D-Link xStack DGS-3620 Series Reference Manual page 240

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xStack DGS-3620 Series Layer 3 Managed Stackable Gigabit Switch Web UI Reference Guide
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.
Parameter 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.
Autonomous Systems External Link Advertisements
Autonomous Systems (AS) link advertisements are Type 5 link state advertisements. These advertisements are
originated by AS boundary routers. A separate advertisement is made for each destination known to the router that
is external to the AS.
AS external link advertisements usually describe a particular external destination. For these advertisements the
Link State ID field specifies an IP network number. AS external link advertisements are also used to describe a
default route. Default routes are used when no specific route exists to the destination. When describing a default
route, the Link State ID is always set with the Default Destination address (0.0.0.0) and the Network Mask is set to
0.0.0.0.
The format of the AS External Link Advertisement is shown below:
E
AS External Link Advertisements
Link-State Age
Link-State Sequence Number
Link-State Checksum
TOS
Forwarding Address
External Route Tag
Figure 6-40 AS External Link Advertisements
Options
Link-State ID
Advertising Router
Network Mask
Metric
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5
Length