3-3-2. Cisco dual BGP with AS prepend (Load-Balancing)
ISPs : 1
WAN links : 2
CPE : 1
PE : 2
Device :Cisco router with LAN/WAN ports.
WAN1 / Serial IP : 12.1.3.0/30 (mask 255.255.255.252)
WAN2 / Serial IP : 12.1.4.0/30 (mask 255.255.255.252)
Destination IP : 100.100.100.100 and 200.200.200.200
LAN / Ethernet IP : 10.1.0.1/24 and 10.1.0.129/25(Secondary)
Receiving Route : No route from ISP
Memory : Full route(recommend 512M), partial route
(recommend 128-256M)
LAN : ISP (AS20) and Customer (AS10)
[Traffic flow]
Outbound traffic on CPE
All outbound traffic will go out thru both links. It is controlled by simple default route statement in this scenario. Outbound traffic won't be divided in exact 50/50. Because traffic will route to the link per destination(default) based, but not per packet. Don't worry, overall it make a balancing traffic. Most of ISPs are not supporting per packet option due to it is CPU intensive command(they said).
Inbound traffic on CPE
In this case we assumed that LAN side IP is fully used(/24). We will split it in half and announce it to Internet thru one of WANs. We will announce /24 thru WAN1 and announce second half of /24, which is /25 block thru other WAN. So traffic targeted to first half of /24 will come thru WAN1 and other traffic will come thru WAN 2.
[CPE/Customer Cisco Router]
Current configuration:
!
version 12.x
!
hostname CPE
!
interface Ethernet0
description to Internal network
ip address 10.1.0.1 255.255.255.0
ip address 10.1.0.129 255.255.255.128 secondary
no ip directed-broadcast
no ip mroute-cache
!
interface Serial0
description to ISP A - WAN 1
ip address 12.1.3.1 255.255.255.252
no ip directed-broadcast
no ip mroute-cache
no fair-queue
!
interface Serial1
description to ISP A - WAN 2
ip address 12.1.4.1 255.255.255.252
no ip directed-broadcast
no ip mroute-cache
no fair-queue
! router bgp 10
no auto-summary
network 10.1.0.0 mask 255.255.255.0
network 10.1.0.128 mask 255.255.255.128
neighbor 12.1.3.2 remote-as 20 neighbor 12.1.3.2 version 4
neighbor 12.1.3.2 prefix-list 1 out
neighbor 12.1.4.2remote-as 20 neighbor 12.1.4.2 version 4 neighbor 12.1.4.2 route-map second_link out
!
ip classless
!
ip route 0.0.0.0 0.0.0.0 Serial0
ip route 0.0.0.0 0.0.0.0 Serial1
!
ip prefix-list 1 seq 5 permit 10.1.0.0/24
! route-map second_link permit 10
set as-path prepend 10 10
!
line con 0
line aux 0
line vty 0 4
login
!
end
[PE1 / ISP Cisco Router]
Current configuration:
!
version 12.x
!
hostname PE
!
!
interface Serial0
description to WAN 1
ip address 12.1.3.2 255.255.255.252
no ip directed-broadcast
no ip mroute-cache
no fair-queue
!
router bgp 20
no auto-summary neighbor 12.1.3.1 remote-as 10 neighbor 12.1.3.1 version 4
neighbor 12.1.3.1 distribute-list 2 out
!
ip classless
!
access-list 2 deny any <-- no route sent
!
line con 0
line aux 0
line vty 0 4
login
!
end
[PE2 / ISP Cisco Router]
Current configuration:
!
version 12.x
!
hostname PE
!
!
interface Serial1
description to WAN 2
ip address 12.1.4.2 255.255.255.252
no ip mroute-cache
no ip directed-broadcast
no fair-queue
!
router bgp 20 no auto-summary neighbor 12.1.4.1 remote-as 10 neighbor 12.1.4.1 version 4
neighbor 12.1.4.1 distribute-list 2 out
!
ip classless
!
access-list 2 deny any <-- no route sent
!
line con 0
line aux 0
line vty 0 4
login
!
end
[Verifing output]
CPE# show ip bgp neighbors 12.1.3.2 ad
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,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 10.1.0.0/24 0.0.0.0 0 32768 i
Total number of prefixes 1
CPE#
CPE# show ip bgp neighbors 12.1.4.2 ad
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,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 10.1.0.0/24 0.0.0.0 0 32768 i
*> 10.1.0.128/25 0.0.0.0 0 32768 i
Total number of prefixes 2
CPE#
CPE# show ip bgp neighbors 12.1.3.2 ro
Total number of prefixes 0
CPE#
CPE# show ip bgp neighbors 12.1.4.2 ro
Total number of prefixes 0
CPE#
PE1#show ip bgp nei 12.1.3.1 ro
BGP table version is 64, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 10.1.0.0/24 12.1.3.1 0 0 10 i
Total number of prefixes 1
PE1#
PE2#show ip bgp nei 12.1.4.1 ro
BGP table version is 104, local router ID is 4.4.4.4
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 10.1.0.0/24 12.1.4.1 0 0 10 10 10 i
*> 10.1.0.128/25 12.1.4.1 0 0 10 10 10 i
Total number of prefixes 2
PE2#
Internet#traceroute 10.1.0.1
Type escape sequence to abort.
Tracing the route to 10.1.0.1
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BGP Configurations 
