Normal BGP practice is to have one router advertising a particular prefix. If two or more do, I think it is reasonably described as "anycast". The principles are the same whether it is 2 or 20,000 routers advertising the same prefix.One short point here. You are incorrect above. In the Internet, almost every prefex/aggregate is originated from more than one router - you'd not want an entire aggregate disappearing if a single router died.Can anyone else comment on this this? I can imagine it is done for some prefixes, but "almost every" prefix?
That's a tough call. Many ISPs have a singly homed customer injected into IBGP from the router that terminates their circuit. However, these are customer specific prefixes that generally aren't leaked into the DFZ.
Certainly aggregates are originated from all domain borders.My recollection is that aggregates are still in the majority, altho the number of leaks for TE purposes is significant. I think "almost every" is overstating things slightly.
In anycast, multiple devices are configured with the same ip address, nothing more. Its quite different than having the same prefix announced from different sources.I disagree with your second sentence. AFAIK, in the context of BGP "anycast" involves multiple routers (perhaps "many" rather than just 2 or a few) advertising the same prefix.
That's correct. In anycast, you can have a prefix originated from different routers, with different origin ASes, and different AS paths.
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