Extreme Networks ExtremeWare XOS 11.1, Руководство пользователя

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Interior Gateway Protocols
ExtremeWare XOS 11.1 Concepts Guide
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Overview of OSPF
OSPF is a link state protocol that distributes routing information between routers belonging to a single
IP domain; the IP domain is also known as an
autonomous system (AS). In a link-state routing protocol,
each router maintains a database describing the topology of the AS. Each participating router has an
identical database maintained from the perspective of that router.
From the link state database (LSDB), each router constructs a tree of shortest paths, using itself as the
root. The shortest path tree provides the route to each destination in the AS. When several equal-cost
routes to a destination exist, traffic can be distributed among them. The cost of a route is described by a
single metric.
Licensing
To use the complete OSPF functionality, you must have a Core license installed on your switch. The
MSM-1 ships with a Core license. The Aspen 8810 switch ships with an Advanced Edge license; you can
obtain a Core License for the switch from Extreme Networks.
A subset of OSPF, called OSPF Edge Mode, is available with an Advanced Edge license.
OSPF Edge Mode
OSPF Edge Mode is a subset of OSPF available on platforms with an Advanced Edge license. There are
two restrictions on OSPF Edge Mode:
●
At most, two Active OSPF VLAN interfaces are permitted. There is no restriction on the number of
Passive interfaces.
●
The OSPF Priority on VLANs is zero, and is not configurable. This prevents the system from acting
as a DR or BDR
Link State Database
Upon initialization, each router transmits a link state advertisement (LSA) on each of its interfaces.
LSAs are collected by each router and entered into the LSDB of each router. Once all LSAs are received,
the router uses the LSDB to calculate the best routes for use in the IP routing table. OSPF uses flooding
to distribute LSAs between routers. Any change in routing information is sent to all of the routers in the
network. All routers within an area have the exact same LSDB.
Table 51 describes LSA type numbers.
Table 51: LSA type numbers
Type Number Description
1 Router LSA
2 Network LSA
3 Summary LSA
4 AS summary LSA
5 AS external LSA
7 NSSA external LSA
9 Link local—Opaque