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6.7 | ![]() |
Stub, Totally Stubby, and Not-So-Stubby Areas | |
6.7.6 | ![]() |
NSSA overview |
NSSAs are a relatively new, standards
based OSPF enhancement. To understand how to use NSSAs, consider the
network shown in Figure
![]() RTA connects to an external RIP domain,
and RTB currently serves as an ABR for Area 0. If the RIP domain is
not under the administrative control, what options are there to
exchange routing information between these two domains? If dynamic
routing is going to be used, an OSPF standard area could be created.
However, what if the routers that are
placed in Area 1 do not have the required processing power or memory
to run OSPF? It has been learned that the burden on OSPF routers can
be reduced by configuring them to participate in a stub or totally
stubby area. Figure
A stub area cannot include an ASBR
because Type 5, external LSAs are not allowed in a stub domain. The
configuration shown in Figure
So, how does external routing information dynamically exchanged without creating a standard OSPF area? Another routing protocol could be configured, such as RIP or IGRP, in place of creating an Area 1. This may prove to be a disadvantage. This is because an additional routing protocol must be maintained and imported into OSPF and because the RIP domain is not under the administrative control. With the introduction of the NSSA,
there is another, easier option. An NSSA acts like a stub network in
the sense that it does not allow Type 5 LSAs. It can also be
configured to prevent floods of Type 3 and Type 4 summary LSAs, just
as a totally stubby area would. However, an NSSA does allow Type 7
LSAs, which can carry external routing information and be flooded
throughout the NSSA.
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