User manual

xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
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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 8- 25. 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.
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: