So we're now going to look at the origin attribute. And this one conveys the origin of the prefix. It may not be quite what you think though. This origin was actually used in the transition from EGP to BGP, so it would tell BGP whether the prefix originated from the old EGP protocol that predated BGP, or from BGP itself. It's a transitive attribute and it's mandatory. And it actually influences the best path selection as well, as we'll see later on. And it has three values and these three values are called, IGP, EGP, and incomplete. Now in this case, IGP means that the prefix has actually been originated by BGP itself. It doesn't mean OSPF or IS-IS. It means that prefix has come from BGP itself. So injected into the local ASes BGP usually by a BGP network statement. The second value EGP means the prefix actually has come from EGP. Now as far as I remember, EGP was probably stopped being used around the early 90s. Indeed Cisco removed it from IOS around about IOS 12.2 release early 2000s. So EGP is very much historical. So you probably find that the origin value EGP is not used as it was originally intended anymore. However, some network operators do use it. It is part of the BGP best path selection process and some operators do use the different origin values to indicate particular policies they might want to implement within their own network. The final value, incomplete, means the prefix has been redistributed from another routing protocol. And if you think back to how we introduced prefixes into BGP using the `redistribute static` command, you will find that if you redistribute static into BGP, all those entries will be marked with an origin incomplete when you view them in the BGP table. Prefixes that are generated by the network statement you find are indicated by origin IGP. And if you wish, you can use policy language on your router platform to manually set the origin EGP if you want to.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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