Analysis of Cisco BGP backdoor Routing

Topology experiment diagram:

650) this. width = 650; "src =" 51cto.com/uploads/allianz 111103/134944bq-0.jpg "title =" BGP backdoor routing "height =" 407 "width =" 606 "/> what is BGP backdoor routing? What is its function? Let's take a look at the brief Network Configuration:

R1

S0/0: ip add 12.1.1.1 255.255.255.0

S0/1: ip add 13.1.1.1 255.255.255.0

L0: ip add 1.1.1.1 255.255.255.0

Router bgp 100

Bgp router-id 1.1.1.1

Neighbor 2.2.2.2 remote-as 200

Neighbor 2.2.2.2 update-source loopback 0

Neighbor 2.2.2.2 ebgp-multihop 2

Router VPN 90

No auto-summary

Network 13.1.1.0 0.0.255

Ip route 2.2.2.0 255.255.255.0 12.1.1.2

R2

S0/0: ip add 12.1.1.2 255.255.255.0

S0/2: ip add 23.1.1.2 255.255.255.0

L0: ip add 2.2.2.2 255.255.255.0

Router bgp 200

Bgp router-id 2.2.2.2

Neighbor 1.1.1.1 remote-as 100

Neighbor 1.1.1.1 update-source loopback 0

Neighbor 1.1.1.1 ebgp-multihop 2

Neighbor 3.3.3.3 remote-as 200

Neighbor 3.3.3.3 update-source loopback 0

Router ospf 110

Router-id 2.2.2.2

Network 2.2.2.0 0.0.0.255 area 0

Network 23.1.1.0 0.0.0.255 area 0

Ip route 1.1.1.0 255.255.255.0 12.1.1.1

R3

S0/2: ip add 23.1.1.3 255.255.255.0

S0/1: ip add 13.1.1.3 255.255.255.0

L0: ip add 3.3.3.3 255.255.255.0

L1: ip add 99.99.99.9 255.255.255.0

Router bgp 200

Bgp router-id 3.3.3.3

Neighbor 2.2.2.2 remote-as 200

Neighbor 2.2.2.2 update-source loopback 0

Network 99.99.99.0 mask 255.255.255.0

Router ospf 110

Router-id 3.3.3.3

Network 3.3.3.0 0.0.0.255 area 0

Network 23.1.1.0 0.0.0.255 area 0

Router VPN 90

No auto-summary

Network 99.99.99.0 0.0.255

Network 13.1.1.0 0.0.255

OK! After the configuration is basically completed, can you guess what routes R1 will learn? Network entries of 99.99.99.0/24 learned from R2 using the BGP protocol, and network entries of 99.99.99.0/24 learned from R2 using the OSPF protocol. In this case, all data destined for the target network 99.99.99.0 will be sent to R2, and R3 will not be taken, because the Management Distance of EBGP is 20, which is far greater than the Management Distance of OSPF, installed in the R1 route table first.

If I have a requirement that the next hop on the target network 99.99.99.0/24 is R3, what can I do? In fact, you may have come up with a solution, that is, to use the "routing policy" to increase the management distance of Route entries sent by EBGP 99.99.99.0, alternatively, the Management Distance from the 99.99.99.0/24 route entries received by the Network in the same way is smaller than that of the EBGP 20.

The routing policy is to make a routing policy based on the content of the route package. The routing package means [data generated by OSPF, OSPF, ISIS, BGP, and other protocols]. The Policy Routing is based on the data content.

Strategy and forwarding. data packets mean end user data. This is my conclusion. If you have any opinions on this, you can contact us.

The above method can solve the problem. In fact, in the BGP design, it provides us with a better and more convenient method, that is, the backdoor routing [backdoor]. the operation is to use the network command to change the 99.99.99.0/24 network learned from R1 to a backdoor route and import it to the local route table. If a BGP Route is manually imported from a local machine, that is, a BGP local route, the AD value of the BGP local route is 200, which is the same as the AD value of the IBGP route, and the AD value is greater than any IGP protocol.

The command is executed on R1:

Router bgp 100

Network 99.99.99.0 mask 255.255.255.0 backdoor

During backdoor routing, note that if you advertise that a master-class network can be configured without a mask, the configuration must be the same as that of the originating BGP Route, that is, the mask must match with the network 99.99.99.99.0 mask 255.255.255.0 network released by R3 in BGP. After completing this configuration, you will find that the route entry "99.99.99.0/24" has already shown the beginning of "D" in the routing table and the next hop is R3. In view BGP, this entry is marked with r at the beginning, meaning that this route exists in IGP.