Allied Telesis AT-9724TS - Manual (Page 117)

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,
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.
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:
Figure 6- 110.AS External Link Advertisements
116 Allied Telesyn AT-9724TS High-Density Layer 3 Stackable Gigabit Ethernet Switch
Octets
AS External Link Advertisements
Link-State ID
Advertising Router
Link-State Checksum Length
Link-State Sequence Number
0 1 2 3 4
Link-State Age Options 5
E TOS Metric
Forwarding Address
External Route Tag
Octets
Summary Link Advertisements
Link-State ID
Advertising Router
Link-State Checksum Length
Link-State Sequence Number
0 1 2 3 4
Link-State Age Options 2
Network Mask
TOS Metric