Backdoor Routes

Two customer sites and a provider

R1 and R2 exchange loopbacks over their direct OSPF link. They also advertise those /32s to provider R3 through separate external sessions. With the default administrative distances in this command family, eBGP distance 20 beats OSPF 110 for the same /32.

hostname R1
ip cef
interface Loopback0
 ip address 10.255.0.1 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.1 255.255.255.252
 no shutdown
exit
interface GigabitEthernet2
 ip address 192.0.2.5 255.255.255.252
 no shutdown
exit
router ospf 1
 router-id 10.255.0.1
 network 10.255.0.1 0.0.0.0 area 0
 passive-interface Loopback0
 network 192.0.2.1 0.0.0.0 area 0
exit
router bgp 65001
 bgp router-id 10.255.0.1
 no bgp default ipv4-unicast
 neighbor 192.0.2.6 remote-as 65003
 address-family ipv4 unicast
  no auto-summary
  neighbor 192.0.2.6 activate
  network 10.255.0.1 mask 255.255.255.255
 exit-address-family
end

hostname R2
ip cef
interface Loopback0
 ip address 10.255.0.2 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.2 255.255.255.252
 no shutdown
exit
interface GigabitEthernet2
 ip address 192.0.2.9 255.255.255.252
 no shutdown
exit
router ospf 1
 router-id 10.255.0.2
 network 10.255.0.2 0.0.0.0 area 0
 passive-interface Loopback0
 network 192.0.2.2 0.0.0.0 area 0
exit
router bgp 65002
 bgp router-id 10.255.0.2
 no bgp default ipv4-unicast
 neighbor 192.0.2.10 remote-as 65003
 address-family ipv4 unicast
  no auto-summary
  neighbor 192.0.2.10 activate
  network 10.255.0.2 mask 255.255.255.255
 exit-address-family
end

hostname R3
ip cef
interface Loopback0
 ip address 10.255.0.3 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.6 255.255.255.252
 no shutdown
exit
interface GigabitEthernet2
 ip address 192.0.2.10 255.255.255.252
 no shutdown
exit
router bgp 65003
 bgp router-id 10.255.0.3
 no bgp default ipv4-unicast
 neighbor 192.0.2.5 remote-as 65001
 neighbor 192.0.2.9 remote-as 65002
 address-family ipv4 unicast
  no auto-summary
  neighbor 192.0.2.5 activate
  neighbor 192.0.2.9 activate
 exit-address-family
end

Inspect the undesired exit

R1# show ip route 10.255.0.2
Routing entry for 10.255.0.2/32
  Known via "bgp 65001", distance 20, metric 0
  * 192.0.2.6, via GigabitEthernet2

The direct physical path may be faster, but route-source preference makes R1 use R3. Administrative distance is local and differs from BGP LOCAL_PREF.

Mark the remote destination backdoor

router bgp 65001
 address-family ipv4 unicast
  network 10.255.0.2 mask 255.255.255.255 backdoor
 exit-address-family

R2

router bgp 65002
 address-family ipv4 unicast
  network 10.255.0.1 mask 255.255.255.255 backdoor
 exit-address-family

The backdoor statement changes treatment of the specified BGP destination to the local/internal distance, normally 200. It does not originate that remote prefix as an ordinary network statement would.

Verify primary and fallback routes

R1# show ip route 10.255.0.2
Routing entry for 10.255.0.2/32
  Known via "ospf 1", distance 110, metric 2
  * 192.0.2.2, via GigabitEthernet1

The IGP copy now wins installation. In the lab, shut the direct R1–R2 interface and wait for OSPF to withdraw; the external BGP candidate can then become the fallback.

interface GigabitEthernet1
 shutdown
exit

R1# show ip route 10.255.0.2
Routing entry for 10.255.0.2/32
  Known via "bgp 65001", distance 200, metric 0
  * 192.0.2.6, via GigabitEthernet2

Restore the direct link and baseline

interface GigabitEthernet1
 no shutdown
exit

router bgp 65001
 address-family ipv4 unicast
  no network 10.255.0.2 mask 255.255.255.255 backdoor
 exit-address-family

R2

router bgp 65002
 address-family ipv4 unicast
  no network 10.255.0.1 mask 255.255.255.255 backdoor
 exit-address-family

The saved baseline again prefers eBGP distance 20. Keep the backdoor setting instead when it is the intended operational policy. Always compare identical prefix lengths before comparing route-source preference.