CONFIGURING BGP WITH TWO DIFFERENT ISPs (MULTIHOMING)

VIDEO CHAPTERS: 0:00 Introduction 1:36 Background Information 4:43 Configuration to Receive Full Internet Routing Table 11:27 Configuration to Receive Directly-Connected Routes 18:10 Configuration to Receive Default Routes Only 22:35 Conclusion Configuring BGP with Two Different ISPs (Multihoming) Border Gateway Protocol (BGP) is one of the key protocols to use to achieve Internet connection redundancy. When you connect your network to two different Internet service providers (ISPs), it is called multihoming. Multihoming provides redundancy and network optimization. It selects the ISP which offers the best path to a resource. When you are running BGP with more than one service provider, you run the risk that your autonomous system (AS) will become a transit AS. This causes Internet traffic to pass through your AS and potentially consume all of the bandwidth and resources on the CPU of your router. CONFIGURE 1. Configuration to Receive Full Internet Routing Table R1: conf t ip as-path access-list 10 permit ^$ !--- Configuring ip as-path access-list 10 permit ^$ means that the router will only send the prefixes which are generated locally in AS 300 and not from any other AS. ! route-map localonly permit 10 match as-path 10 ! router bgp 300 network 1.0.0.0 network 2.0.0.0 neighbor 10.10.10.2 remote-as 100 neighbor 20.20.20.2 remote-as 200 neighbor 10.10.10.2 route-map localonly out !--- Outgoing policy route-map that filters routes to ISP_1. neighbor 20.20.20.2 route-map localonly out !--- Outgoing policy route-map that filters routes to ISP_2. end wr 2. Configuration to Receive Directly-Connected Routes R1: conf t ip as-path access-list 10 permit ^$ !--- Configuring ip as-path access-list 10 permit ^$ means that the router will only send the prefixes which are generated locally in AS 300 and not from any other AS. ! route-map localonly permit 10 match as-path 10 ! ip as-path access-list 20 permit ^100$ ! route-map as100only permit 10 match as-path 20 ! ip as-path access-list 30 permit ^200$ ! route-map as200only permit 10 match as-path 30 ! router bgp 300 network 1.0.0.0 network 2.0.0.0 neighbor 10.10.10.2 remote-as 100 neighbor 20.20.20.2 remote-as 200 ! neighbor 10.10.10.2 route-map localonly out !--- Outgoing policy route-map that filters routes to ISP_1. neighbor 10.10.10.2 route-map as100only in !--- Incoming policy route-map that filters routes from ISP_1. ! neighbor 20.20.20.2 route-map localonly out !--- Outgoing policy route-map that filters routes to ISP_2. neighbor 20.20.20.2 route-map as200only in !--- Incoming policy route-map that filters routes from ISP_2. end ip route 0.0.0.0 0.0.0.0 10.10.10.2 ip route 0.0.0.0 0.0.0.0 20.20.20.2 wr 3. Configuration to Receive Default Routes Only R1: conf t ip as-path access-list 10 permit ^$ !--- Configuring ip as-path access-list 10 permit ^$ means that the router will only send the prefixes which are generated locally in AS 300 and not from any other AS. ! route-map localonly permit 10 match as-path 10 ! ip prefix-list ABC seq 5 permit 0.0.0.0/0 !--- Prefix list to allow only default route updates. ! router bgp 300 network 1.0.0.0 network 2.0.0.0 neighbor 10.10.10.2 remote-as 100 neighbor 20.20.20.2 remote-as 200 ! neighbor 10.10.10.2 route-map localonly out !--- Outgoing policy route-map that filters routes to ISP_1. neighbor 10.10.10.2 prefix-list ABC in ! neighbor 20.20.20.2 route-map localonly out !--- Outgoing policy route-map that filters routes to ISP_2. neighbor 20.20.20.2 prefix-list ABC in VERIFY show ip bgp sum show ip bgp show ip route References: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/23675-27.html https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-16/irg-xe-16-book/configuring-a-basic-bgp-network.html https://archive.nanog.org/meetings/nanog32/presentations/smith.pdf #cisco #ccna #ccnp #ccie