| As the center of the network, the core layer is designed to be
fast and reliable. Access lists are avoided in the core because they
add latency, or delay. Moreover, end users should not access the
core directly. Consider an apple; you can not get to the seeds in an
apple's core without going through the skin first. In a hierarchical
network, end users' traffic should reach core routers only after
those packets have passed through the distribution and access
layers, where access lists may be applied.
Because core routing is done without access lists, address
translation, or other packet manipulation, it may seem as though the
least powerful routers would work well for so simple a task.
However, the
opposite is true. The most powerful Cisco routers serve the core
because they have the fastest switching technologies and the largest
capacity for physical interfaces.
Marketed by Cisco as enterprise core routers, the 7000, 7200, and
7500 series routers feature the fastest switching modes available.
The 12000 series router is also a core router, but it is designed to
meet the core routing needs of Internet service providers (ISPs).
Unless your company is in the business of providing Internet access
to other companies, you are not likely to see a 12000 series router
in your telecommunications closet.
Unlike some routers, such as the Cisco 2500 series, the 7000,
7200, and 7500 series routers are modular, so interface modules can
be added as needed. The large chassis of this series can accommodate
dozens of interfaces on multiple modules for virtually any media
type, which makes these routers scalable, reliable core solutions.
One way that core routers achieve reliability is through using
redundant links, usually to all other core routers. When possible,
these redundant links should be symmetrical (i.e., they should
have equal throughput) so that equal-cost load balancing can be
used. That is why core routers need a relatively large number of
interfaces. Another way that core routers achieve reliability is
through redundant power supplies. Core routers usually feature two
or more "hot-swappable" power supplies, which may be
removed and replaced individually without bringing down the router.
The figure presents a simple core topology using 7507 routers
at three key sites in an enterprise. Each Cisco 7507 is directly
connected to every other router by two links, which makes this
configuration a full mesh. Core links should be the fastest, most
reliable, and most expensive leased lines in the WAN: T1, T3, OC3,
or better. If redundant T1s are used for this WAN core, each router
needs four serial interfaces for two point-to-point connections to
each site. Ultimately, the design requires even more than this
because other routers at the distribution layer will also need to
connect to the core routers. Fortunately, you can easily add
interfaces to the 7507s because they are modular.
You can see that with the high-end routers and WAN links
involved, the core can become a huge expense, even in a simple
example such as this. Some designers will choose not to use
symmetrical links in the core to reduce cost. In place of redundant
lines, packet-switched and dial-on-demand technologies, such as
Frame Relay and ISDN, may be used as backup links. The trade-off for
saving money by using such technologies is performance. For
instance, if you use ISDN BRIs as backup links, you lose the
capability to do equal-cost load balancing.
The core of a network does not have to exist in the WAN. In some
cases, a LAN backbone may also be considered to belong to the core
layer. Campus networks, or large networks that span an office
complex or adjacent buildings, might have a LAN-based core. In this
case, switched Fast Ethernet and Gigabit Ethernet are the most
common core technologies, and they are usually run over fiber.
Enterprises switches, such as the Catalyst 4000, 5000, and 6000
series, shoulder the load in LAN cores because they switch frames at
Layer 2 much faster than routers can switch packets at Layer 3. In
fact, as modular devices, these switches can be equipped with route
switch modules (RSMs), adding Layer 3 routing functionality to the
switch chassis.
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