WAN / Serial IP : 12.1.3.0/30 (mask 255.255.255.252)
LAN / Ethernet IP : 10.1.0.0/24 (mask 255.255.255.0)
Receiving Route : Default route + full route from ISP
Memory : Full route(recommend 512M), partial route
(recommend 128-256M)
LAN : ISP (AS20) and Customer (AS10)
Comments:If you have only single link with ISP, you don't really need BGP protocol at all. Better to use just default route to save even bandwidth. Consult with professionals if really need it. This article is shown for educational purpose.
[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 no ip directed-broadcast no ip mroute-cache ! interface Serial0 description to ISP ip address 12.1.3.1 255.255.255.252 no ip directed-broadcast no ip mroute-cache no fair-queue ! interface Serial1 no ip mroute-cache no ip directed-broadcast shutdown ! router bgp 10
no auto-summary
network 10.1.0.0 mask 255.255.255.0
neighbor 12.1.3.2 remote-as 20 <---- 20 is ISP's ASN neighbor 12.1.3.2 version 4
neighbor 12.1.3.2 route-map To_ISP out ! ip classless ip route 0.0.0.0 0.0.0.0 Serial0 <----- Default route, if partial routes are receiving from ISP, you must have this default route statement. You can request ISP to send default route through BGP. ! ip prefix-list 1 description announcing routes to ISP ip prefix-list 1 seq 5 permit 10.1.0.0/24 le 32 ! route-map To_ISP permit 10 match ip address prefix-list 1 set as-path prepend 10 10 10 <---- AS prepended three times ! line con 0 line aux 0 line vty 0 4 login ! end
[PE / ISP Cisco Router]
Current configuration: ! version 12.x ! hostname PE ! ! interface Serial0 description to ISP ip address 12.1.3.2 255.255.255.252 no ip directed-broadcast no ip mroute-cache no fair-queue ! interface Serial1 no ip mroute-cache no ip directed-broadcast shutdown ! router bgp 20 neighbor 12.1.3.1 remote-as 10 <---- 10 is other side's ASN neighbor 12.1.3.1 version 4
neighbor 12.1.3.1 default-originate <---- Sending default route to customer through BGP.
neighbor 12.1.3.1 prefix-list 2 in ! ip classless ! ip prefix-list 2 description from customer or peer ip prefix-list 2 seq 5 permit 10.1.0.0/24 le 32 <---- Accepting smaller block than /24 ! 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 8, local router ID is x.x.x.x
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
*> 10.1.0.0/24 0.0.0.0 0 0 32768 ? CPE#
PE#show ip bgp neighbors 12.1.3.1 ro BBGP table version is 245, local router ID is x.x.x.x 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 *> 10.1.0.0/24 12.1.3.1 0 0 10 10 10 10 ? <---- AS prepended three times
Total number of prefixes 1
CPE#show ip bgp neighbors 12.1.3.2 ro <---- Getting default route from ISP BGP table version is 10, local router ID is x.x.x.x Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete
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BGP Configurations 
