Download Areas The reader should by now have a good feel for why OSPF

The reader should by now have a good feel for why OSPF, with its multiple
databases and complex algorithms, can put greater demands on the
memory and processors of a router than the previously examined
protocols can. As an internet work grows, these demands can become
significant or even crippling. And although flooding is more efficient than
the periodic, full-table updates of RIP and IGRP, it can still place an
unacceptable burden on the data links of a large internet work. Contrary to
popular belief, the SPF algorithm itself is not particularly processor
intensive. It is the related processes, such as flooding and database
maintenance, that burden the CPU.
Note
Benefits of areas
OSPF uses areas to reduce these adverse effects. In the context of OSPF,
an area is a logical grouping of OSPF routers and links that effectively
divide an OSPF domain into sub-domains (Figure 9.20). Routers within an
area will have no detailed knowledge of the topology outside of their area.
Because of this condition:
A router must share an identical link state database only with the other
routers in its area, not with the entire internet work. The reduced size of
the database reduces the impact on a router's memory.
The smaller link state databases mean fewer Lass to process and
therefore less impact on the CPU.
Because the link state database must be maintained only within an area,
most flooding is also limited to the area.
Areas
Virtual-link
• Virtual link is a link backbone through nonbackbone area
• Virtual Links
• A virtual link is a link to the backbone through a
non-backbone area. Virtual links are used for the
following purposes
• To link an area to the backbone through a nonbackbone area .
• To connect the two parts of a partitioned
backbone through a non-backbone area
What is backbone area?
Area ID 0 (or 0.0.0.0) is reserved for the backbone.
The backbone is responsible for summarizing the
topographies of each area to every other area. For
this reason, all inter-area traffic must pass through
the backbone; non-backbone areas cannot exchange
packets directly
What is Stubby Area?
• Several rules are associated with the
configuration of virtual links:
• Virtual links must be configured between
two ABRs.
• The area through which the virtual link is
configured, known as the transit area,
must have full routing information.
• The transit area cannot be a stub area.
• Ospf Areas.
• A stub area is an area into which AS
external lsas are not flooded.
• If type 5 lsas are not known inside area
type 4 lsas are unnecessary.
• For communication purpose stub area will
use network summary LSAs to advertise a
single default route into the area.
4.
Restrictions on stub areas
5.
As in any area, all routers in a stub area must have identical link state
databases. To ensure this condition, all stub routers will set a flag (the Ebit) in their Hello packets to zero; they will not accept any Hello from a
router in which the E-bit is set to one. As a result, adjacencies will not be
established with any router that is not configured as a stub router
6.
7.
Virtual links cannot be configured within, or transit, a stub area
No router within a stub area can be an ASBR. This restriction is intuitively
understandable because ASBRs produce type 5 LSAs and type 5 LSAs
cannot exist within a stub area.
8.
A stub area may have more than one ABR, but because of the default
route, the Internal Routers cannot determine which router is the optimal
gateway to the ASBR.
• Not-so-stubby areas
• allow external routes to be advertised into the
OSPF autonomous system while retaining the
characteristics of a stub area to the rest of the
autonomous system. To do this, the ASBR in an
NSSA will originate type 7 LSAs to advertise the
external destinations. These NSSA External
LSAs are flooded throughout the NSSA but are
blocked at the ABR
• Totally stubby area
• It will block the LSA 3 also.
• Totally stubby areas use a default route to reach
not only destinations external to the autonomous
system but also all destinations external to the
area. The ABR of a totally stubby area will block
not only AS External LSAs but also all Summary
LSAs—with the exception of a single type 3 LSA
to advertise the default route
• Ospf LSAs --- 11.
• 1 Router LSA
• Which is produced by every Router.this LSA will flooded
only with in the area.
• To cheack this lsa show ip ospf database router.
• 2.NETWORK LSA;-this LSA will produced by DR router
on every multi-access network.This LSAs will flooded in
the with in the orginated area.
• The network lsa lists all attached routers, DR itself.
• Command is show ip ospf database network.
Area type
1&2
3&4
5
7
Backbone area
yes
yes
yes no
Non-backbone,non-stub yes yes
yes
no
Stub
yes
yes n0
no
Totstub
yes
no no
no
Non-so-stub
yes
yes no
yes
• Network summary Lsa:---network
summary lsa will produced by ABR.and its
advertise single area to destinations
outside that area.
• Show ip ospf database summary.
• ASBR summary LSA;--its also orginated
by ABRs , ASBR summary LSAs are
identical to network summary LSAs expect
that destination they advertise is an ASBR
not network.
• Show ip ospf database asbr-summary.
•
•
•
•
•
Ospf areas.
Back-bone area
Stub area
Not-so-stubby area
Totally stubby area.
• The NSSA External LSA has a flag in its header
known as the P-bit. The NSSA ASBR has the
option of setting or clearing the P-bit. If the
NSSA's ABR receives a type 7 LSA with the Pbit set to one, it will translate the type 7 LSA into
a type 5 LSA and flood it throughout the other
areas If the P-bit is set to zero, no translation will
take place and the destination in the type 7 LSA
will not be advertised outside of the NSSA
• Nssa external LSAs orginated by ASBR
with in not-so-stubby areas its only flooded
in the nssa only
• Show ip ospf database nssa-external
• External attributes proposed as an
alternative to running internal BGP(ibgp) in
order to transport BGP information across
an OSPF domain.
• Group membership LSA; its for
enhancement of ospf known multicast ospf
packets from single source to multiple
destinations, or group members which
share a class D multicast address.
• As external LSA;--orginated by ASBRs
and advertise either destination external to
the ospf autonomous system,or a default
route external to the ospf AS
• External LSAs are flooded throughout the
autonomous system.
• Show ip ospf database external.