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Two links to different ISPs with Backup

TWO LINKS TO DIFFERENT ISPs WITH BACKUP
 
Two links to different ISPs One link primary, the other link backup only
 
• Announce /19 aggregate on each link primary link makes standard announcement
  backup link lengthens the AS PATH by using AS PATH prepend
 
• When one link fails, the announcement of the /19 aggregate via the other link ensures 
  continued connectivity
 
 
• RouterA Configuration 
 
router bgp 100 
    network 221.10.0.0 mask 255.255.224.0 
    neighbor 222.222.10.1 remote-as 200 
    neighbor 222.222.10.1 prefix-list aggregate out 
    neighbor 222.222.10.1 prefix-list default in 
 
 
ip prefix-list aggregate permit 221.10.0.0/19 
ip prefix-list default permit 0.0.0.0/0 
 
 
• RouterB Configuration 
 
router bgp 100 
    network 221.10.0.0 mask 255.255.224.0 
    neighbor 220.1.5.1 remote-as 300 
    neighbor 220.1.5.1 prefix-list aggregate out 
    neighbor 220.1.5.1 route-map routerD-out out 
    neighbor 220.1.5.1 prefix-list default in 
    neighbor 220.1.5.1 route-map routerD-in in 
 
 
ip prefix-list aggregate permit 221.10.0.0/19 
ip prefix-list default permit 0.0.0.0/0 
 
 
route-map routerD-out permit 10 
    set as-path prepend 100 100 100
 
route-map routerD-in permit 10 
    set local-preference 80 
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