Filter
Top Products
xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
151
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:
Link-State Age
Options
Link-State ID
Advertising Router
Link-State Sequence Number
Link-State Checksum
Length
Network Mask
TOS Metric
AS External Link Advertisements
5
Forwarding Address
External Route Tag
E
Figure 8- 29. AS External Link Advertisements
Field Description
Network Mask
The IP address mask for the advertised destination.
E - bit
The type of external metric. If the E - bit is set, the metric specified
is a Type 2 external metric. This means the metric is considered
larger than any link state path. If the E - bit is zero, the specified
metric is a Type 1 external metric. This means that is comparable
directly to the link state metric.
Forwarding Address
Data traffic for the advertised destination will be forwarded to this
address. If the Forwarding Address is set to 0.0.0.0, data traffic will
be forwarded instead to the advertisement’s originator.
TOS
The Type of Service that the following cost is relevant to.
Metric
The cost of this route. The interpretation of this metric depends on
the external type indication (the E - bit above).
External Route Tag
A 32-bit field attached to each external route. This is not used by
the OSPF protocol itself.