DES-3326 Layer 3 Fast Ethernet Switch User’s Guide

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

Link-State Age

Options2

 

Link-State ID

 

 

Advertising Router

 

 

 

Link-State Sequence Number

 

 

 

 

Link-State Checksum

Length

 

 

Network Mask

 

 

 

 

TOS

Metric

 

 

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 per-area basis. Default summary routes are used in stub area instead of flooding a complete set of external routes. When describing a default summary route, the advertisement’s Link State ID is always set to the Default Destination 0.0.0.0, and the Network Mask is set to 0.0.0.0.

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 T-bit is reset in the advertisement’s Option field, only a route for TOS 0 is described by the advertisement. 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.

 

 

84

Page 84
Image 84
D-Link DES-3326 manual Summary Link Advertisements