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Two Link to The Same ISP with Loadsharing

TWO LINK TO THE SAME ISP WITH LOADSHARING
 
• AS109 removes private AS and any customer subprefixes from Internet announcement
 
• Announce /19 aggregate on each link
 
• Split /19 and announce as two /20s, one on each link basic inbound loadsharing
  assumes equal circuit capacity and even spread of traffic across address block.
 
• Vary the split until “perfect” loadsharing achieved
 
• Accept the default from upstream basic outbound loadsharing by nearest exit
  okay in first approx as most ISP and end-site traffic is inbound
 
 
• RouterA Configuration 
 
router bgp 200 
    network 221.10.0.0 mask 255.255.224.0 
    network 221.10.0.0 mask 255.255.240.0 
    neighbor 222.222.10.2 remote-as 109 
    neighbor 222.222.10.2 prefix-list routerC out 
    neighbor 222.222.10.2 prefix-list default in 
ip prefix-list default permit 0.0.0.0/0 
ip prefix-list routerC permit 221.10.0.0/20 
ip prefix-list routerC permit 221.10.0.0/19 
ip route 221.10.0.0 255.255.240.0 null0 
ip route 221.10.0.0 255.255.224.0 null0 
 
 
• RouterB Configuration 
 
router bgp 200
    network 221.10.0.0 mask 255.255.224.0 
    network 221.10.0.0 mask 255.255.240.0 
    neighbor 222.222.10.6 remote-as 109 
    neighbor 222.222.10.6 prefix-list routerC out 
    neighbor 222.222.10.6 prefix-list default in 
ip prefix-list default permit 0.0.0.0/0 
ip prefix-list routerC permit 221.10.0.0/20 
ip prefix-list routerC permit 221.10.0.0/19 
 
ip route 221.10.0.0 255.255.240.0 null0 
ip route 221.10.0.0 255.255.224.0 null0
 
 
 
• RouterC Configuration 
 
router bgp 300 
    neighbor 222.222.10.1 remote-as 65534 
    neighbor 222.222.10.1 default-originate 
    neighbor 222.222.10.1 prefix-list Customer in 
    neighbor 222.222.10.1 prefix-list default out 
 
 
ip prefix-list Customer permit 221.10.0.0/19 le 20 
ip prefix-list default permit 0.0.0.0/0 
 
• Router C only allows in /19 and /20 prefixes from customer block
• Router D configuration is identical
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