BGP Configuration for Enterprise Internet Redundancy

Dual-homed BGP gives an enterprise two independent internet paths with automatic failover and optional load balancing.

Lab Topology

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ISP1 (AS 65001) ─── [RTR1 - AS 65100] ─── Enterprise LAN
ISP2 (AS 65002) ─── [RTR1]                 10.0.0.0/8

Basic eBGP Configuration (Cisco IOS-XE)

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router bgp 65100
 bgp router-id 203.0.113.1
 bgp log-neighbor-changes

 ! ISP1 neighbor
 neighbor 198.51.100.1 remote-as 65001
 neighbor 198.51.100.1 description ISP1-Primary
 neighbor 198.51.100.1 password ISPsecret1
 neighbor 198.51.100.1 update-source GigabitEthernet0/0

 ! ISP2 neighbor
 neighbor 198.51.100.5 remote-as 65002
 neighbor 198.51.100.5 description ISP2-Secondary
 neighbor 198.51.100.5 password ISPsecret2

 ! Advertise enterprise prefix
 network 203.0.113.0 mask 255.255.255.0

Path Selection: Prefer ISP1

Use local preference to prefer ISP1:

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route-map ISP1-IN permit 10
 set local-preference 200

route-map ISP2-IN permit 10
 set local-preference 100

router bgp 65100
 neighbor 198.51.100.1 route-map ISP1-IN in
 neighbor 198.51.100.5 route-map ISP2-IN in

Outbound Load Balancing

To split outbound traffic between ISPs, use AS-PATH prepending on ISP1:

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route-map OUT-ISP2 permit 10
 set as-path prepend 65100 65100  ! Makes ISP2 path look shorter to ISP2

Prefix Filtering (Security)

Never accept a full routing table if you don't need it. Use prefix-lists:

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ip prefix-list DEFAULT-ONLY seq 5 permit 0.0.0.0/0
ip prefix-list DEFAULT-ONLY seq 10 deny 0.0.0.0/0 le 32

router bgp 65100
 neighbor 198.51.100.1 prefix-list DEFAULT-ONLY in

Verification

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show bgp summary
show bgp ipv4 unicast 0.0.0.0
show ip route bgp
debug ip bgp 198.51.100.1 events