of Contents

iBGP Multipath Load Sharing

Finding Feature Information

Contents

iBGP Multipath Load Sharing Overview

Benefits of iBGP Multipath Load Sharing

Restrictions on iBGP Multipath Load Sharing

How To Configure iBGP Multipath Load Sharing

Configuring iBGP Multipath Load Sharing

Verifying iBGP Multipath Load Sharing

Monitoring and Maintaining iBGP Multipath Load Sharing

Configuration Examples

Example: iBGP Multipath Load Sharing in a Non-MPLS Topology

Example: iBGP Multipath Load Sharing in an MPLS VPN Topology

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Feature Information for iBGP Multipath Load Sharing


iBGP Multipath Load Sharing


First Published: 2001
Last Updated: May 4, 2009

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

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 for iBGP Multipath Load Sharing" section.

Use Cisco Feature Navigator to find information about platform support and Cisco IOS XE software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents

iBGP Multipath Load Sharing Overview

How To Configure iBGP Multipath Load Sharing

Monitoring and Maintaining iBGP Multipath Load Sharing

Configuration Examples

Additional References

Feature Information for iBGP Multipath Load Sharing

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 Figure 1, 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 Figure 2, 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 Figure 3, 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 (required)

Verifying iBGP Multipath Load Sharing (optional)

Monitoring and Maintaining 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:


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-prefix EXEC command to display attributes for a network in an MPLS VPN:

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:10.0.0.5, Cluster list:10.0.0.4
  22
    10.2.1.9 (metric 11) from 10.1.1.2 (10.0.0.9)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath
      Originator:10.0.0.9, Cluster list:10.0.0.2
  22
    10.2.5.10 (metric 11) from 10.1.5.6 (10.0.0.10)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath
      Originator:10.0.0.10, Cluster list:10.0.0.6
  22
    10.2.4.10 (metric 11) from 10.1.4.5 (10.0.0.10)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath
      Originator:10.0.0.10, Cluster list:10.0.0.5
  22
    10.2.6.10 (metric 11) from 10.1.6.7 (10.0.0.10)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath
      Originator:10.0.0.10, Cluster list:10.0.0.7

Router# show ip bgp vpnv4 all 10.22.22.0

BGP routing table entry for 10:1:10.22.22.0/24, version 50
Paths:(6 available, best #1)
Multipath:iBGP
  Advertised to non peer-group peers:
  10.1.12.12 
  22
    10.22.7.8 (metric 11) from 10.11.3.4 (10.0.0.8)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath, best
      Extended Community:RT:100:1
      Originator:10.0.0.8, Cluster list:10.1.1.44
  22
    10.22.1.9 (metric 11) from 10.11.1.2 (10.0.0.9)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath
      Extended Community:RT:100:1
      Originator:10.0.0.9, Cluster list:10.1.1.22
  22
    10.22.6.10 (metric 11) from 10.11.6.7 (10.0.0.10)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath
      Extended Community:RT:100:1
      Originator:10.0.0.10, Cluster list:10.0.0.7
  22
    10.22.4.10 (metric 11) from 10.11.4.5 (10.0.0.10)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath
      Extended Community:RT:100:1
      Originator:10.0.0.10, Cluster list:10.0.0.5
  22
    10.22.5.10 (metric 11) from 10.11.5.6 (10.0.0.10)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath
      Extended Community:RT:100:1
      Originator:10.0.0.10, Cluster list:10.0.0.6

Step 2 In the display resulting from the show ip bgp network-number EXEC command or the show ip bgp vpnv4 all ip-prefix EXEC 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:

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-prefix EXEC 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

Example: iBGP Multipath Load Sharing in a Non-MPLS Topology

Example: iBGP Multipath Load Sharing in an MPLS VPN Topology

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 Figure 4).

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 Figure 5).

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

Table 1 lists the release history for this feature.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS XE software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.


Note Table 1 lists only the Cisco IOS XE software release that introduced support for a given feature in a given Cisco IOS XE software release train. Unless noted otherwise, subsequent releases of that Cisco IOS XE software release train also support that feature.


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.