Load Sharing When Dual-Homed to One Internet Service Provider (ISP) Through a Single Local Router

This scenario shows how to achieve load sharing when multiple links exist between a remote AS and a local AS. These links are terminated in one router at the local AS and on multiple routers at remote ASs in a single-homed BGP environment. The Network Diagram is an example of such a network.
This sample configuration uses the maximum-paths command. By default, BGP chooses one best path among the possible equal-cost paths that are learned from one AS. However, you can change the maximum number of parallel equal-cost paths that are allowed. In order to make this change, include the maximum-paths paths command under the BGP configuration. Use a number between 1 and 6 for the paths argument.

Network Diagram

This section uses this network setup:

Configurations

This section uses these configurations:

• RouterA
  
• RouterB
  
• RouterC 

RouterA
interface Loopback0 ip address 1.1.1.1 255.255.255.0 ! interface Serial 0 ip address 160.20.20.1 255.255.255.0 ! ! interface Serial 1 ip address 150.10.10.1 255.255.255.0 ! ! router bgp 11 neighbor 160.20.20.2 remote-as 10 neighbor 150.10.10.2 remote-as 10 network 1.0.0.0 maximum-paths 2

 

RouterB
interface Ethernet0 ip address 2.2.2.1 255.255.255.0 ! interface Serial 0 ip address 160.20.20.2 255.255.255.0 ! ! router bgp 10 neighbor 160.20.20.1 remote-as 11 network 2.0.0.0 auto-summary

RouterC
interface Ethernet0 ip address 2.2.2.2 255.255.255.0 ! interface Serial 1 ip address 150.10.10.2 255.255.255.0 ! ! router bgp 10 neighbor 150.10.10.1 remote-as 11 network 2.0.0.0 auto-summary

Verify

Use this section to confirm that your configuration works properly.
The Output Interpreter Tool (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 BGP. 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-destination basis. The show ip bgp command gives the valid entries for the 2.0.0.0 network.

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
B    2.0.0.0/8 [20/0] via 150.10.10.2, 00:04:23
               [20/0] via 160.20.20.2, 00:04:01
     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 * 

RouterA# show ip bgp

BGP table version is 3, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete 
 
Network  Next Hop  Metric  LocPrf  Weight Path    
*> 1.0.0.0  0.0.0.0           0  32768  i    
*> 2.0.0.0  160.20.20.2  0    0  10 i    
*               150.10.10.2  0    0  10 i