Asynchronous Transfer Mode (ATM) is the first networking
architecture developed specifically for supporting multiple
services. ATM networks are capable of supporting audio (voice),
video and data simultaneously. ATM is currently architected to
support up to 2.5 Gbps bandwidth. Data networks immediately get a
performance enhancement when moving to ATM due to the increased
bandwidth over a WAN. Voice networks realize a cost savings due in
part to sharing the same network with data and through voice
compression, silence compression, repetitive pattern suppression,
and dynamic bandwidth allocation. The ATM fixed-size 53-byte cell
enables ATM to support the isochronicitiy of a time-division
multiplexed (TDM) private network with the efficiencies of public
switched data networks (PDSN).
Most network designers are first challenged by the integration of
ATM with the data network. Data network integration requires legacy
network protocols to traverse a cell-based switched network. ATM can
accomplish this in several ways. The first of these is LAN
- LAN emulation (LANE)
ATM employs a standards based specification for enabling the
installed base of legacy LANs and the legacy network protocols
used on these LANs to communicate over an ATM network. This
standard is known as LAN emulation (LANE). LANE uses the Media
Access Control (MAC) sublayer of the OSI data link control Layer
2. Using MAC encapsulation techniques enables ATM to address the
majority of Layer 2 and Layer 3 networking protocols. ATM LANE
logically extends the appearance of a LAN thereby providing legacy
protocols with equivalent performance characteristics as are found
in traditional LAN environments. Figure 6.1 illustrates a typical
ATM topology with LANE support.
LANE can use ATM emulated LANs (ELANs).. Using ELANs, a LAN in
one location is logically connected to a LAN in another location.
This allows a network designer to extend a LAN over an ATM WAN
avoiding the need for routing packets between the two locations.
LANE services can be employed by ATM attached serves or
workstations, edge devices such as switches, and routers when
routing between ELANs is required. ATM LANE uses four components
to establish end-to-end connectivity for legacy protocols and
devices. These are LAN Emulation Client, LAN emulation
configuration server (LECS), LAN emulation server (LES), and
Broadcast and Unknown Server (BUS).
- LAN Emulation Client (LEC)
Any end system that connects using ATM require a LAN emulation
Client (LEC). The LEC performs the emulation necessary in support of
the legacy LAN. The functions of the LEC are:
- Data forwarding
- Address resolution
- Registering MAC addresses with the LANE server
- Communication with other LECs using ATM virtual channel
End systems that support the LEC functions are:
- ATM-attached workstations
- ATM-attached servers
- ATM LAN switches (Cisco Catalyst family)
- ATM attached routers (Cisco 12000, 7500, 7000, 4700, 4500 and
- LAN Emulation Configuration Server (LECS)
The ELAN database is maintained by the LAN emulation
configuration server (LECS). In addition, the LECS builds and
maintains an ATM address database of LAN Emulation Servers (LES).
The LECS maps an ELAN name to a LES ATM address. The LECS performs
the following LANE functions:
- Accepts queries from a LEC
- Responds to LEC query with an ATM address of the LES for the
- Serves multiple emulated LANs
- Manually defined and maintained
The LECS assigns individual clients to a ELAN by directing them
to the LES that corresponds to the ELAN.
- LAN Emulation Server (LES)
LECs are controlled from a central control point called a
LAN Emulation Server (LES). LECs communicate with the LES
using a Control Direct Virtual Channel Connection (VCC). The
Control Direct VCC is used for forwarding registration and
control information. The LES uses a Control Distribute VCC, a
point-to-multipoint VCC, enabling the LES to forward control
information to all the LECs. The LES services the LAN
Emulation Address Resolution Protocol (LE_ARP) request which
it uses to build an maintain a list of LAN destination MAC
- Broadcast Unknown Server (BUS)
ATM is based on the notion that the network is
point-to-point. Therefore, there is no inherent support for
broadcast or any-to-any services. LANE provides this type of
support over ATM by centralizing broadcast and multicast
functions on a Broadcast And Unknown Server (BUS). Each LEC
communicates with the BUS using a Multicast Send VCC. The BUS
communicates with all LECs using point-multipoint VCC known as
the Multicast Forward VCC. A BUS reassembles received cells on
each Multicast Send VCC in sequence to create the complete
frame. Once a frame is complete is then sent to all the LECs
on a Multicast Forward VCC. This ensures the proper sequence
of data between LECs.
- LANE Design Considerations
The following are guidelines for designing LANE services on Cisco
- The AIP has a bi-directional limit of 60 thousand packets per
- The ATM interface on a Cisco router has the capability of
supporting up to 255 subinterfaces.
- Only one active LECS can support all the ELANs. Other LECS
operate in backup mode.
- Each ELAN has one LES/BUS pair and one or more LECs.
- LES and BUS must be defined on the same subinterface of the
- Only one LES/BUS pair per ELAN is permitted.
- Only one active LES/BUS pair per subinterface is allowed.
- LANE Phase 1 standard does not provide for LES/BUS redundancy.
- The LECS can reside on a different router than the LES/BUS
- VCCs are supported over switched virtual circuits (SVCs) or
permanent virtual circuits (PVCs).
- A subinterface supports only one LEC.
- Protocols such as , AppleTalk, IP and IPX are routable over a
LEC if they are defined on the AIP subinterface.
- AN ELAN should be in only one subnet for IP.
- Network Support
The LANE support in Cisco IOS enables legacy LAN protocols to
utilize ATM as the transport mechanism for inter-LAN communications.
The following features highlight the Cisco IOS support for LANE:
- Support for Ethernet-emulated LANs only. There is currently no
token-ring LAN emulation support.
- Support for routing between ELANs using IP, IPX or AppleTalk.
- Support for bridging between ELANs
- Support for bridging between ELANs and LANs
- LANE server redundancy support through simple server
redundancy protocol (SSRP)
- IP gateway redundancy support using hot standby routing
- DECnet, Banyan VINES, and XNS routed protocols
LANE requires MAC addressing for every client. LANE clients
defined on the same interface or subinterface automatically
have the same MAC address. This MAC address is used as the end
system identifier (ESI) value of the ATM address. Though the
MAC address is duplicated the resulting ATM address
representing each LANE client is unique. All ATM addresses
must be unique for proper ATM operations. Each LANE services
component has an ATM address unique form all other ATM
- LANE ATM Addresses
LANE uses the NSAP ATM address syntax however it is not a Layer 3
network address. The address format used by LANE is :
- A 13-byte prefix that includes the following fields defined by
the ATM Forum:
- AFI (Authority and Format Identifier) field (1 byte)
- DCC (Data Country Code) or ICD (International Code Designator)
field (2 bytes)
- DFI field (Domain Specific Part Format Identifier) (1 byte)
- Administrative Authority field (3 bytes)
- Reserved field (2 bytes)
- Routing Domain field (2 bytes)
- Area field (2 bytes)
- A 6-byte end-system identifier (ESI)
- Cisco's Method of Automatically Assigning ATM Addresses
The Cisco IOS supports an automated function of defining ATM and
MAC addresses. Theses addresses are used in the LECS database. The
automation process uses a pool of eight MAC address that are
assigned to each router ATM interface. The Cisco IOS applies the
addresses to the LANE components using the following
- All LANE components on the router use the same prefix value.
The prefix value identifies a switch and must be defined within
- The first address in the MAC address pool becomes the ESI
field value for every LANE client on the interface.
- The second address in the MAC address pool becomes the ESI
field value for every LANE server on the interface.
- The third address in the MAC address pool becomes the ESI
field value for the LANE broadcast-and-unknown server on the
- The fourth address in the MAC address pool becomes the ESI
field value for the LANE configuration server on the interface.
- The selector field for the LANE configuration server is set to
a 0 value. All other components use the subinterface number of
interface to which they are defined as the selector field.
The requirement that the LANE components be defined on different
subinterfaces of an ATM interface results in a unique ATM address
due to the use of the selector field value being set to the
- Using ATM Address Templates
ATM address definitions is greatly simplified through the
use of address templates. However, these templates are not
supported for the E.164 ATM address format. The address
templates used for LANE ATM addressing can use either an
asterisk (*) or an ellipsis (å) character. An asterisk is used
for matching any single character. An ellipsis is used for
matching leading or trailing characters. Table 6.1 lists the
address template value determination.
Unspecified Digits In
Resulting Value Is
Prefix (first 13 bytes)
Obtained from ATM switch via Interim Local Management
ESI (next 6 bytes)
Filled using the first MAC address of the MAC address
2-LANE broadcast-and-unknown server
Selector field (last 1 byte)
Subinterface number, in the range 0 through
The ATM address templates can be either a prefix, or ESI
template. When using a prefix template, the first 13 bytes
match the defined prefix for the switch but uses wildcards for
the ESI and selector fields. An ESI template matches the ESI
field but uses wildcards for the prefix and selector
- Rules for Assigning Components to Interfaces and
The LANE components can be assigned to the primary ATM interface
as well as the subinterfaces. The following are gudielines for
applying LANE components on a Cisco router ATM interface.
- The LECS always runs on the primary interface.
- Assignment a component to the primary interface falls through
to assigning that component on the 0 subinterface.
- The LES and LEC of the same emulated LAN can be configured on
the same subinterface in a router.
- LECs of two different emulated LANs must be defined on a
different subinterface in a router.
- LESs of two different emulated LANs must be defined on a
different subinterface in a router.
- Redundancy in LANE environments
The ATM LANE V 1.0 specification does not provide for redundancy
of the LANE components. High avialbility is always a goal for
network designers and the single point of failure in the LANE
specification requires a technique for redundancy. Cisco IOS
supports LANE redundancy through the implmenentation of Simple
Server Replicatoin Protocol (SSRP).
SSRP supports redundancy for LECS and LES/BUS services. LECS
redundancy is provided by configuring multiple LECS address in the
ATM switches. Each defined LECS is defined with a rank. The rank is
the index (number of the entry in the LECS address table) of the
LECS address in the table. At iitialization the LECS requests the
LECS address table form the ATM swixth. The requesting LECs
onreceipt of the LECS addres table tries to connect to all the LECSs
with a lower rank. In this way the LECS learns of its role in the
redundancy hierarchy. A LECS that connects with a LECS whose rank is
higher places itself in a backup mode. The LECS that connects to all
other LECS and does not find a ranking higher than its own assumes
the responsibility of the primary LECS. In this hierarchy, as shown
in Figure 6.2, the failure of a primary LECS does not result in a
LANE failure. Rather , the second highest ranking LECS assumes the
primary LECS role. Loss of the VCC between the primary and highest
ranking secondary signals the highest secondary ranking LECS that it
is now the primary LECS.
In theory any number of LECS can be designed using SSRP. However,
Cisco recommends that no more than three LECS be designed into SSRP.
The recommendation is based on adding a degree of complexity to the
network design which can lead to an increase in the time it takes
for resolving problems.
LES/BUS redundancy using SSRP is similar in that it uses a
primary-secondary hierarchy however, the primary LES/BUS pair is
assigned by the primary LECS. The LECS determines the primary
LES/BUS pair by determining the LES/BUS pair having the highest
priority with an open VCC to the primary LECS. The LES/BUS pair
priority is assigned during configuration into the LECS
The following guidelines are highly recommended for desinging the
LECS redundancy scheme and ensuring a properly running SSRP
- Each LECS must maintain the same ELAN database.
- Configure the LECS addresses in the LECS address table in the
same order on each ATM switch in the network.
- Do not define two LECSs on the same ATM switch when using the
Well Known Address. Only one of the LECS will register the Well
Known Address with the switch which may led to initialization
A second type of redundancy mechanism used in LANE is specific to
ELANS using IP protocol. The Host Standby Router Protocol (HSRP)
enables two routers to share a common virtual IP address using a
virtual MAC address assigned to the resulting virtual interface.
This enables two routers to respond as the single IP gateway address
for IP end stations. Figure 6.3 illustrates the use of HSRP with
LANE. The primary and secondary router interface is determined by
definition of HSRP on interface or subinterface. HSRP exchanges
definition information between the two routers to determine which
interface is the primary gateway address. The secondary then sends
HELLO messages to the primary to determine its viability. When the
secondary does not receive a HELLO message from the primary HSRP
router it assumes the primary role.
- Data Exchange Interface (DXI)
ATM networks connect to serial attached routers by
implementing the ATM data exchange interface (DXI)
specification. The DXI specification enables ATM user-network
interface (UNI) connectivity between a Cisco router with only a
serial interface to the ATM network. This is accomplished using
an ATM Data Service Unit (ADSU). As shown in Figure 6.4, router
R1 connects to the ADSU using a High Speed Serial Interface
(HSSI) connection. The ADSU recevies data from the router in the
ATM DXI format. The ADSU then converts the data into ATM cells
and forwards them to the ATM network. The ADSU performs the
opposite function for data going to the router.
- Supported Modes
While there are three modes of ATM DXI the Cisco IOS supports
only mode 1a. The three modes are:
- Mode 1a-Supports AAL5 only, a 9232 octet maximum, a 16-bit
FCS, up to 1023 virtual circuits.
- Mode 1b-Supports AAL3/4 and AAL5, a 9224 octet maximum, a
16-bit FCS. AAL5 support up to 1023 virtual circuits. AAL3/4 is
supported on one virtual circuit.
- Mode 2-Supports AAL3/4 and AAL5 with 16,777,215 virtual
circuits, a 65535 octet maximum, and 32-bit FCS.
- DXI Addressing
The DXI addressing using a value which is equivalent to a frame
relay data link connection identifier. In DXI this field is called a
DFA. The ADSU maps the DFA to the appropriate ATM Virtual Path
Identifier (VPI) and Virtual Connection Identifier (VCI). Figure 6.5
illustrates the bytes and position mapping of the DXI DFA address to
the ATM cell VPI and VCI values.
- Classical IP
Cisco routers are configurable as both an IP client and IP
server in support of Classical IP. Classical IP enables the
routers to view the ATM network as a Logical IP Subnet (LIS).
Configuring the routers as an ATM ARP server enables classical
IP networks to communicate over an ATM network. The benefit to
this is a simplified configuration. Classical IP support using
an ATM ARP server alleviates the need to define the IP network
address and ATM address of each end device connecting through
the router in the router configuration.
ATM uses PVCs and SVCs. The ATM ARP server feature of
Classical IP is specific to using SVCs. Using the ATM ARP server
feature each end device only configures its own ATM address and
the address of the ATM ARP server. Since RFC 1577 allows for
only one ATM ARP server address there is no redundancy available
for Classical IP. As shown in Figure 6.6, the ATM ARP server
address can point to a Cisco router. IP clients using Classical
IP make a connection to the ATM ARP server address defined in
their configuration. The server then sends an ATM Inverse ARP
(InARP) request to the client. The client responds with its IP
network address and ATM address. The ATM ARP server places these
addresses in its cache. The cache is used to resolve ATM ARP
requests from IP clients. The IP client established a connection
to the IP-ATM address provided in the ATM ARP server reply.
- Multiprotocol over ATM (MPOA)
MPOA provides a single solution for transporting all protocols
through an ATM network. MPOA V1.0 in concert with LANE
User-to-Network Interface (UNI) V2.0 allows routers and other ATM
networking devices to fully exploit VLANs, QoS and
high-availability. These network enhancements enable designers to
add services while relieving traffic congestion and flexibility to
the network. The key benefits to MPOA are:
- Inter-VLAN "cut-through" which maximizes bandwidth and
- Robust Layer 3 QoS features to support packetized traffic
such as video or voice, while ensuring data service levels.
- Software only upgrade which minimizes the cost and
The MPOA specification is built on four components. These
- MPOA Client (MPC)
- MPOA Server (MPS)
- Next Hop Resolution Protocol (NHRP)
- LAN Emulation (LANE)
Both MPC and MPS functions are supported on Cisco routers. MPOA
uses a direct virtual channel connection (VCC) between the ingress
(inbound) and egress (outbound) edge or host device. Direct VCCs are
also termed shortcut VCCs. The direct VCC enables the forwarding of
Layer-3 packets, normally routed through intermediate routers,
between source and destination host thereby increasing performance
and reducing latency.
Figure 6.7, illustrates the use of MCP, MPS, and NHRP for
establishing a direct VCC between two edge devices servicing two end
- Multiprotocol Client (MPC)
Typically, the Multiprotocol client (MPC) will reside on an ATM
edge device such as a Cisco Catalyst family of switches. However, a
Cisco router can perform the functions of an MPC or MPS. An MPC
provides the following functions:
- Ingress/egress cache management
- ATM data-plane and control-plane VCC management
- MPOA frame processing
- MPOA protocol and flow detection
- Identifies packets sent to an MPOA-capable router
- Attempts to establish a direct VCC with the egress
- Multiprotocol Server (MPS)
The Multiprotocol server (MPS) provides the forwarding
information used by the MPCs. The MPS maintains the information by
using Next Hop Resolution Protocol (NHRP). MPS interacts with the
NHRP module running in the router. MPS interacts with NHRP in the
- The MPS converts the MPOA resolution request to a NHRP
request. The MPS then sends the NHRP request to either the next
hop MPS or the Next Hop server (NHS) based on the results form
the next hop information search through the MPS tables. MPS
ensures that the correct encapsulation is used depending on the
next hop server type.
- If the next hop is determined to be on a LANE cloud the NHS
sends resolution requests to the MPS. Likewise, the NHS sends
resolution requests when the destination of the packet is
unknown. The MPS may also request the NHS to terminate the
request or discard the packet.
- If the replies terminate in the router or the next hop
interface uses LANE, resolution replies are sent from the NHS to
- Upon receiving resolution replies from the NHS the MPS sends
a MPOA resolution reply to the MPC.
MPS uses a network ID. The default nework ID for all MPSs is 1.
Using different network IDs allows the network designer to segregate
traffic. This enables the designer to permit direct VCCs between
groups of LECs and deny direct VCCs between others. The network ID
of an MPS and NHRP on the same router must be the same in order for
reqeusts, replies and shortcuts across the MPS and NHRP.
- MPOA Guidelines
The following is a list of guidelines for designing MPOA:
- An ELAN identifier must be defined for each ELAN.
- An MPC/MPS can serve as a single LEC or multiple LECs.
- A LEC can associate with any MPC/MPS.
- A LEC can attach to only one MPC and one MPS at a time.
- A LEC must break its attachment to the current MPC or MPS
before attaching another MPC or MPS.
- A primary ATM interface can have multiple MPCs or MPSs
defined with different control ATM addresses.
- Multiple MPCs or MPSs can be attached to the same interface.
- The interface attached to the MPC or MPS must be reachable
through the ATM network by all LECs that bind to it.
- Bandwidth support on routers
ATM is supported on the Cisco 7500 and 7000 series routers using
the ATM Interface Processor (AIP). In designing the ATM internetwork
in support of LANE the total ATM bandwidth support for the entire
router should not exceed 200 Mbps in full duplex mode. This results
in the following possible hardware configurations:
- Two Transparent Asynchronous Transmitter/Receiver Interface
- One OC-3 Synchronous Optical Network (SONET) and one E3
- One OC-3 SONET and one low-use OC-3 SONET connections
- Five E3 connections
- Configurable Traffic Parameters
The AIP provides the ability to shape various traffic. The AIP
supports up to eight rate queues. Each queue is programmed for a
different peak rate. The ATM virtual circuits can be assigned to one
of the eight rate queues. A virtual circuit can have an average rate
and a burst size defined. The AIP supports the following
configurable traffic rate parameters:
- Forward peak cell rate
- Backward peak cell rate
- Forward sustainable cell rate
- Backward sustainable cell rate
- Forward maximum burst
- Backward maximum burst