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

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

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 broad-

cast. 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 tracing

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 interior

gateway protocol that distributes routing information between routers in a single autonomous system.

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 describ-

ing 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 designated 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 Auto-

nomous 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 information

appears on the tree as leaves. The link-state advertisement format distinguishes between information

acquired from external sources and information acquired from internal routers, so there is no ambiguity

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 advertising router,

enabling a passing of additional information between routers on the borders of the autonomous 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 OSPF uses

the destination address and the type of service to choose the best route to the destination.

OSPF intra- and inter-area routes are always imported into the GateD routing database with a preference

of 10. It would be a violation of the protocol if an OSPF router did not participate fully in the OSPF of the

area, so it is not possible to override this. Although it is possible to give other routes lower preference

values explicitly, it is ill-advisedto do so.

Hardware multicast capabilities are also used where possible to deliver link-status messages. OSPF areas

are connected by the backbone area, the area with identiﬁer 0.0.0.0. All areas must be logically contiguous

HP-UX Release 11i: December 2000 − 18 − Section 4−−89

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