Transcript Multi-Area OSPF - Strona startowa

Multi-Area OSPF
Multi-area OSPF networks can be
difficult to design, and typically
demand more administrative
attention than any other popular
interior gateway protocol.
OSPF Advantages
• OSPF routers in a heavily populated
OSPF network can be overwhelmed by:
high demand for router processing and
memory resources, large routing tables,
and large topology tables.
• OSPF allows splitting the network into
manageable pieces, or areas, thereby
allowing tremendous scalability.
Hierarchical Routing
• OSPF's capability to separate a large
internetwork into multiple smaller
internetworks, called areas, is referred to as
hierarchical routing.
• With this technique interarea routing occurs,
which is simply the process of exchanging
routing information between OSPF areas.
• Interarea routing allows OSPF to summarize
and contain area-specific information so that
many of the smaller internal routing
operations are restricted to one area.
Hierarchical Routing Advantages
The hierarchical topology possibilities of
OSPF yield several important
advantages:
• Reduced frequency of SPF calculations
• Smaller routing tables
• Reduced LSU overhead
OSPF Router Types
There are four different types of OSPF Routers:
• Internal router - routers that have all their interfaces within the
same area with identical Link State databases and running the
same routing algorithm.
• Backbone router - Routers attached to the backbone area of the
OSPF network with at least one interface connected to Area 0.
• Area Border Router - routers with interfaces attached to multiple
areas that maintain separate link-state databases for each area
they are connected to. Areas can have one or more ABRs.
• Autonomous System Boundary Router - ASBRs - routers that
have at least one interface connected to an external
internetwork (another autonomous system), such as a nonOSPF network.
A router can be more than one router type. If a router interconnects to
Area 0 and Area 1, as well as to a non-OSPF network, it would be both
an ABR and an ASBR.
OSPF Area Types
• Backbone area (Area 0) – A transit area, the central entity to
which all other areas connect, all other areas must connect to
this area to exchange route information. Accepts all LSAs.
• Stub area - Area that does not accept information about routes
external to the autonomous system (AS), such as routes from
non-OSPF sources. If routers need to reach networks outside
the AS, they use a default route. Does not accept LSAs.
• Totally stubby area - Area that does not accept external AS
routes and summary routes from other areas internal to the AS.
Does not accept external or summary LSAs. A totally
proprietary Cisco feature that uses a default route to reach
networks outside the AS.
• Not-so-stubby areas - Area that is similar to a stub area but
allows for importing external routes as Type 7 LSAs and
translation of specific Type 7 LSA routes into Type 5 LSAs.
LSA types
• LSA1s build the Router Link States
• LSA2s build the Net Link States (O)
• LSA3s build the Summary Net Link
States (O IA)
• LSA4s build the AS Summary Net Link
States (IA)
• LSA5s build external route information
(E1 or E2)
Stub and Totally Stub Areas
• You can configure an OSPF router interface to either
operate in a stub area (does not accept information about
routes external to the AS) or as a totally stubby area (does
not accept external AS routes and summary routes from
other areas internal to the AS).
• By configuring an area as stub, you can greatly reduce the
size of the link-state database inside that area and, as a
result, reduce the memory requirements of area routers.
• Stub areas are typically created when you have a huband-spoke topology, with the spokes (such as branch
offices) configured as stub areas. Branch offices may not
need to know about every network at the headquarters
site but can instead use default routes to get there.
Is it Stubby? Or Totally Stubby?
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An area qualifies as stub or totally stubby
when it meets the following criteria:
There is a single exit point from that area.
The area is not needed as a transit area for
virtual links.
No ASBR is internal to the stub area.
The area is not the backbone area (Area 0).
All these criteria are important because a
stub/totally stubby area is configured primarily
to exclude external routes.
Configuring Stub/Totally Stub Areas
(See curriculum 5.4.4 and 5.4.5)
To configure a stub or totally stubby area, use the following
on all router interfaces in that area:
Router(config-router)#area area-id stub
An optional no-summary keyword is added only on ABRs.
This configures the ABR to block interarea routes (Type 3
and Type 4 LSAs). The no-summary creates a totally
stubby area.
The area stub command is configured on each router in
the stub location and is essential for the routers to
become neighbors and exchange routing information.
On ABRs only, there is an option of defining a cost of the
default route to be automatically injected in the stub/totally
stubby area:
Router(config-router)#area area-id default-
Not-so-stubby Areas (NSSAs)
• NSSAs are a relatively new, standards-based
OSPF enhancement.
• Generally, NSSAs are the area located
between a RIP/IGRP network and Area 0 of
an OSPF network.
• This area allows LSA5s in, in a limited
fashion. Data updates about the RIP or IGRP
networks come in as LSA7s and are then
switched into Area 0 via an ABR as LSA5s.
Configuring NSSAs
To configure an area as a NSSA, you must configure all
OSPF router interfaces that belong to the area using the
following command syntax:
Router(config-router)#area area-id nssa [nosummary]
The no-summary keyword is used on the ABR and
typically makes the NSSA totally stubby.
To control the summarization or filtering during the
translation using the following syntax:
Router(config)#summary-address prefix mask
[not-advertise] [tag tag]
The not-advertise keyword is used to suppress routes
that match the prefix/mask pair. This keyword applies to
OSPF only. The tag value can also be assigned but is not
Configuring NSSAs (continued)
The ABR does not generate default routes in an NSSA. To
force the ABR to generate the default route, use this
command only on the ABR for the NSSA:
router ospf pid
area id nssa default-information originate
To verify that NSSA is defined on a given router, you
can use the show ip ospf command
Virtual Links
If a new area is added after the OSPF internetwork
has been designed, and it is not possible to provide
that new area with direct access to the backbone, a
virtual link can be defined to provide the needed
connectivity to the backbone area. Because all areas
must be connected to Area 0, the virtual link provides
the disconnected area a logical path to the backbone.
The virtual link has the two requirements:
• It must be established between two routers that share
a common area.
• One of these two routers must be connected to the
backbone.
Reasons for Virtual Links
• They can link an area that does not have a physical
connection to the backbone. For example, when two
organizations merge.
• They can patch the backbone if discontinuity in Area
0 occurs. For example, if two companies merge their
two separate OSPF networks into a single one with a
common Area 0, the only alternative for the
companies is to redesign the entire OSPF network
and create a unified backbone.
• Virtual links can add redundancy in cases when
router failure might cause the backbone to be split
into two.
Configuring Virtual Links
On each router that will use the virtual
link, create the "virtual link"
configuration. The routers that make
the links are the ABR that connects
the remote area to the transit area and
the ABR that connects the transit area
to the backbone area:
router(config-router)#area area-id
virtual-link router-id
Summarization
• Summarization is the consolidation of
multiple routes into one single, supernet
advertisement to conserve bandwidth,
CPU, and memory resources .
• Summarization is done only on the ABR
and ASBR.
Summarization commands:
• On the ABR –
(config-router)#area id range address mask
• On the ASBR –
(config-router)#summary-address address
mask