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Documentation update.

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Ondrej Zajicek 2010-08-03 00:59:13 +02:00
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@ -882,6 +882,14 @@ undefined value is regarded as empty clist for most purposes.
<tag><m/enum/ dest</tag>
Type of destination the packets should be sent to (<cf/RTD_ROUTER/ for forwarding to a neighboring router, <cf/RTD_DEVICE/ for routing to a directly-connected network, <cf/RTD_BLACKHOLE/ for packets to be silently discarded, <cf/RTD_UNREACHABLE/, <cf/RTD_PROHIBIT/ for packets that should be returned with ICMP host unreachable / ICMP administratively prohibited messages). Read-only.
<tag><m/int/ igp_metric</tag>
The optional attribute that can be used to specify a distance
to the network for routes that do not have a native protocol
metric attribute (like <cf/ospf_metric1/ for OSPF routes). It
is used mainly by BGP to compare internal distances to boundary
routers (see below). It is also used when the route is exported
to OSPF as a default value for OSPF type 1 metric.
</descrip>
<p>There also exist some protocol-specific attributes which are described in the corresponding protocol sections.
@ -916,12 +924,16 @@ making it possible to implement any routing policy at any router in the
network, the only restriction being that if a router advertises a route,
it must accept and forward packets according to it.
<p>BGP works in terms of autonomous systems (often abbreviated as AS). Each
AS is a part of the network with common management and common routing policy. It is identified by a unique 16-bit number.
Routers within each AS usually communicate with each other using either a interior routing
protocol (such as OSPF or RIP) or an interior variant of BGP (called iBGP).
Boundary routers at the border of the AS communicate with their peers
in the neighboring AS'es via exterior BGP (eBGP).
<p>BGP works in terms of autonomous systems (often abbreviated as
AS). Each AS is a part of the network with common management and
common routing policy. It is identified by a unique 16-bit number
(ASN). Routers within each AS usually exchange AS-internal routing
information with each other using an interior gateway protocol (IGP,
such as OSPF or RIP). Boundary routers at the border of
the AS communicate global (inter-AS) network reachability information with
their neighbors in the neighboring AS'es via exterior BGP (eBGP) and
redistribute received information to other routers in the AS via
interior BGP (iBGP).
<p>Each BGP router sends to its neighbors updates of the parts of its
routing table it wishes to export along with complete path information
@ -962,13 +974,25 @@ among them and so on.
<itemize>
<item>Prefer route with the highest Local Preference attribute.
<item>Prefer route with the shortest AS path.
<item>Prefer IGP origin over EGP and EGP over incomplete.
<item>Prefer IGP origin over EGP and EGP origin over incomplete.
<item>Prefer the lowest value of the Multiple Exit Discriminator.
<item>Prefer internal routes over external ones.
<item>Prefer routes received via eBGP over ones received via iBGP.
<item>Prefer routes with lower internal distance to a boundary router.
<item>Prefer the route with the lowest value of router ID of the
advertising router.
</itemize>
<sect1>IGP routing table
<p>BGP is mainly concerned with global network reachability and with
routes to other autonomous systems. When such routes are redistributed
to routers in the AS via BGP, they contain IP addresses of a boundary
routers (in route attribute NEXT_HOP). BGP depends on existing IGP
routing table with AS-internal routes to determine immediate next hops
for routes and to know their internal distances to boundary routers
for the purpose of BGP route selection. In BIRD, there is usually
one routing table used for both IGP routes and BGP routes.
<sect1>Configuration
<p>Each instance of the BGP corresponds to one neighboring router.
@ -1035,7 +1059,7 @@ for each neighbor using the following configuration parameters:
how it is computed. Direct mode means that the IP address from
<cf/bgp_next_hop/ is used if it is directly reachable,
otherwise the neighbor IP address is used. Recursive mode
means that the gateway is computed by a IGP routing table
means that the gateway is computed by an IGP routing table
lookup for the IP address from <cf/bgp_next_hop/. Recursive
mode is the behavior specified by the BGP standard. Direct
mode is simpler, does not require any routes in a routing
@ -1044,8 +1068,8 @@ for each neighbor using the following configuration parameters:
<cf/direct/ for singlehop eBGP, <cf/recursive/ otherwise.
<tag>igp table <m/name/</tag> Specifies a table that is used
in a recursive gateway mode for computing <cf/gw/ attributes.
Default: the same as the table BGP is connected to.
as an IGP routing table. Default: the same as the table BGP is
connected to.
<tag>password <m/string/</tag> Use this password for MD5 authentication
of BGP sessions. Default: no authentication. Password has to be set by
@ -1157,6 +1181,9 @@ for each neighbor using the following configuration parameters:
<tag>path metric <m/switch/</tag> Enable comparison of path lengths
when deciding which BGP route is the best one. Default: on.
<tag>igp metric <m/switch/</tag> Enable comparison of internal
distances to boundary routers during best route selection. Default: on.
<tag>prefer older <m/switch/</tag> Standard route selection algorithm
breaks ties by comparing router IDs. This changes the behavior
to prefer older routes (when both are external and from different