gated.conf.4 (2010 09)

g
gated.conf(4) gated.conf(4)
metricout metric
Specifies the HELLO metric to be added to routes that are send via the specified
interface(s). The default is zero. This option is used to make other routers prefer other
sources of HELLO routes over this router.
trustedgateways gateway_list
Defines the list of gateways from which HELLO will accept updates. The gateway_list is sim-
ply a list of host names or IP addresses. By default, all routers on the shared network are
trusted to supply routing information. But if the trustedgateways clause is specified only
updates from the gateways in the list are accepted.
sourcegateways gateway_list
Defines a list of routers to which HELLO sends packets directly, not through multicast or
broadcast. This can be used to send different routing information to specific gateways.
Updates to gateways in this list are not affected by noripout on the interface.
traceoptions trace_options
Specifies the tracing options for HELLO. (See Trace Statements and the HELLO specific trac-
ing options below.)
The default preference is 90. The default metric is 30000.
Tracing options
The policy option logs info whenever a new route is announce, the metric being announced changes or
a route goes or leaves holddown.
Packet tracing options (which may be modified with detail, send and/or recv):
packets
All HELLO packets
The OSPF Protocol
Open Shortest Path Routing (OSPF) is a shortest path first (SPF) or link-state protocol. OSPF is an inte-
rior gateway protocol that distributes routing information between routers in a single autonomous sys-
tem. OSPF chooses the least cost path as the best path. Suitable for complex networks with a large
number of routers, OSPF provides equal cost multipath routing where packets to a single destination can
be sent via more than one interface simultaneously. In a link-state protocol, each router maintains a
database describing the entire AS topology, which it builds out of the collected link state advertisements
of all routers. Each participating router distributes its local state (the usable interfaces and reachable
neighbors of the router) throughout the AS by flooding. Each multiaccess network that has at least two
attached routers has a designated router and a backup designated router. The designated router floods a
link state advertisement for the multiaccess network and has other special responsibilities. The desig-
nated router concept reduces the number of adjacencies required on a multiaccess network.
OSPF allows networks to be grouped into areas. Routing information passed between areas is abstracted,
potentially allowing a significant reduction in routing traffic. OSPF uses four different types of routes,
listed in order of preference: intra-area, inter-area, type 1 external and type 2 external. Intra-area paths
have destinations within the same area, inter-area paths have destinations in other OSPF areas and
Autonomous System External (ASE) routes are routes to destinations external to the AS. Routes imported
into OSPF as type 1 routes are supposed to be from igps whose external metrics are directly comparable
to OSPF metrics. When a routing decision is being made, OSPF will add the internal cost to the AS
Border router to the external metric. Type 2 ASEs are used for egps whose metrics are not comparable to
OSPF metrics. In this case, only the internal OSPF cost to the AS Border router is used in the routing
decision.
From the topology database, each router constructs a tree of the shortest paths with itself as the root.
This shortest-path tree gives the route to each destination in the AS. Externally derived routing informa-
tion appears on the tree as leaves. The link-state advertisement format distinguishes between informa-
tion acquired from external sources and information acquired from internal routers, so there is no ambi-
guity about the source or reliability of routes. Externally derived routing information (for example,
routes learned from EGP or BGP) is passed transparently through the autonomous system and is kept
separate from the OSPF internally derived data. Each external route can also be tagged by the advertis-
ing router, enabling a passing of additional information between routers on the borders of the auto-
nomous system.
OSPF optionally includes type of service (TOS) routing and allows administrators to install multiple
routes to a given destination for each type of service (low delay or high throughput.) A router running
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