[leoc]

If you're talking about the restriction that prevents the forwarding of routes in one iBGP connection and out another, I think that's a requirement of the standard rather than something vendors have imposed. Because iBGP hops don't prepend anything to the AS path, there would be no mechanism to prevent routing loops if multi-hop iBGP route forwarding were enabled. (I'm just a CCNP sprog, not an expert.)