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:

AS External Link Advertisements

Link-State Age

Options5

 

 

 

Link-State ID

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Advertising Router

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Link-State Sequence Number

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Link-State Checksum

Length

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Network Mask

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

E

TOS

Metric

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Forwarding Address

 

 

 

 

 

 

 

 

 

 

 

 

 

 

External Route Tag

 

 

 

 

 

 

 

 

 

Figure 5-36. AS External Link Advertisement

 

 

Field

Description

 

 

Network Mask

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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D-Link DES-3326 manual Autonomous Systems External Link Advertisements, AS External Link Advertisements