KTI KGS-3120, Руководство по эксплуатации

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Summary Link Advertisements
Summary link advertisements are Type 3 and 4 link state advertisements. These advertisements are
originated by Area Border routers. A separate summary link advertisement is made for each destina-
tion known to the router, that belongs to the Autonomous System (AS), yet is outside the area.
Type 3 link state advertisements are used when the destination is an IP network. In this case the
advertisement
’
s Link State ID field is an IP network number. When the destination is an AS boundary
router, a Type 4 advertisement is used, and the Link State ID field is the AS boundary router
’
s OSPF
Router ID. Other that the difference in the Link State ID field, the format of Type 3 and 4 link state
advertisements is identical.
Summary Link Advertisements
For stub area, Type 3 summary link advertisements can also be used to describe a default route on a
per-area basis. Default summary routes are used in stub area instead of flooding a complete set of
external routes. When describing a default summary route, the advertisement
’
s Link State ID is al-
ways set to the Default Destination - 0.0.0.0, and the Network Mask is set to 0.0.0.0.
Separate costs may be advertised for each IP Type of Service. Note that the cost for TOS 0 must be
included, and is always listed first. If the T-bit is reset in the advertisement
’
s Option field, only a route
for TOS 0 is described by the advertisement. Otherwise, routes for the other TOS values are also
described. If a cost for a certain TOS is not included, its cost defaults to that specified for TOS 0.
Field Description
Network Mask For Type 3 link state advertisements, this indicates the destination network
’
s IP
address mask. For example, when advertising the location of a class A network
the value 0xff000000
TOS The Type of Service that the following cost is relevant to.
Metric The cost of this route. Expressed in the same units as the interface costs in the
router links advertisements.