WB1 7.28 BGP Backdoor

7.28 BGP Backdoor
• Remove the BGP configuration applied in the previous task.
• Shutdown the BGP peering link between AS 100 and AS 300
• Create a new Loopback1 interface in SW1 with the IP address 150.1.77.77/24 and advertise it into BGP.
• Configure R1 and R4 so that they prefer reaching the new subnet via EIGRP as opposed to eBGP.

Note

To resolve this issue you may change the AD of eBGP routes in R1 and R4, but this may increase the risk of routing loops. There is a special command in the BGP configuration mode used to explicitly change the distance of an eBGP prefix: network <subnet> mask <netmask> backdoor. Keep in mind that the purpose of this command is to change the AD of a particaulr eBGP prefix from 20 to 200, not to advertise a new network. Thus, the command applies to non-local prefixes as well. When the command is entered, the eBGP speakers will prefer paths learned via IGP, and utilize the backdoor link.

Configuration

SW1:
router bgp 300
neighbor 155.1.67.6 shutdown
network 150.1.77.0 mask 255.255.255.0
!
interface Loopback1
ip address 150.1.77.77 255.255.255.0
R1, R4::
router bgp 100
network 150.1.77.0 mask 255.255.255.0 backdoor

-------------------------------------

變更前

Rack1R1#show ip route 150.1.77.0
Routing entry for 150.1.77.0/24
  Known via "bgp 100", distance 20, metric 0
  Tag 200, type external
  Last update from 155.1.13.3 00:03:55 ago
  Routing Descriptor Blocks:
  * 155.1.13.3, from 155.1.13.3, 00:03:55 ago
      Route metric is 0, traffic share count is 1
      AS Hops 2
      Route tag 200
Rack1R1#

變更後

Rack1R1#show ip route 150.1.77.0
Routing entry for 150.1.77.0/24
  Known via "eigrp 1", distance 90, metric 158720, type internal
  Redistributing via eigrp 1
  Last update from 155.1.146.6 on FastEthernet0/0, 00:00:25 ago
  Routing Descriptor Blocks:
  * 155.1.146.6, from 155.1.146.6, 00:00:25 ago, via FastEthernet0/0
      Route metric is 158720, traffic share count is 1
      Total delay is 5200 microseconds, minimum bandwidth is 100000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 2
Rack1R1#

Rack1R1#show ip bgp 150.1.77.0
BGP routing table entry for 150.1.77.0/24, version 210
Paths: (2 available, best #2, table Default-IP-Routing-Table, RIB-failure(17))
Flag: 0x820
  Advertised to update-groups:
        1
  200 300, (Received from a RR-client)
    155.1.45.5 (metric 27260160) from 155.1.146.4 (150.1.4.4)
      Origin IGP, metric 0, localpref 100, valid, internal
  200 300
    155.1.13.3 from 155.1.13.3 (150.1.3.3)
      Origin IGP, localpref 100, valid, external, best
Rack1R1#