When ISP has different router for both IIG

1. Assume ISP has own ip block which is 121.10.0.0/19
2. Announce /19 aggregate on each link
3. Split /19 and announce as two /20s, one on each link basic inbound loadsharing
4. When one link fails, the announcement of the /19 aggregate via the other IIG ensures continued connectivity

Router A Configuration

router bgp 130
network 121.10.0.0 mask 255.255.224.0
network 121.10.0.0 mask 255.255.240.0
neighbor 192.168.10.2 remote-as 130
neighbor 122.102.10.1 remote-as 17806
neighbor 122.102.10.1 prefix-list firstblock out
neighbor 122.102.10.1 prefix-list default in
!
ip prefix-list default permit 0.0.0.0/0
!
ip prefix-list firstblock permit 121.10.0.0/20
ip prefix-list firstblock permit 121.10.0.0/19

Router B Configuration

router bgp 130
network 121.10.0.0 mask 255.255.224.0
network 121.10.16.0 mask 255.255.240.0
neighbor 192.168.10.1 remote-as 130
neighbor 120.1.5.1 remote-as 500
neighbor 120.1.5.1 prefix-list secondblock out
neighbor 120.1.5.1 prefix-list default in
!
ip prefix-list default permit 0.0.0.0/0
!
ip prefix-list secondblock permit 121.10.16.0/20
ip prefix-list secondblock permit 121.10.0.0/19

When ISP has one router for both IIG

Router A Configuration

router bgp 130
network 121.10.0.0 mask 255.255.224.0
network 121.10.0.0 mask 255.255.240.0
network 121.10.16.0 mask 255.255.240.0
neighbor 122.102.10.1 remote-as 17806
neighbor 120.1.5.1 remote-as 500
neighbor 122.102.10.1 prefix-list firstblock out
neighbor 122.102.10.1 prefix-list default in
neighbor 120.1.5.1 prefix-list secondblock out
!
ip prefix-list default permit 0.0.0.0/0
!
ip prefix-list firstblock permit 121.10.0.0/20
ip prefix-list firstblock permit 121.10.0.0/19
!
ip prefix-list secondblock permit 121.10.16.0/20
Ip prefix-list secondblock permit 121.10.0.0/19
			

1. Announce /19 aggregate on each link
   • On first link, announce /19 as normal
   • On second link, announce /19 with longer AS PATH, and announce one /20 subprefix
   • Controls load sharing between up streams and the Internet
2. Vary the subprefix size and AS PATH length until “perfect” load sharing achieved

Router A Configuration

router bgp 130
network 121.10.0.0 mask 255.255.224.0
Neighbor 192.168.10.2 remote-as 130
neighbor 122.102.10.1 remote-as 17806
neighbor 122.102.10.1 prefix-list default in
neighbor 122.102.10.1 prefix-list aggregate out
!
ip prefix-list aggregate permit 121.10.0.0/19

Router B Configuration

router bgp 130
network 121.10.0.0 mask 255.255.224.0
network 121.10.16.0 mask 255.255.240.0 
Neighbor 192.168.10.1 remote-as 130
neighbor 120.1.5.1 remote-as 500
neighbor 120.1.5.1 prefix-list default in
neighbor 120.1.5.1 prefix-list subblocks out
neighbor 120.1.5.1 route-map routerD out
!
route-map routerD permit 10
match ip address prefix-list aggregate
set as-path prepend 130 130
route-map routerD permit 20
!
ip prefix-list subblocks permit 121.10.0.0/19 le 20
ip prefix-list aggregate permit 121.10.0.0/19
	

1. Specify which IP block will go through which IIG
2. Use route-map for tuning the outgoing traffic

Router A Configuration

!
interface GigabitEthernet0/1
 description Main LAN
 (ip and other config)…
 ip policy route-map uplink
!
router bgp 130
network 121.10.0.0 mask 255.255.224.0
network 121.10.0.0 mask 255.255.240.0
network 121.10.16.0 mask 255.255.240.0
neighbor 122.102.10.1 remote-as 17806
neighbor 120.1.5.1 remote-as 500
neighbor 122.102.10.1 prefix-list firstblock out
neighbor 120.1.5.1 prefix-list secondblock out
!
ip prefix-list firstblock permit 121.10.0.0/20
ip prefix-list firstblock permit 121.10.0.0/19
!
ip prefix-list secondblock permit 121.10.16.0/20
ip prefix-list secondblock permit 121.10.0.0/19
!
ip access-list 1 permit 121.10.16.0 0.0.15.255
!
route-map uplink permit 10
match ip address 1
set default next-hop 120.1.5.1 122.102.10.1
route-map uplink permit 20
! 
	

1. ISP can take full or partial internet routing table from both IIG and traffic engineering as per their requirement.
2. Assume isp want to take partial routes from one IIG and default route from another IIG. So load balancing is possible by varying received partial routes from one IIG.

	
Router A Configuration

!
router bgp 130
network 121.10.0.0 mask 255.255.224.0
network 121.10.0.0 mask 255.255.240.0
network 121.10.16.0 mask 255.255.240.0
neighbor 122.102.10.1 remote-as 17806
neighbor 120.1.5.1 remote-as 500
neighbor 122.102.10.1 prefix-list firstblock out
neighbor 122.102.10.1 filter-list 10 in
Neighbor 120.1.5.1 prefix-list default in
neighbor 120.1.5.1 prefix-list secondblock out
!
ip prefix-list firstblock permit 121.10.0.0/20
ip prefix-list firstblock permit 121.10.0.0/19
!
ip prefix-list secondblock permit 121.10.16.0/20
ip prefix-list secondblock permit 121.10.0.0/19
!
ip access-list 1 permit 121.10.16.0 0.0.15.255
!
ip as-path access-list 10 permit ^_701_$
ip prefix-list default permit 0.0.0.0/0

!