BGP Flow Control

Usually, if the route comes via BGP, it is written in the IP routing table, because its "distance" = 20. However, sometimes two AS are like "back door" to each other, resulting in IGP, but the path obtained by EBGP. In this case, using certain commands in one of these routers, you can control the routing so that the best way would be the way, obtained through the IGP, and the siding (to be used in IP route table in the event of a primary path) will be the path obtained a BGP update.

Figure = 12-19: Back Door Example

In this figure, router "A" and "C", as well as routers "C" and "B", to interact with EBGP. Router "A" and "B" interact in IGP (RIP, IGRP, Enhanced IGRP, or OSFP). The values ??of the default distance for RIP, IGRP, Enhanced IGRP and OSPF are 120,100,90, and 110, respectively. Since all of these default distance is greater than def distance to EBGP (20), according to the rule described above, for a table of IP routing on the router "A" must be specified route, resulting in EBGP.

More details: router "A" receives updates about network 160.10.0.0 from two protocols: EBGP and IGP. Because the default distance for EBGP less than IGP, the router "A" to choose the path of the network had received from the router "C". But we can change this picture (just as this is the use of the back door): for example, if we want a router "A" used the path to 160.10.0.0, obtained by the IGP from the router "B", you can use one of three the following methods:

*) Change the external distance from EBGP. But it is not recommended because distance affects all updates between routers, and it may be confusion among multiple protocols between them;

*) Change the distance from IGP. Not recommended for the same reason;

*) Use the configuration of BGP back door. That is correct.

So, for the use of IGP connection as back door, there is a router configuration command:
network [net] backdoor

With the help of Configured Router "A" so that it will be in its IP table prescribe update'y received from the router "B".
! Router A
router eigrp 10
network 150.10.0.0
router bgp 100
neighbor 2.2.2.1 remote-as 300
network 160.10.0.0 backdoor

The command network 160.10.0.0 backdoor on a router "A" provides a route to network 160.10.0.0, both local and registers it in the IP routing table with the value of distance = 200. Because the network 160.10.0.0 as advertised and Enhanced IGRP (with the router "B", and with distance = 90) is precisely obtained by Enhanced IGRP route will link registered in the IP route table will be used as primary. If IGRP route will fall, the EBGP route will be registered in the IP routing table and will be used for passing traffic.
Note: Despite the fact that BGP treats network 160.10.0.0 as a local,
This does not mean that the network is advertised as a local
for the router.