[vc_row][vc_column][vc_column_text]We’ll assume you have two (Cisco) routers connecting to two ISPs. This means that each router speaks eBGP (external BGP) to one ISP an iBGP (internal BGP) towards your other BGP router. And they’ll use OSPF to distribute the subnet prefixes used to connect to each ISP to the other router so the BGP next hop can be resolved as well as the router’s loopback addresses so iBGP can be configured to/ from loopback addresses and thus not depend on any particular physical interface.
!
interface Loopback0
ip address10.0.19.253 255.255.255.255
!
interface GigabitEthernet0/0
description ISP A
ip address 10.93.194.26 255.255.255.252
!
router ospf 1
redistribute connected subnets
network 10.0.16.0 0.0.3.255 area 0
!
router bgp 64496
network 10.0.16.0 mask 255.255.252.0
timers bgp 10 30
!
! reducing the timers from default 60 / 180 so BGP will
! detect a dead neighbor in 30 second rather than 180
!
neighbor 10.0.19.254 remote-as 64496
neighbor 10.0.19.254 description iBGP to router 2
neighbor 10.0.19.254 update-source Loopback0
!
! update-source makes sure we use the loopback address
! for iBGP messages
!
neighbor 10.93.194.25 remote-as 65550
neighbor 10.93.194.25 description ISP A
neighbor 10.93.194.25 prefix-list infilter in
neighbor 10.93.194.25 prefix-list outfilter out
neighbor 10.93.194.25 filter-list 1 out
interface Loopback0
ip address10.0.19.253 255.255.255.255
!
interface GigabitEthernet0/0
description ISP A
ip address 10.93.194.26 255.255.255.252
!
router ospf 1
redistribute connected subnets
network 10.0.16.0 0.0.3.255 area 0
!
router bgp 64496
network 10.0.16.0 mask 255.255.252.0
timers bgp 10 30
!
! reducing the timers from default 60 / 180 so BGP will
! detect a dead neighbor in 30 second rather than 180
!
neighbor 10.0.19.254 remote-as 64496
neighbor 10.0.19.254 description iBGP to router 2
neighbor 10.0.19.254 update-source Loopback0
!
! update-source makes sure we use the loopback address
! for iBGP messages
!
neighbor 10.93.194.25 remote-as 65550
neighbor 10.93.194.25 description ISP A
neighbor 10.93.194.25 prefix-list infilter in
neighbor 10.93.194.25 prefix-list outfilter out
neighbor 10.93.194.25 filter-list 1 out
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