Syntax of GateD Configuration File Policy Statements for BGP

For BGP, the general GateD policy syntax is as follows:

import proto bgp ( as <ASN> | aspath <aspath_spec> ) restrict;

import proto bgp ( as <ASN> | aspath <aspath_spec> ) 
 	[ preference <GateD preference> ] {

	#Here is the "restrict" form for a single NRLI:
 	<NLRI specification> restrict;

	#Here is the non-"restrict" form for a single NRLI:
	<NLRI specification> [ preference <GateD preference> ] ;
	...
};

export proto bgp as <ASN> restrict; 

export proto bgp as <ASN> 
 	[ metric <BGP metric> ] {

	#Here is the "restrict" form for an entire source:
	proto <source> restrict; 

	#Here is the non-"restrict" form for a source:
	proto <source> [ metric <BGP metric> ] {

		#Here is the "restrict" form for a single NRLI:
 		<NLRI specification> restrict;

		#Here is the non-"restrict" form for a single NRLI:
 		<NLRI specification> metric <BGP metric>;
		...
	};
	...
};

where, for both import and export we have:

• <ASN> is an IP AS number,
• <aspath_spec> is an AS path specification,
• <NLRI specification> is any of:
  • an IP prefix (possibly with a mask specification), with an optional exact or refines match modifier
  • an IP host specification: host ( <IP address> | <DNS name> )
  • the specification of all or default.

  Also, import can have a <GateD preference>, a number which GateD uses as an internal, GateD-specific metric, different from protocol-specific metrics in that GateD's preference is used during route selection; it allows for comparison of routes on a "normalized" scale, so that one need not compare different, generally incompatible ideas of "metric" which exist internal to the various routing protocols. Thus, GateD tags inbound routing information, where routes come from various routing protocols, with a preference, which is really a metric internal to GateD. (For more about "preference" as used in IDRP and GateD, and the "metrics" used by routing protocols, with an explanation of the differences between them, see this note about the difference between the IDRP preference and the GateD preference.)

  Upon export, one can set the <BGP metric>; for outbound routing information, where routes are going back into a specific routing protocol, the use of that protocol's idea of metric is the appropriate parallel to GateD's internal preference.