User manual

xStack DGS/DXS-3300 Series Layer 3 Stackable Gigabit Ethernet Switch User Manual
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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 destination known to the router, which 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 6- 138. 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 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.