Load Sharing with BGP in Single and Multihomed Environments

Introduction

Load sharing allows a router to distribute the outgoing and incoming traffic among multiple paths. The paths are derived either statically or with dynamic protocols, such as:

• Routing Information Protocol (RIP)
  
• Enhanced Interior Gateway Routing Protocol (EIGRP)
  
• Open Shortest Path First (OSPF) Protocol
  
• Interior Gateway Routing Protocol (IGRP)
  

By default, Border Gateway Protocol (BGP) selects only a single best path and does not perform load balancing. This document shows how to perform load sharing in different scenarios with the use of BGP. For additional information about load balancing.

Requirements

Ensure that you meet these requirements before you attempt this configuration:

         Knowledge of BGP Best Path Selection Algorithm

Load Sharing with the Loopback Address as a BGP Neighbor

This scenario shows how to achieve load sharing when there are multiple (up to a maximum of six), equal-cost links. The links are terminated in one router at a local autonomous system (AS) and in another router at a remote AS in a single-homed BGP environment. The Network Diagram serves as an example.
Note: Use the Command Lookup Tool (registered customers only) to find more information on the commands used in this document.

Network Diagram

This section uses this network setup:

Configurations

This section uses these configurations:

• RouterA
  
• RouterB
  

RouterA

interface loopback 0 ip address 1.1.1.1 255.255.255.0  interface serial 0 ip address 160.20.20.1 255.255.255.0 no ip route-cache interface serial 1 ip address 150.10.10.1 255.255.255.0 no ip route-cache router bgp 11 neighbor 2.2.2.2 remote-as 10 neighbor 2.2.2.2 update-source loopback 0 !--- Use the IP address of the loopback interface for TCP connections. neighbor 2.2.2.2 ebgp-multihop !--- You must configure ebgp-multihop whenever the router eigrp 12 network 1.0.0.0 network 150.10.0.0 network 160.20.0.0 no auto-summary

RouterB

interface loopback 0 ip address 2.2.2.2 255.255.255.0 interface serial 0 ip address 160.20.20.2 255.255.255.0 no ip route-cache interface serial 1 ip address 150.10.10.2 255.255.255.0 no ip route-cache router bgp 10 neighbor 1.1.1.1 remote-as 11 neighbor 1.1.1.1 update-source loopback 0 !--- Use the IP address of the loopback interface for TCP connections. neighbor 1.1.1.1 ebgp-multihop !--- You must configure ebgp-multihop whenever the eBGP connections !-- - are not on the same network address. router eigrp 12 network 2.0.0.0 network 150.10.0.0 network 160.20.0.0 no auto-summary

Note: You can use static routes in the place of a routing protocol in order to introduce two equal-cost paths to reach the destination. In this case, the routing protocol is EIGRP.

Verify

Use this section to confirm that your configuration works properly.
The Output Interpreter Tool (registered customers only) (OIT) supports certain show commands. Use the OIT to view an analysis of show command output.
The output of the show ip route command shows that both of the paths to the 2.2.2.0 network are learned via EIGRP. The output of the traceroute command indicates that the load is distributed between two serial links. In this scenario, load sharing occurs on a per-packet basis. You can issue the ip route-cache command on the serial interfaces to do load sharing on a per-destination basis. You can also configure per-packet and per-destination load balancing with Cisco Express Forwarding.

RouterA# show ip route

!--- Output suppressed.

Gateway of last resort is not set

     1.0.0.0/24 is subnetted, 1 subnets
C       1.1.1.0 is directly connected, Loopback0
     2.0.0.0/24 is subnetted, 1 subnets
D       2.2.2.0 [90/2297856] via 150.10.10.2, 00:00:45, Serial1
                [90/2297856] via 160.20.20.2, 00:00:45, Serial0
     160.20.0.0/24 is subnetted, 1 subnets
C       160.20.20.0 is directly connected, Serial0
     150.10.0.0/24 is subnetted, 1 subnets
C       150.10.10.0 is directly connected, Serial1

RouterA# traceroute 2.2.2.2 

Type escape sequence to abort.
Tracing the route to 2.2.2.2

1 160.20.20.2 16 msec
    150.10.10.2 8 msec *