Allied Telesis AT-9724TS specifications Autonomous Systems External Link Advertisements

Summary Link Advertisements
Octets
0	1	2	3	4
	Link-State Age		Options	2
		Link-State ID

Advertising Router

Link-State Sequence Number

Link-State Checksum

Length

	Network Mask
TOS	Metric
	Figure 6- 109. Summary Link Advertisements

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,
TOS	when advertising the location of a class A network the value 0xff000000
	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.

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
Octets
0	1	2	3	4
	Link-State Age		Options	5
		Link-State ID

Advertising Router

Link-State Sequence Number

Link-State Checksum

Length

E

TOS Metric

Forwarding Address

External Route Tag

Figure 6- 110.AS External Link Advertisements

Allied Telesyn AT-9724TS High-Density Layer 3 Stackable Gigabit Ethernet Switch 116