As technologies such as ISL became more
common, network designers began to use trunk links to connect
routers to a campus backbone. Figure
illustrates an example of this approach.
Although any trunking technology such
as ISL, 802.1Q, 802.10, or LAN Emulation (LANE) can be used,
Ethernet-based approaches are most common (ISL and 802.1Q). Figure
uses ISL running over Fast Ethernet. The solid lines refer to the
single physical link running between the Catalyst Switch and the
router. The dashed lines refer to the multiple logical links running
over this physical link using subinterfaces.
The primary advantage of using a
trunk link is a reduction in router and switch ports. Not only can
this save money, it can also reduce configuration complexity.
Consequently, the trunk-connected router approach can scale to a
much larger number of VLANs than a one-link-per-VLAN design.
However, the trunk-connected router
configuration has disadvantages, including the following:
- There is a possibility of
inadequate bandwidth for each VLAN.
- Additional overhead on the router
can occur.
- Older versions of the Cisco IOS
Software support only a limited set of features on ISL
interfaces.
With regard to inadequate bandwidth
for each VLAN, consider, for example, the use of a Fast Ethernet
link where all VLANs must share 100 Mbps of bandwidth. A single VLAN
could easily consume the entire capacity of the router or the link
(especially in the event of a broadcast storm or Spanning-Tree
problem).
With regard to the additional
overhead on the router caused by using a trunk-connected router, not
only must the router perform normal routing and data-forwarding
duties, it must also handle the additional encapsulation used by the
trunking protocol. Consider ISL running on a high-end router as an
example. These software-based routers have many different switching
modes, a term that Cisco uses to refer generically to the process of
data forwarding in a router. Don't confuse the term switching here
with how it is normally used throughout this course. These
software-based routers use the term switching to refer to the
process of forwarding frames through the box, regardless of whether
the frames are routed or bridged.
Every Cisco router supports multiple
forwarding techniques. Although a full discussion of these is not
appropriate here, an analogy can be made to make the point:
think of switching modes as gears in an automobile transmission. For
example, just as every car has a first gear, every Cisco router
(including low-end routers) supports a feature called process
switching. Process switching relies on the CPU to perform
brute-force routing on every packet. Just as first gear is useful in
all situations (uphill, flat roads, rain, snow, dry, and so on),
process switching can route all packets and protocols. However, just
as first gear is the slowest in a car, process switching is the
slowest forwarding technique for a router.
Every router also has a second
gear-this is referred to as fast switching. Taking advantage of
software-based caching techniques provides faster data forwarding.
However, just as second gear is not useful in all situations (going
up a steep hill, starting at a traffic stop, and so on), fast
switching cannot handle all types of traffic (for example, many
types of Systems Network Architecture [SNA] traffic).
Finally, just as high-end automobiles
offer fancy five-speed transmissions, high-end routers offer a
variety of other switching modes. These switching modes are known by
names such as autonomous switching, silicon switching, optimum
switching, and distributed switching. Think of these as gears three,
four, and five in a transmission-they can allow you to move very
quickly, but can be useful only in ideal conditions and very limited
situations (that is, dry pavement, a long country road, and no
police!).
Getting back to the example of an ISL
interface on a high-end router, these routers normally use
techniques such as optimum switching and distributed switching to
achieve data-forwarding rates from 300,000 to over 1,000,000 packets
per second (pps).
Several performance figures are
included in this chapter to allow you to develop a general sense of
the throughput you can expect from the various Layer 3 switching
options. Keep in mind that throughput numbers are dependent on many
factors such as configuration options, software version, and
hardware revision.
When running ISL on an interface,
that interface becomes limited to second gear (fast switching).
Because of this restriction, ISL routing is limited to approximately
50,000 to 100,000 pps on a high-end router (and considerably less on
many lower-end platforms).
The third disadvantage of the
trunk-connected router design is that older versions of the Cisco
IOS Software support only a limited set of features on ISL
interfaces. Although most limitations were removed in 11.3 and some
later 11.2 images, networks using older images need to carefully
plan the inter-VLAN routing in their network. Some of the more
significant limitations prior to 11.3 include the following:
- Earlier versions support only IP
and IPX. All other protocols (including AppleTalk and DECnet)
must be bridged. Inter-VLAN bridging is almost always a bad idea
because IPX supports only the novell_ether encapsulation (Novell
refers to this as Ethernet_802.3).
- Hot Standby Router Protocol (HSRP)
is not supported. This can make it very difficult or impossible
to provide default gateway redundancy.
- Secondary IP addresses are not
supported.
The example in Figure
configures a Fast Ethernet port to perform ISL Routing for three
VLANs.
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