Networking Concepts: BGP MPLS

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

The BGP Multipath Load Sharing for eBGP and iBGP feature allows you to configure multipath load balancing with both external BGP (eBGP) and internal BGP (iBGP) paths in Border Gateway Protocol (BGP) networks that are configured to use Multiprotocol Label Switching (MPLS) Virtual Private Networks (VPNs). This feature provides improved load balancing deployment and service offering capabilities and is useful for multi-homed autonomous systems and Provider Edge (PE) routers that import both eBGP and iBGP paths from multihomed and stub networks.

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

Load Balancing is Configured Under CEF
Cisco Express Forwarding (CEF) or distributed CEF (dCEF) must be enabled on all participating routers.

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

Address Family Support
This feature is configured on a per VPN routing and forwarding instance (VRF) basis. This feature can be configured under only the IPv4 VRF address family.

Memory Consumption Restriction
Each BGP multipath routing table entry will use additional memory. We recommend that you do not use this feature on a router with a low amount of available memory and especially if router is carries full Internet routing tables.

Route Reflector Limitation
When multiple iBGP paths installed in a routing table, a route reflector will advertise only one paths
(next hop). If a router is behind a route reflector, all routers that are connected to multihomed sites will
not be advertised unless a different route distinguisher is configured for each VRF.

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

To configure the BGP Multipath Load Sharing for Both eBGP and iBGP in an MPLS-VPN feature, you must understand the following concepts:

• Multipath Load Sharing Between eBGP and iBGP
• eBGP and iBGP Multipath Load Sharing in a BGP MPLS Network,
• eBGP and iBGP Multipath Load Sharing With Route Reflectors
• Benefits of Multipath Load Sharing for Both eBGP and iBGP

Multipath Load Sharing Between eBGP and iBGP

A BGP routing process will install a single path as the best path in the routing information base (RIB) by default. The maximum-paths command allows you to configure BGP to install multiple paths in the RIB for multipath load sharing. BGP uses the best path algorithm to still select a single multipath as the best path and advertise the best path to BGP peers.

Load balancing over the multipaths is performed by CEF. CEF load balancing is configured on a per-packet round robin or on a per session (source and destination pair) basis. For information about CEF, refer to Cisco IOS Switching Services Configuration Guide documentation:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123cgcr/swit_vcg.htm

The BGP Multipath Load Sharing for Both eBGP and iBGP in an MPLS VPN feature is enabled only under the IPv4 VRF address family configuration mode. When enabled, this feature can perform load balancing on eBGP and/or iBGP paths that are imported into the VRF. The number of multipaths is configured on a per VRF basis. Separate VRF multipath configurations are isolated by unique route distinguisher.

eBGP and iBGP Multipath Load Sharing in a BGP MPLS Network

Figure 1 shows a service provider BGP MPLS network that connects two remote networks to PE router 1 and PE router 2. PE router 1 and PE router 2 are both configured for VPNv4 unicast iBGP peering. Network 2 is a multihomed network that is connected to PE router 1 and PE router 2. Network 2 also has extranet VPN services configured with Network 1. Both Network 1 and Network 2 are configured for eBGP peering with the PE routers.

PE router 1 can be configured with the BGP Multipath Load Sharing for Both eBGP and iBGP in an MPLS VPN feature so that both iBGP and eBGP paths can be selected as multipaths and imported into the VRF of Network 1. The multipaths will be used by CEF to perform load balancing. IP traffic that is sent from Network 2 to PE router 1 and PE router 2 will be sent across the eBGP paths as IP traffic. IP traffic that is sent across the iBGP path will be sent as MPLS traffic, and MPLS traffic that is sent across an eBGP path will be sent as IP traffic. Any prefix that is advertised from Network 2 will be received by PE router 1 through route distinguisher (RD) 21 and RD 22.The advertisement through RD 21 will be carried in IP packets, and the advertisement through RD 22 will be carried in MPLS packets. Both paths can be selected as multipaths for VRF1 and installed into the VRF1 RIB.

eBGP and iBGP Multipath Load Sharing With Route Reflectors

Figure 2 shows a topology that contains three PE routers and a route reflector, all configured for iBGP peering. PE router 2 and PE router 3 each advertise an equal preference eBGP path to PE router 1. By default, the route reflector will choose only one path and advertise PE router 1.

For all equal preference paths to PE router 1 to be advertised through the route reflector, you must configure each VRF with a different RD. The prefixes received by the route reflector will be recognized differently and advertised to PE router 1.

Benefits of Multipath Load Sharing for Both eBGP and iBGP
The BGP Multipath Load Sharing for Both eBGP and iBGP in an MPLS VPN feature allows multihomed autonomous systems and PE routers to be configured to distribute traffic across both eBGP and iBGP paths.

How to Configure BGP Multipath Load Sharing for Both eBGP and iBGP in an MPLS-VPN

This section contains the following procedures:
• Configuring Multipath Load Sharing for Both eBGP an iBGP
• Verifying Multipath Load Sharing for Both eBGP an iBGP

Configuring Multipath Load Sharing for Both eBGP an iBGP
To configure this feature, perform the steps in this section.

SUMMARY STEPS

enable
configure {terminal | memory | network}
router bgp as-number
address-family ipv4 [mdt | multicast | tunnel | unicast [vrf vrf-name] | vrf vrf-name] | ipv6 [multicast | unicast] | vpnv4 [unicast]
maximum-paths eibgp number [import number]
end

DETAILED STEPS

Command or Action	Purpose
enable Example: Router> enable	Enables higher privilege levels, such as privileged EXEC mode. • Enter your password if prompted.
configure {terminal \| memory \| network} Example: Router# configure terminal	Enters global configuration mode.
router bgp as-number Example: Router(config)# router bgp 40000	Enters router configuration mode to create or configure a BGP routing process.
address-family ipv4 vrf vrf-name Example: Router(config-router)# address-family ipv4 vrf RED	Places the router in address family configuration mode. • Separate VRF multipath configurations are isolated by unique route distinguisher.
maximum-paths eibgp number [import number] Example: Router(config-router-af)# maximum-paths eibgp 6	Configures the number of parallel iBGP and eBGP routes that can be installed into a routing table. Note: The maximum-paths eibgp command can be configured only under the IPv4 VRF address family configuration mode and cannot be configured in any other address family configuration mode.
end Example:Router(config-router-af)# end	Exits address family configuration mode, and enters Privileged EXEC mode.

Verifying Multipath Load Sharing for Both eBGP an iBGP

To verify this feature, perform the steps in this section

SUMMARY STEPS

enable
show ip bgp neighbors [neighbor-address [advertised-routes | dampened-routes | flap-statistics | paths [regexp] | received prefix-filter | received-routes | routes]]
show ip bgp vpnv4 {all | rd route-distinguisher | vrf vrf-name} ip-prefix/length [longer-prefixes] [output-modifiers] | network-address [mask] [longer-prefixes] [output-modifiers] [cidr-only] | [community [number | exact-match | local-as | no-advertise | no-export]] | [community-list name | number [exact-match]] | [dampening dampened-paths | flap-statistics | parameters] | [filter-list regexp-acl] | [inconsistent-as] | [injected-paths] | [labels] | [neighbors] | [paths [regexp]] | [peer-group [name [summary]]] | [quote-regexp [regexp]] | [regexp string] | [replication [update-group [ip-address]] | [ip-address]] | [rib-failure] | [route-map name] | [summary] | [templates peer-policy [name] | peer-session [name]] | update-group [update-group [ip-address]] | [ip-address]]
show ip route vrf vrf-name [connected] [protocol [process-number] [tag] [output-modifiers]] [ip-prefix] [list number [output-modifiers]] [profile] [static [output-modifiers]] [summary [output-modifiers]] [supernets-only [output-modifiers]] [traffic-engineering [output-modifiers]]

DETAILED STEPS

Command or Action	Purpose
enable Example: Router> enable	Enables higher privilege levels, such as privileged EXEC mode. • Enter your password if prompted.
show ip bgp neighbors [neighbor-address [advertised-routes \| dampened-routes \| flap-statistics \| paths [regexp] \| received prefix-filter \| received-routes \| routes]] Example: Router# show ip bgp neighbors	Displays information about the TCP and BGP connections to neighbors.
show ip bgp vpnv4 {all \| rd route-distinguisher \| vrf vrf-name} ip-prefix/length [longer-prefixes] [output-modifiers] \| network-address [mask] [longer-prefixes] [output-modifiers] [cidr-only] \| [community [number \| exact-match \| local-as \| no-advertise \| no-export]] \| [community-list name \| number [exact-match]] \| [dampening dampened-paths \| flap-statistics \| parameters] \| [filter-list regexp-acl] \| [inconsistent-as] \| [injected-paths] \| [labels] \| [neighbors] \| [paths [regexp]] \| [peer-group [name [summary]]] \| [quote-regexp [regexp]] \| [regexp string] \| [replication [update-group [ip-address]] \| [ip-address]] \| [rib-failure] \| [route-map name] \| [summary] \| [templates peer-policy [name] \| peer-session[name]] \| update-group [update-group [ip-address]] \| [ip-address]] Example: Router# show ip bgp vpnv4 vrf RED	Displays VPN address information from the BGP table. This command is used to verify that the VRF has been received by BGP.
show ip route vrf vrf-name [connected] [protocol [process-number] [tag] [output-modifiers]] [ip-prefix] [list number [output-modifiers]] [profile] [static [output-modifiers]] [summary [output-modifiers]] [supernets-only [output-modifiers]] [traffic-engineering [output-modifiers]] Example: Router# show ip route vrf RED	Displays the IP routing table associated with a VRF instance. The show ip route vrf command is used to verify that the VRF is in the routing table.

Configuration Examples for the BGP Multipath Load Sharing for Both eBGP and iBGP in an MPLS-VPN Feature
The following examples show how to configure and verify this feature:

• eBGP and iBGP Multipath Load Sharing Configuration Example
• eBGP and iBGP Multipath Load Sharing Verification Examples

eBGP and iBGP Multipath Load Sharing Configuration Example
This following configuration example configures a router in address-family mode to select six BGP routes (eBGP or iBGP) as multipaths:

Router(config)# router bgp 40000 Router(config-router)# address-family ipv4 vrf RED
Router(config-router-af)# maximum-paths eibgp 6 Router(config-router-af)# end
eBGP and iBGP Multipath Load Sharing Verification Examples
To verify that iBGP and eBGP routes have been configured for load sharing, use the show ip bgp vpnv4 EXEC command or the show ip route vrf EXEC command.

In the following example, the show ip bgp vpnv4 command is entered to display multipaths installed in the VPNv4 RIB:

Router# show ip bgp vpnv4 all 10.22.22.0 BGP routing table entry for 10:1:22.22.22.0/24, version 19 Paths:(5 available, best #5) Multipath:eiBGPAdvertised to non peer-group peers: 10.0.0.2 10.0.0.3 10.0.0.4 10.0.0.5 22

10.0.0.2 (metric 20) from 10.0.0.4 (10.0.0.4) Origin IGP, metric 0, localpref 100, valid, internal, multipath Extended Community:0x0:0:0 RT:100:1 0x0:0:0 Originator:10.0.0.2, Cluster list:10.0.0.4

10.0.0.2 (metric 20) from 10.0.0.5 (10.0.0.5) Origin IGP, metric 0, localpref 100, valid, internal, multipath Extended Community:0x0:0:0 RT:100:1 0x0:0:0 Originator:10.0.0.2, Cluster list:10.0.0.5

10.0.0.2 (metric 20) from 10.0.0.2 (10.0.0.2) Origin IGP, metric 0, localpref 100, valid, internal, multipath Extended Community:RT:100:1 0x0:0:0

10.0.0.2 (metric 20) from 10.0.0.3 (10.0.0.3) Origin IGP, metric 0, localpref 100, valid, internal, multipath Extended Community:0x0:0:0 RT:100:1 0x0:0:0 Originator:10.0.0.2, Cluster list:10.0.0.3

10.1.1.12 from 10.1.1.12 (10.22.22.12) Origin IGP, metric 0, localpref 100, valid, external, multipath, best Extended Community:RT:100:1

In the following example, the show ip route vrf command is entered to display multipath routes in the
VRF table:
Router# show ip route vrf PATH 10.22.22.0 Routing entry for 10.22.22.0/24 Known via "bgp 1", distance 20, metric 0 Tag 22, type external Last update from 10.1.1.12 01:59:31 ago Routing Descriptor Blocks:

* 10.0.0.2 (Default-IP-Routing-Table), from 10.0.0.4, 01:59:31 ago
Route metric is 0, traffic share count is 1
AS Hops 1
10.0.0.2 (Default-IP-Routing-Table), from 10.0.0.5, 01:59:31 ago
Route metric is 0, traffic share count is 1
AS Hops 1
10.0.0.2 (Default-IP-Routing-Table), from 10.0.0.2, 01:59:31 ago
Route metric is 0, traffic share count is 1
AS Hops 1
10.0.0.2 (Default-IP-Routing-Table), from 10.0.0.3, 01:59:31 ago
Route metric is 0, traffic share count is 1
AS Hops 1
10.1.1.12, from 10.1.1.12, 01:59:31 ago
Route metric is 0, traffic share count is 1
AS Hops 1

maximum-paths eibgp

To configure multipath load sharing for external BGP (eBGP) and internal (iBGP) routes, use the maximum-paths eibgp command in address family configuration mode. To disable multipath load sharing for eBGP and iBGP routes, use the no form of this command.

maximum-paths eibgp number [import number] no maximum-paths eibgp number [import number]
Syntax Description

number: Specifies the number of routes to install to the routing table. See the usage guidelines for the number of paths that can be configured with this argument.

import number : (Optional) Specifies the number of redundant paths that can be configured as back up multipaths for a VRF. This keyword can only be configured under a VRF in address family configuration mode.

Defaults Border Gateway Protocol (BGP) by default will install only one best path in the routing table.

Command Modes IPv4 VRF Address family configuration

The maximum-paths eibgp command cannot be configured with the maximum-paths ormaximum-paths ibgp command because the maximum-paths eibgp command is a superset of these commands.

Configuring VRF Import Paths
A VRF will import only one path (best path) per prefix from the source VRF table, unless the prefix is exported with a different route-target. If the best path goes down, the destination will not be reachable until the next import event occurs, and then a new best path will be imported into the VRF table. The import event runs every 15 seconds by default.

The import keyword allows you to configure the VRF table to accept multiple redundant paths in addition to the best path. An import path is a redundant path, and it can have a next hop that matches an installed multipath.This feature should be used when there are multiple paths with identical next hops available to ensure optimal convergence times. A typical application of this feature is to configure redundant paths in a network that has multiple route reflectors for redundancy.

Note Configuring redundant paths with the import keyword can increase CPU and memory utilization significantly, especially in a network where there are many prefixes to learn and a large number of configured VRFs. It is recommended that this feature is only configured as necessary and that the minimum number of redundant paths are configured (Typically, not more than two).

Examples
In the following example, the router is configured to install 6 eBGP or iBGP routes into the VRF routing table:

Router(config)# router bgp 40000 Router(config-router)# address-family ipv4 vrf YELLOW Router(config-router-af)# maximum-paths eibgp 6
In the following example, the router is configured to install 4 equal-cost routes and 2 import routes (backup) in the VRF routing table:

Router(config)# router bgp 45000
Router(config-router)# address-family ipv4 vrf GREEN
Router(config-router-af)# maximum-paths eibgp 4 import 2
In the following example, the router is configured to install 2 import routes in the VRF routing table:
Router(config)# router bgp 50000
Router(config-router)# address-family ipv4 vrf ORANGE
Router(config-router-af)# maximum-paths eibgp import 2

Note: Separate VRFs must be configured with different route distinguisher to support separate multipath configurations.

Related Commands: maximum-paths, maximum-paths ibgp, show ip bgp