D-Link xStack DXS-3350SR Installation & User Manual - page 166
DXS-3350SR Gigabit Layer 3 Switch
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:
Link-State Age
Options
Link-State ID
Advertising Router
Link-State Sequence Number
Link-State Checksum
Length
Network Mask
TOS
Metric
AS External Link Advertisements
5
Forwarding Address
External Route Tag
E
Field Description
Network Mask
The IP address mask for the advertised destination.
E – bit
The type of external metric. If the E – bit is set, the metric specified is
a Type 2 external metric. This means the metric is considered larger
than any link state path. If the E – bit is zero, the specified metric is a
Type 1 external metric. This means that is comparable directly to the
link state metric.
Forwarding Address
Data traffic for the advertised destination will be forwarded to this
address. If the Forwarding Address is set to 0.0.0.0, data traffic will
be forwarded instead to the advertisement’s originator.
TOS
The Type of Service that the following cost is relevant to.
Metric
The cost of this route. The interpretation of this metric depends on
the external type indication (the E – bit above).
External Route Tag
A 32-bit field attached to each external route. This is not used by the
OSPF protocol itself.
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