Including The Nssa - D-Link xStack DGS-3600 User Manual

xStack DGS-3600 Series Layer 3 Gigabit Ethernet Managed Switch

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:
Figure 8- 38. NSSA Area 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
Type-7 LSA (Link State Advertisement) packets are
used to import external routes into the NSSA. These
packets can originate from NSSA ASBRs or NSSA
ABRs and are defined by setting the P-Bit in the LSA
type-7 packet header. Each destination network
learned from external routes is converted into Type-7
LSA packets. These packets are specific for NSSA
switches and the route information contained in these
packets cannot leave the area unless translated into
Type-5 LSA packets by Area Border Routers. See
the following table for a better description of the
LSA type-7 packet seen here.
Figure 8- 39. LSA Type-7 Packet
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