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| -rw-r--r-- | doc/bird.sgml | 49 |
1 files changed, 38 insertions, 11 deletions
diff --git a/doc/bird.sgml b/doc/bird.sgml index c848a79..266c449 100644 --- a/doc/bird.sgml +++ b/doc/bird.sgml @@ -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 |