Table
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
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| External Route Tag |
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Network Mask | e IP address mask for the advertised destination. | |||||||
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E – bit | The type of external metric. If the E – bit is set, the metric | |||||||
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| specified is a Type 2 external metric. This means the metric is | ||||||
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| considered larger than any link state path. If the E – bit is | ||||||
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| zero, the specified metric is a Type 1 external metric. This | ||||||
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| means that is comparable directly to the link state metric. | ||||||
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Forwarding Address | Data traffic for the advertised destination will be forwarded to | |||||||
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| this address. If the Forwarding Address is set to 0.0.0.0, data | ||||||
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| traffic will be forwarded instead to the advertisement’s | ||||||
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| originator. |
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TOS | The Type of Service that the following cost is relevant to. | |||||||
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Metric | The cost of this route. The interpretation of this metric | |||||||
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