HP-UX Reference (11i v1 00/12) - 4 File Formats (vol 8)

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STANDARD Printed by: Nora Chuang [nchuang] STANDARD

/build/1111/BRICK/man4/!!!intro.4

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gated.conf(4) gated.conf(4)

are used. See the section on Route Aggregation for more information.

Exporting by AS path

proto proto | all aspath aspath_regexp

origin any |([igp ][egp ][incomplete ])

restrict ;

proto proto | all aspath aspath_regexp

origin any |([igp ][egp ][incomplete ])

[ metric metric ] {

route_ﬁlter [ restrict |(metric metric )];

};

When BGP is conﬁgured, all routes are assigned an AS path when they are added to the routing table. For

all interior routes this AS path speciﬁes IGP as the origin and no ASes in the AS path (the current AS is

added when the route is exported). For EGP routes this AS path speciﬁes EGP as the origin and the source

AS as the AS path. For BGP routes, the AS path is stored as learned from BGP.

AS path regular expressions are documented in the section on Matching AS paths.

Exporting by route Tag

proto proto | all tag tag restrict ;

proto proto | all tag tag

[ metric metric ] {

route_ﬁlter [ restrict |(metric metric )];

};

Both OSPF and RIP version 2 currently support tags, all other protocols always have a tag of zero. The

source of exported routes may be selected based on this tag. This is useful when routes are classiﬁed by tag

when the are exported into a given routing protocol.

Route Aggregation

Route aggregation is a method of generating a more general route given the presence of a speciﬁc route. It

is used, for example, at an autonomous system border to generate a route to a network to be advertised via

EGP given the presence of one or more subnets of that network learned via RIP. Older versions of GateD

automatically performed this function, generating an aggregate route to a natural network (using the old

Class A, B and C concept) given an interface to a subnet of that natural network. However that was not

always the correct thing to do, and with the advent of classless interdomain routing it is even more fre-

quently the wrong thing to do, so aggregation must be explicitly conﬁgured. No aggregation is performed

unless explicitly requested in an aggregate statement.

Route aggregation is also used by regional and national networks to reduce the amount of routing informa-

tion passed around. With careful allocation of network addresses to clients, regional networks can just

announce one route to regional networks instead of hundreds.

Aggregate routes are not actually used for packet forwarding by the originator of the aggregate route, only

by the receiver (if it wishes). A router receiving a packet which does not match one of the component routes

which led to the generation of an aggregate route is supposed to respond with an ICMP network unreach-

able message. This is to prevent packets for unknown component routes from following a default route into

another network where they would be forwarded back to the border router, and around and around again

and again, until their TTL expires. Sending an unreachable message for a missing piece of an aggregate is

only possible on systems with support for reject routes.

A slight variation of aggregation is the generation of a route based on the existence of certain conditions.

This is sometimes known as the route of last resort. This route inherits the next hops and aspath from the

contributor speciﬁed with the lowest (most favorable) preference. The most common usage for this is to gen-

erate a default based on the presence of a route from a peer on a neighboring backbone.

Aggregation and Generation syntax

aggregate default

|(network [(mask mask )|(masklen number )])

[ preference preference ][brief ] {

[(as autonomous system )|(tag tag )

|(aspath aspath_regexp )]

restrict ;

HP-UX Release 11i: December 2000 − 48 − Section 4−−119

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