WAN Support
First, take a look at Table 11.1.
TABLE 1 1 . 1
Setting the Encapsulation on an Interface
Command Meaning
configure terminal
Takes you to global configuration mode
interface
interface
Chooses your interface to configure
encapsulation
encapsulation
Sets the WAN encapsulation type
Basically, Cisco just supports HDLC, PPP, and Frame Relay on its serial interfaces, and you
can see this with the
encapsulation ?
command from any serial interface (your output may
vary depending on the IOS version you are running):
Corp#
config t
Corp(config)#
int s0/0/0
Corp(config-if)#
encapsulation ?
atm-dxi ATM-DXI encapsulation
frame-relay Frame Relay networks
hdlc Serial HDLC synchronous
lapb LAPB (X.25 Level 2)
ppp Point-to-Point protocol
smds Switched Megabit Data Service (SMDS)
x25 X.25
Understand that if I had other types of interfaces on my router, I would have other encapsulation
options, such as ISDN or ADSL. And remember, you can’t configure Ethernet or
Token Ring encapsulation on a serial interface.
Next, I’m going to define the most prominently known WAN protocols used today: Frame
Relay, ISDN, LAPB, LAPD, HDLC, PPP, PPPoE, cable, DSL, MPLS, and ATM. Just so you
know, the only WAN protocols you’ll usually find configured on a serial interface are HDLC,
PPP, and Frame Relay, but who said we’re stuck with using only serial interfaces for wide area
connections?
Frame Relay
A packet-switched technology that made its debut in the early 1990s,
Frame
Relay
is a high-performance Data Link and Physical layer specification. It’s pretty much a successor
to X.25, except that much of the technology in X.25 that used to compensate for physical
errors (noisy lines) has been eliminated. An upside to Frame Relay is that it can be more cost
effective than point-to-point links; plus, it typically runs at speeds of 64Kbps up to 45Mbps
(T3). Another Frame Relay benefit is that it provides features for dynamic bandwidth allocation
and congestion control.
ISDN
Integrated Services Digital Network (ISDN)
is a set of digital services that transmit
voice and data over existing phone lines. ISDN offers a cost-effective solution for remote users
who need a higher-speed connection than analog dial-up links can give them, and it’s also
a good choice to use as a backup link for other types of links such as Frame Relay or T1
connections.
LAPB
Link Access Procedure, Balanced (LAPB)
was created to be a connection-oriented
protocol at the Data Link layer for use with X.25, but it can also be used as a simple data link
transport. A not-so-good characteristic of LAPB is that it tends to create a tremendous amount
of overhead because of its strict timeout and windowing techniques.
LAPD
Link Access Procedure, D-Channel (LAPD)
is used with ISDN at the Data Link layer
(layer 2) as a protocol for the D (signaling) channel. LAPD was derived from the Link Access
Procedure, Balanced (LAPB) protocol and is designed primarily to satisfy the signaling
requirements of ISDN basic access.
HDLC
High-Level Data-Link Control (HDLC)
was derived from Synchronous Data Link
Control (SDLC), which was created by IBM as a Data Link connection protocol. HDLC
works at the Data Link layer and creates very little overhead compared to LAPB.
It wasn’t intended to encapsulate multiple Network layer protocols across the same link—the
HDLC header doesn’t contain any identification about the type of protocol being carried
inside the HDLC encapsulation. Because of this, each vendor that uses HDLC has its own way
of identifying the Network layer protocol, meaning each vendor’s HDLC is proprietary with
regard to its specific equipment.
PPP
Point-to-Point Protocol (PPP)
is a pretty famous, industry-standard protocol. Because
all multiprotocol versions of HDLC are proprietary, PPP can be used to create point-to-point
links between different vendors’ equipment. It uses a Network Control Protocol field in the
Data Link header to identify the Network layer protocol and allows authentication and
multilink connections to be run over asynchronous and synchronous links.
PPPoE
Point-to-Point Protocol over Ethernet encapsulates PPP frames in Ethernet frames
and is usually used in conjunction with ADSL services. It gives you a lot of the familiar PPP
features such as authentication, encryption, and compression, but there’s a downside—it has
a lower maximum transmission unit (MTU) than standard Ethernet does, and if your firewall
isn’t solidly configured, this little attribute can really give you some grief!
Still somewhat popular in the United States, PPPoE on Ethernet’s main feature is that it adds
a direct connection to Ethernet interfaces while providing DSL support as well. It’s often used
by many hosts on a shared Ethernet interface for opening PPP sessions to various destinations
via at least one bridging modem.
Cable
In a modern HFC network, typically 500 to 2,000 active data subscribers are connected
to a certain cable network segment, all sharing the upstream and downstream bandwidth.
(
Hybrid fiber-coaxial
, or HFC, is a telecommunications industry term for a network
that incorporates both optical fiber and coaxial cable to create a broadband network.) The
actual bandwidth for Internet service over a cable TV (CATV) line can be up to about 27Mbps
on the download path to the subscriber, with about 2.5Mbps of bandwidth on the upload
path. Typically users get an access speed from 256Kbps to 6Mbps. This data rate varies greatly
throughout the United States.
DSL
Digital subscriber line
is a technology used by traditional telephone companies to
deliver advanced services (high-speed data and sometimes video) over twisted-pair copper telephone
wires. It typically has lower data carrying capacity than HFC networks, and data speeds
can range by line lengths and quality. DSL is not a complete end-to-end solution but rather aMPLS
MultiProtocol Label Switching (MPLS)
is a data-carrying mechanism that emulates
some properties of a circuit-switched network over a packet-switched network. MPLS is a
switching mechanism that imposes labels (numbers) to packets and then uses those labels to
forward packets. The labels are assigned on the edge of the MPLS of the network, and forwarding
inside the MPLS network is done solely based on labels. Labels usually correspond to
a path to layer-3 destination addresses (equal to IP destination-based routing). MPLS was
designed to support forwarding of protocols other than TCP/IP. Because of this, label switching
within the network is performed the same regardless of the layer-3 protocol. In larger networks,
the result of MPLS labeling is that only the edge routers perform a routing lookup. All
the core routers forward packets based on the labels, which makes forwarding the packets
through the service provider network faster. (Most companies are replacing their Frame Relay
networks with MPLS today.)
ATM
Asynchronous Transfer Mode
(ATM) was created for time-sensitive traffic, providing
simultaneous transmission of voice, video, and data. ATM uses cells that are a fixed 53 bytes
long instead of packets. It also can use isochronous clocking (external clocking) to help the
data move faster. Typically, if you are running Frame Relay today, you will be running Frame
Relay over ATM.e, or wireless. DSL connections are
deployed in the last mile of a local telephone network—the local loop. The connection is set
up between a pair of modems on either end of a copper wire that is between the CPE and the
DSLAM. A DSLAM is the device located at the provider’s CO and concentrates connections
from multiple DSL subscribers.