The Lowdown on Committed Information Rate (CIR)

We’ve talked formally about CIR—we even presented a calculation—but what does it really mean?
As we have said, the acronym itself stands for committed information rate, which really doesn’t
seem that difficult to understand. But there seems to be widespread misinterpretation of this concept,
especially by some service providers, so let’s attempt to figure the whole thing out.
We’ve discussed terms such as burst rate and phrases such as bursting above your CIR, but
these terms can be misleading. They were devised by network engineers who assumed—you
know what that leads to—that we wouldn’t understand hardcore network-engineering concepts,
so they tried to put them in layman’s terms and botched the whole thing up. In reality,
you’re always “bursting” to your line speed because Frame Relay is an HDLC protocol and
there’s no other way to make it work.
HDLC is a synchronous protocol (which means that the data is synchronized to a clock) that
sends data with a standardized framing and checksum technique. When a frame is transmitted,
the data must be contiguous; that is, there cannot be any holes or spaces between bytes of
data. So if you’re transmitting 500 bytes of data, you can’t send 250 and then wait for a while
and then send the rest. It has to go out as one big chunk. The Frame Relay expression bursting
over your CIR comes into play because there is no way to slow down the data or to change the
length of the chunk after you start transmitting; you just send until you are finished. If you happen
to send too much data because your data chunk is larger than the allotment, you’ve
bursted over your CIR.
So what is the big deal about CIR, then? And why, when you buy Frame Relay from a company
like Qwest, do they quote you a CIR? CIR is the “worst-case” throughput that the Frame Relay
network provider attempts to guarantee. It’s like a restaurant guaranteeing that you’ll always
be able to eat a certain amount of food from its buffet. Like the restaurant, the Frame Relay network
provider can’t guarantee that you’ll always be able to transmit at the CIR (take the case
when everyone on the network happens to be transmitting at once), but they can guarantee it
over a reasonable time span (usually over a span of seconds). Basically, the network backbone
is engineered to handle reasonable loads—just like the number of lanes in a highway. Given a
certain amount of traffic, the data should flow through the backbone without delay. At times,
when unusually heavy traffic exists, you have what is called congestion.

Frame Relay Summary

Frame Relay is one of the most popular WAN protocols in the world. This technology
will become even more critical as corporations stretch their networks globally and the Internet
becomes more pervasive.
To become a successful CCNP, you need to understand the Frame Relay protocol. This technology
makes up the majority of the world’s non-dedicated circuits, and its importance cannot
be underestimated.
Frame Relay is the distant cousin to X.25, without some of X.25’s overhead. It does provide
congestion notification, which can be used with traffic shaping to help traffic response. Like
X.25, Frame Relay provides for permanent and switched virtual circuits.
LMI is an extension to the Frame Relay protocol developed by the Frame Relay Forum and
is used to provide management for virtual circuits. This makes the management of DLCI information
easier for the network administrator.
Cisco provides for a mechanism to enable a multipoint interface such as Frame Relay to look
like multiple virtual point-to-point or multipoint interfaces called subinterfaces. Point-to-point
subinterfaces can be used to solve problems caused by distance-vector routing protocols running
over multipoint interfaces.
Setting up a Cisco router as a Frame Relay switch is not something that you would do often,
but it is a useful feature when you are working in a lab environment. There are many troubleshooting
commands that can be used to verify the configuration of Frame Relay on a Cisco
router. They can be used to see the DLCI-to-Network-layer-address mapping and the current
state of LMI on the router. Frame Relay is a technology used in many networks, and mastering
its configuration and operation will take you far in your networking career.