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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
Table 5-21. Summary Link Advertisement
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
Figure 5-36. AS External Link Advertisement
Field Description
Network Mask
h
e 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).
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