D-Link DGS-3620-52P Reference Manual page 190

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xStack DGS-3620 Series Layer 3 Managed Stackable Gigabit Switch Web UI Reference Guide
Parameter
Network Mask
E-bit
Forwarding Address
TOS
Metric
External Route Tag
Including the NSSA
The NSSA or Not So Stubby Area is a feature that has been added to OSPF so external routes from ASs
(Autonomous Systems) can be imported into the OSPF area. As an extension of stub areas, the NSSA feature
uses a packet translation system used by BRs (Border Routers) to translate outside routes into the OSPF area.
Consider the following example:
The NSSA ASBR (Not So Stubby Area Autonomous System Border Router) is receiving External Route information
and translating it as an LSA Type-7 packet that will be distributed ONLY to switches within the NSSA (Area 2 in the
example above). For this route's information to enter another area, the LSA Type-7 packet has to be translated into
an LSA Type-5 packet by the NSSA ABR (Area Border Router) and then is distributed to other switches within the
other OSPF areas (Area 1 and 2 in the example above). Once completed, new routes are learned and new
shortest routes will be determined.
To alleviate any problems with OSPF summary routing due to new routes and packets, all NSSA area border
routers (ABR) must support optional importing of LSA type-3 summary packets into the NSSA.
Type-7 LSA Packets
Description
The IP address mask for the advertised destination.
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.
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.
The Type of Service that the following cost is relevant to.
The cost of this route. The interpretation of this metric depends on the external type
indication (the E - bit above).
A 32-bit field attached to each external route. This is not used by the OSPF protocol
itself.
Figure 5-32 NSSA Area example
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