Autonomous Systems External Link Advertisements - D-Link DXS-3326GSR Manual

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
Network Mask
TOS
Metric

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 Stat ID is always
set 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:
DXS-3326GSR Gigabit Layer 3 Switch
Summary Link Advertisements
Link-State Age
Advertising Router
Link-State Sequence Number
Link-State Checksum
Network Mask
TOS
Figure 7- 27. Summary Link Advertisements
Description
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
The Type of Service that the following cost is relevant to.
The cost of this route. Expressed in the same units as the interface
costs in the router links advertisements.
Options
Link-State ID
Length
Metric
149
2