What backdoor command does ?

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Backdoor only makes the IGP learned route the preferred route. To specify a backdoor route to a BGP border router that will provide better information about the network, use the network backdoor router configuration command. To remove an address from the list, use the no form of this command.
By definition, eBGP updates have a distance of 20 that is lower than the IGP distances. Default distance is 120 for RIP, 100 for IGRP, 90 for EIGRP, and 110 for OSPF. By default, BGP has the following distances, but that could be changed by the distance command

It will change the distance of an eBGP route to 200

References:
http://www.cisco.com/en/US/products/sw/iosswrel/ps1826/products_command_summary_chapter09186a00800d9c5b.html#xtocid197442
http://www.cisco.com/en/US/tech/tk365/tk80/technologies_tech_note09186a00800c95bb.shtml#bgpbackdoor