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OSPF Hierarchical Topology, Areas and Router Roles
(Page 2 of 3)
Router Roles in OSPF Hierarchical Topology
The topology described above is hierarchical
because the routers in the AS are no longer all peers in a single group.
The two-level hierarchy consists of the lower level containing individual
areas, and the higher level that connects them together, which is called
the backbone and is designated as Area 0. The routers
are no longer all peers, but in fact play different roles depending
on where they are located and how they are connected. There are three
different labels applied to routers in this configuration:
- Internal Routers: These are routers that
are only connected to other routers or networks within a single area.
They maintain an LSDB for only that area, and really have no knowledge
of the topology of other areas.
- Area Border Routers: These are routers
that connect to routers or networks in more than one area. They maintain
an LSDB for each area of which they are a part. They also participate
in the backbone.
- Backbone Routers: These are routers that
are part of the OSPF backbone. By definition, this includes all area
border routers, since those routers pass routing information between
areas. However, a backbone router may also be a router that connects
only to other backbone (or area border) routers, and is therefore not
part of any area (other than Area 0).
To summarize: an area border router is always also a backbone router,
but a backbone router is not necessarily an area border router.
Note: The classifications above are independent of the designation of a router as being a boundary router or not, as described in the previous topic. A boundary router is one that talks to routers or networks outside the AS. Now a boundary router will also often be an area border router or a backbone router, but this is not necessarily the casea boundary router could be an internal router in one area.
Okay, I bet you are now wondering
what is the point of all this? Well, the point is exactly the same as
point of using autonomous system architecture in the first place.
The topology of each area matters only to the devices in that area.
This means that changes in that topology only need to be propagated
within the area. It also means that internal routers within Area 1 don't
need to know about anything that goes on within Area 2, and don't need
to maintain information about any area other than their own. Only the
backbone routers (which include at least one area border router within
each area) need to know the details of the entire autonomous system.
These backbone routers condense information about the areas so that
only a summary of each area's topology needs to be advertised
on the backbone.
Routing in a hierarchical topology
AS is performed in one of two ways, depending on the location of the
devices. If the source and destination are in the same area, then routing
occurs only over networks and routers in that area. If they are in a
different area, then the datagram is routed from the source to an area
border router in the source's area, over the backbone to an area border
router in the destination's area, and then finally, delivered to the
destination. Again, this is analogous to how routing works between ASes
in the big-picture internetwork.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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