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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
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 destination
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.
Link-State Age
Options
Link-State ID
Advertising Router
Link-State Sequence Number
Link-State Checksum
Length
2
Network Mask
Summary Link Advertisements
TOS Metric
Figure 5-35. Summary Link Advertisement
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 always
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.
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