3Com Switch 7757, Configuration Manual

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OSPF Overview 303
some non-backbone areas on the edge of the AS, you can configure these areas as
stub areas.
A stub area cannot import any external route. For this reason the concept NSSA
area (not-so-stubby area) is introduced. In an NSSA area, type 7 LSAs are allowed
to be propagated. A type 7 LSA is generated by an ASBR (autonomous system
boundary router) in a NSSA area. A type 7 LSA reaching an ABR in the NSSA area
is transformed into an AS-external LSA, which is then advertised to other areas.
Backbone area and virtual link
Backbone Area
With OSPF area partition, not all areas are equal. One of the areas is different from
any other area. Its area ID is 0 and it is usually called the backbone area.
Virtual link
Since all areas must be connected to the backbone area, the concept virtual link is
introduced to maintain logical connectivity between the backbone area and any
other area physically separated from the backbone area.
Route summary
After an AS is divided into different areas that are interconnected through OSPF
ABRs, The routing information between areas can be reduced through route
summary. This reduces the size of routing tables and improves the calculation
speed of routers.
After an ABR in an area calculates the intra-area routes in the area, the ABR
aggregates multiple OSPF routes into one LSA (based on the summary
configuration) and sends the LSA outside the area.
For example, as shown in Figure 71, there are three intra-area routes in Area 1:
19.1.1.0/24, 19.1.2.0/24, and 19.1.3.0/24. If route summary is configured, the
three routes are aggregated into one route 19.1.0.0/16, and only one
corresponding LSA is generated on Router A into Area 0.
Figure 71 Area partition and route aggregation
OSPF Network Type Four OSPF network types
OSPF divides networks into four types by link layer protocols:
■
Broadcast: If Ethernet or FDDI is adopted, OSPF defaults the network type to
broadcast. In a broadcast network, protocol packets are sent in multicast
(224.0.0.5 and 224.0.0.6) by default.
Router A
ABR
Router B
ABR
Area 0
19.1.1.0/24
19.1.2.0/24
19.1.3.0/24
ĂĂ
Area 1
19.1.0.0/16