
Summary Link Advertisements
Summary link advertisements are Type 3 and 4 link state advertisements. These advertisements are originated by Area Border routers. A separate summary link advertisement is made for each destination known to the router, that belongs to the Autonomous System (AS), yet is outside the area.
Type 3 link state advertisements are used when the destination is an IP network. In this case the advertisement’s Link State ID field is an IP network number. When the destination is an AS boundary router, a Type 4 advertisement is used, and the Link State ID field is the AS boundary router’s OSPF Router ID. Other that the difference in the Link State ID field, the format of Type 3 and 4 link state advertisements is identical.
Summary Link Advertisements
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Advertising Router | |
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Length | |
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Network Mask | |
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TOS | Metric |
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Figure 5-35. Summary Link Advertisement
For stub area, Type 3 summary link advertisements can also be used to describe a default route on a
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
Field | Description |
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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 |
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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. |
| Expressed in the same units |
| as the interface costs in the |
| router links advertisements. |
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