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Chap 3 – Frame Relay
Learning Objectives
•
•
•
•
Describe the fundamental concepts of Frame Relay technology in terms of
Enterprise WAN services including Frame Relay operation, Frame Relay
implementation requirements, Frame Relay maps, and LMI operation.
Configure a basic Frame Relay PVC including configuring and troubleshooting
Frame Relay on a router serial interface and configuring a static Frame Relay
map.
Describe advanced concepts of Frame Relay technology in terms of
Enterprise WAN services including Frame Relay sub-interfaces, Frame Relay
bandwidth and flow control.
Configure an advanced Frame Relay PVC including solving reachability issues,
configuring Frame Relay sub-interfaces, verifying and troubleshooting Frame
Relay configuration.
1
Chapter 3
Requirement for Frame Relay
•A dedicated line provides
little practical opportunity
for a one-to-many
connection without getting
more lines from the
network provider.
•Inefficient use of
bandwidth.
•Cost increases are
proportional to the
distance between sites.
2
Chapter 3
Requirement for Frame Relay
•Frame Relay customers
only pay for the local
loop, and for the
bandwidth they purchase
from the network
provider.
•Distance between nodes
is not important.
• Efficient use of
bandwidth as Frame Relay
shares bandwidth across
a larger base of
customers.
3
Chapter 3
Frame Relay
•
•
•
•
Frame Relay is an ITU-T and ANSI standard.
Frame Relay is a packet-switched, connection-oriented,
WAN service, operating at the data link layer of the OSI
reference model.
Frame Relay uses a subset of the high-level data-link
control (HDLC) protocol called Link Access Procedure for
Frame Relay (LAPF).
Frames carry data between user devices called data
terminal equipment (DTE), and the data communications
equipment (DCE) at the edge of the WAN
4
Chapter 3
Frame Relay Overview
Frame Relay is used between the customer premises equipment
(CPE) device and the Frame Relay switch. It does not affect how
packets get routed within the Frame Relay ‘cloud’.
5
Chapter 3
Frame Relay Overview
•Packet Switched
•STDM
•Uses Virtual Circuits through the network
•Bandwidth is not allocated until required
•Buffering and congestion control mechanisms
•Relies on upper layer protocols (e.g. TCP) for error recovery
•Supports data rates up to 45Mbps
6
Chapter 3
Terminology
•
•
•
The connection through the Frame Relay network
between two DTEs is called a VC (Virtual Circuit).
Virtual circuits may be established dynamically by
sending signaling messages to the network. In this
case they are called SVC (Switched Virtual Circuits)
Generally PVC (Permanent Virtual Circuits) that have
been pre-configured by the carrier are used. The
switching information for a VC is stored in the
memory of the Frame relay switches and/or
infrastructure switches.
7
Chapter 3
Data Link Connection
Identifier (DLCI)
A
121
0
B
579
C
319
432
119
D
•Virtual circuits sharing a single access line can be distinguished
because each VC has a separate DLCI.
•The DLCI is stored in the address field of every frame transmitted,
and usually has only local significance.
8
Chapter 3
Multiple Virtual Circuits
DLCI 101
DLCI 100
DLCI 300
DLCI 200
Frame Relay
Network
DLCI 201
DLCI 301
•The Frame Relay Access Device (FRAD) or router connected to the
Frame Relay network may have multiple VCs connecting it to various
endpoints.
•Multiple VCs on a single physical line are distinguished because each
VC has its own DLCI.
9
Chapter 3
Frame Relay Stack Layered
Support
IP Packet
Layer 3
Layer 2
Layer 1
Flag
Address
Data
FCS
Flag
Line Coding = 10101010100100101011111100101
10
Chapter 3
Frame Relay Header
Flag
Address
Data
FCS
Flag
•DLCI - Data Link Connection Identifier
•DE – Discard Eligible
•FECN – Forward Explicit Congestion Identifier
•BECN- Backwards Explicit Congestion Identifier
11
Chapter 3
Star Topology
In a hub and spoke topology the location of the hub is
chosen to give the lowest leased line cost
12
Chapter 3
Frame Relay Star Topology
HUB
Because Frame Relay tariffs are not distance related, the hub
does not need to be in the geographical centre of the network.
13
Chapter 3
Full-Mesh Topology
A full mesh topology is chosen when services to
be accessed are geographically dispersed and
highly reliable access to them is required
14
Chapter 3
Frame Relay Partial Mesh
With partial mesh, there are more interconnections
than required for a star arrangement, but not as
many as for a full mesh
15
Chapter 3
Frame Relay
bandwidth and
flow control
= E1
E1
•
•
Local access rate or port speed – This is the clock speed or port
speed of the connection or local loop to the Frame Relay cloud. It is
the rate at which data travels into or out of the network, regardless
of other settings.
Committed Information Rate (CIR) – This is the rate, in bits per
second, at which the Frame Relay switch agrees to transfer data.
The rate is usually averaged over a period of time, referred to as
the Committed Time Interval (Tc).
16
Chapter 3
Frame Relay bandwidth and flow
control
•
•
•
•
Committed Burst Information Rate (CBIR) is a negotiated
rate above the CIR which the customer can use to
transmit for short burst.
It allows traffic to burst to higher speeds, as available
network bandwidth permits.
However, it cannot exceed the port speed of the link. A
device can burst up to the CBIR and still expect the data
to get through.
The duration of a burst transmission should be short, less
than three or four seconds. If long bursts persist, then a
higher CIR should be purchased.
17
Chapter 3
Frame Relay Bursting
•Frames submitted above the CIR marked as Discard
Eligible (DE) in the frame header, indicating that they may
be dropped if there is congestion or there is not enough
capacity in the network
18
Chapter 3
Frame Relay Header
Flag
Address
Data
FCS
Flag
•DE – Discard Eligible
•FECN – Forward Explicit Congestion Identifier
•BECN- Backwards Explicit Congestion Identifier
19
Chapter 3
Frame Relay
bandwidth and
flow control
= E1
E1
•
•
Forward Explicit Congestion Notification (FECN) – When a Frame
Relay switch recognizes congestion in the network, it sends an FECN
packet to the destination device, indicating that congestion has
occurred.
Backward Explicit Congestion Notification (BECN) – When a Frame
Relay switch recognizes congestion in the network, it sends a BECN
packet to the source router, instructing the router to reduce the rate
at which it is sending packets.
20
Chapter 3
Frame Relay
bandwidth and
flow control
= E1
E1
•
•
Discard eligibility (DE) bit is set on the traffic that was received
after the CIR was met – i.e. Burst Excess (BE).
Data frames with the DE bit set are normally delivered. However, in
periods of congestion, the Frame Relay switch will drop packets with
the DE bit set first.
21
Chapter 3
Frame Relay bandwidth
•
•
•
•
Several factors determine the rate at which a customer can send data
on a Frame Relay network. foremost in limiting the maximum
transmission rate is the capacity of the local loop to the provider.
If the local loop is an E1, no more than 2.048 Mbps can be sent. In
Frame Relay terminology, the speed of the local loop is called the local
access rate.
Providers use the CIR parameter to provision network resources and
regulate usage.
For example, a company with an E1 connection to a packet-switched
network (PSN) may agree to a CIR of 1024 Kbps. This means that the
provider guarantees 1024 Kbps of bandwidth to the customer’s link at
all times.
22
Chapter 3
Frame Relay bandwidth
•
•
•
•
Typically, the higher the CIR, the higher the cost of
service.
Customers can choose the CIR that is most appropriate to
their bandwidth needs, as long as the CIR is less than or
equal to the local access rate.
If the CIR of the customer is less than the local access
rate, the customer and provider agree on whether
bursting above the CIR is allowed.
If the local access rate is E1 or 2.048 Mbps, and the CIR
is 1024 Kbps, half of the potential bandwidth (as
determined by the local access rate) remains available.
23
Chapter 3
Frame Relay bandwidth
•
•
•
•
•
Many providers allow their customers to purchase a CIR
of 0 (zero) - This means that the provider does not
guarantee any throughput.
In practice, customers usually find that their provider
allows them to burst over the 0 (zero) CIR virtually all of
the time.
If a CIR of 0 (zero) is purchased, carefully monitor
performance in order to determine whether or not it is
acceptable.
Frame Relay allows a customer and provider to agree that
under certain circumstances, the customer can “burst”
over the CIR.
Since burst traffic is in excess of the CIR, the provider
does not guarantee that it will deliver the frames.
24
Chapter 3
Network Address Mapping
•
•
•
A mapping is needed in each FRAD or router between a
data link layer Frame Relay address (DLCI) and a
network layer address, such as an IP address.
The DLCI for each VC must be associated with the
network address of its remote router. This information
can be configured statically by using map commands.
The DLCI can also be configured automatically using
Inverse ARP.
25
Chapter 3
LMI – Local Management Interface
•
•
•
LMI is a signaling standard between the DTE and the Frame Relay
switch.
LMI is responsible for managing the connection and maintaining
the status between devices.
Cisco supports 3 LMI standards – ANSI, Q933a, Cisco
LMI includes:
•
•
•
•
A keepalive mechanism, which verifies that data is flowing.
A multicast mechanism, which provides the network server (router) with
its local DLCI.
Multicast addressing, which can give DLCIs global rather than local
significance in Frame Relay networks (not common).
A status mechanism, which provides an ongoing status on the DLCIs known
to the switch.
26
Chapter 3
LMI – Local Management Interface
LMI
•
•
•
Frame Relay
Network
There are three types of LMI, none of which is compatible with the
others.
Cisco, StrataCom, Northern Telecom, and Digital Equipment Corporation
(Gang of Four) released one type of LMI, while the ANSI and the ITUT each released their own versions.
The LMI type must match between the provider Frame Relay switch and
the customer DTE device.
27
Chapter 3
LMI – Local Management Interface
LMI
•
•
•
Frame Relay
Network
In Cisco IOS releases prior to 11.2, the Frame Relay interface must be
manually configured to use the correct LMI type.
If using Cisco IOS Release 11.2 or later, the router attempts to
automatically detect the type of LMI used by the provider switch.
This automatic detection process is called LMI autosensing.
28
Chapter 3
LMI – Local Management Interface
•
•
The 10-bit DLCI field allows VC identifiers 0 through 1023. The LMI
extensions reserve some of these identifiers. This reduces the
number of permitted VCs.
LMI messages are exchanged between the DTE and DCE using these
reserved DLCIs.
29
VC IDent
VC Types
0
LMI (ANSI, ITU)
1-15
Reserved
992-1007
CLLM
1008-1022
Reserved (ANSI, ITU)
1019-1022
Cisco Multicasting
1023
LMI (Cisco)
Chapter 3
LMI Extensions
In addition to the Frame Relay protocol functions for transferring data, the Frame
Relay specification includes optional LMI extensions that are extremely useful in an
Internetworking environment. Some of the extensions include:
•
•
•
•
VC status messages - Provide information about PVC integrity by communicating and
synchronizing between devices, periodically reporting the existence of new PVCs and
the deletion of already existing PVCs. VC status messages prevent data from being
sent into black holes (PVCs that no longer exist).
Multicasting - Allows a sender to transmit a single frame that is delivered to
multiple recipients. Multicasting supports the efficient delivery of routing protocol
messages and address resolution procedures that are typically sent to many
destinations simultaneously.
Global addressing - Gives connection identifiers global rather than local significance,
allowing them to be used to identify a specific interface to the Frame Relay network.
Global addressing makes the Frame Relay network resemble a LAN in terms of
addressing, and ARPs perform exactly as they do over a LAN.
Simple flow control - Provides for an XON/XOFF flow control mechanism that
applies to the entire Frame Relay interface. It is intended for those devices whose
higher layers cannot use the congestion notification bits and need some level of flow
control.
30
Chapter 3
Frame Relay Address Mapping
DLCI 101
R1
DLCI 102
1.
R1 connects to the Frame Relay network, it sends an LMI status
inquiry message (75) to the network.
2. Network replies with an LMI status message (7D) containing details
of every VC configured on the access link.
3. If the router needs to map the VCs to network layer addresses, it
sends an Inverse ARP message on each VC.
4. The Inverse ARP message includes the network layer address of the
router, so the remote DTE, or router, can also perform the mapping.
31
Chapter 3
Configuring Frame Relay maps
Hub City
Spokane
172.16.1.2
DLCI 101
•
•
Frame Relay
Network
172.16.1.1
DLCI 102
If the environment does not support LMI autosensing and Inverse ARP, a
Frame Relay map must be manually configured.
Once a static map for a given DLCI is configured, Inverse ARP is disabled
on that DLCI.
HubCity(config)# interface serial 0
HubCity(config-if)# frame-relay map ip 172.16.1.1 101 broadcast
Spokane(config)# interface serial 0
Spokane(config-if)# frame-relay map ip 172.16.1.2 102 broadcast
32
Chapter 3
Inverse ARP
Hub City
Spokane
172.16.1.2
DLCI 101
Frame Relay
Network
172.16.1.1
DLCI 102
HubCity# show frame-relay map
Serial0 (up): ip 172.16.1.1 dlci 101,
dynamic, broadcast, status defined, active
• dynamic refers to the router learning the IP address via Inverse
•
ARP
DLCI 101 is configured on the Frame Relay Switch by the provider.
33
Chapter 3
Inverse ARP Limitations
Hub City
Spokane
172.16.1.2
DLCI 101
•
•
•
•
Frame Relay
Network
172.16.1.1
DLCI 102
Inverse ARP only resolves network addresses of remote Frame-Relay
connections that are directly connected via frame-relay nodes.
Inverse ARP does not work with Hub-and-Spoke connections.
Dynamic address mapping is enabled by default for all protocols
enabled on a physical interface.
If the Frame Relay environment supports LMI autosensing and
Inverse ARP, dynamic address mapping takes place automatically.
34
Chapter 3
Frame Relay Configuration
R2
S0/0/0
10.1.1.2 / 24
S0/0/0
10.1.1.1 / 24
S0/0/0
10.1.1.3 / 24
R1
R3
DLCI 103
Fa0/1
192.168.10.1 / 24
Fa0/1
192.168.30.1 / 24
•Default Frame Relay
encapsulation enabled on
supported interfaces is the
Cisco encapsulation.
35
•Use Cisco encapsulation if
connecting to another Cisco
router, use IETF if connecting
to non-Cisco devices.
Chapter 3
Frame Relay Configuration
R2
S0/0/0
10.1.1.2 / 24
S0/0/0
10.1.1.1 / 24
S0/0/0
10.1.1.3 / 24
R1
R3
DLCI 103
Fa0/1
192.168.10.1 / 24
Fa0/1
192.168.30.1 / 24
Static Map
.0
•If the environment does not
support LMI autosensing and
Inverse ARP, a Frame Relay map
must be manually configured.
•Once a static map for a given
DLCI is configured, Inverse ARP
is disabled on that DLCI.
36
Chapter 3
Reachability issues
with routing
updates
•
•
•
An None Broadcast Multiple Access (NBMA) network is a multi-access
network, which means more than two nodes can connect to the network.
Frame Relay is a NBMA, and its nodes cannot see broadcasts of other
nodes unless they are directly connected by a virtual circuit.
This means that Branch A cannot directly see the broadcasts from
Branch B, because they are connected using a hub and spoke topology.
37
Chapter 3
Reachability issues
with routing
updates
•
•
•
The Central router must receive the routing update broadcast from
Branch A and then send its own update broadcast to Branch B & C.
In this example, there are problems with routing protocols because
of the split horizon rule.
Split Horizon prohibits routing updates received on an interface
from exiting that same interface.
38
Chapter 3
Frame Relay
Network
172.30.3.2/24
S0
DLCI 101
172.30.3.1/24
S3
DLCI 100
172.30.1.1/24
S0
DLCI 300
172.30.2.1/24
S2
DLCI 200
172.30.2.2/24
S0
DLCI 201
172.30.1.2/24
S0
DLCI 301
•
•
Use of multiple point-to-point serial interfaces solves the problem
of split-horizon, as routing updates are not being sent from the
same physical interface.
Expensive to implement, as multiple serial interfaces and point-topoint connections are required.
39
Chapter 3
Point-to-Point Sub-interface
•
•
•
A single point-to-point sub-interface is used to
establish one PVC connection to another physical
interface or sub-interface on a remote router.
In this case, each pair of the point-to-point routers is
on its own subnet and each point-to-point sub-interface
would have a single DLCI.
In a point-to-point environment, each sub-interface is
acting like a point-to-point interface. Therefore,
routing update traffic is not subject to the splithorizon rule.
40
Chapter 3
Frame
Relay
Network
Interface
S0
S0.100
S0.200
S0.300
172.30.3.2/24
S0
DLCI 101
172.30.3.1/24
DLCI 100
172.30.2.1/24
DLCI 200
172.30.2.2/24
S0
DLCI 201
172.30.1.1/24
DLCI 300
172.30.1.2/24
S0
DLCI 301
•
Point-to-point sub-interfaces are equivalent to using
multiple physical “point to point” interfaces.
41
Chapter 3
Point-to-point Sub-interfaces
172.30.3.2/24
S0
DLCI 101
Interface
S0
S0.100
S0.200
S0.300
172.30.3.1/24
DLCI 100
172.30.2.1/24
DLCI 200
172.30.2.2/24
S0
DLCI 201
172.30.1.1/24
DLCI 300
Router(config)# interface Serial 0
Router(config-if)# encapsulation frame-relay ietf
172.30.1.2/24
S0
Router(config-if)#frame-relay lmi-type ansi
DLCI 301
Router(config-if)# interface Serial0.100 point-to-point
Router(config-if)# ip address 172.30.3.1 255.255.255.0
Router(config-subif)# frame-relay interface-dlci 100
Router(config-subif)# exit
Router(config-if)# interface Serial0.200 point-to-point
Router(config-if)# ip address 172.30.2.1 255.255.255.0
Router(config-subif)# frame-relay interface-dlci 200
42
Chapter 3
Point-to-point Sub-interfaces
Frame-Relay service supplies multiple PVCs over a single
physical interface and point-to-point sub-interfaces
subdivide each PVC as if it were a physical point-to-point
interface.
Point-to-point sub-interfaces are like conventional pointto-point interfaces and do not need or allow:
•
•
•
Inverse-ARP
Frame-relay map statements
Multiple DLCIs for a sub-interface
Point-to-point sub-interfaces completely bypass the
local DLCI to remote network address mapping issue.
43
Chapter 3
Multipoint Sub-interface
•
•
•
A single multipoint sub-interface is used to
establish multiple PVC connections to multiple
physical interfaces or sub-interfaces on
remote routers.
All the participating interfaces would be in the
same subnet.
The sub-interface acts like an NBMA Frame
Relay interface so routing update traffic is
subject to the split-horizon rule.
44
Chapter 3
Frame
Relay
Network
Interface
S0
S0.1
172.30.1.1/24
Multipoint
172.30.1.2/24
S0
DLCI 101
Point-to-Point
DLCI 100
DLCI 200
DLCI 300
172.30.1.3/24
S0
DLCI 201
Point-to-Point
172.30.1.4/24
S0
DLCI 301
Point-to-Point
•
Multipoint sub-interfaces are equivalent to using
multiple physical “hub to spoke” interfaces
45
Chapter 3
Frame
Relay
Network
Interface
S0
S0.1
172.30.1.1/24
Point-to-Point
DLCI 100
DLCI 200
DLCI 300
Multipoint
•
•
•
•
•
•
•
Router(config)# interface Serial 0
Router(config-if)# encapsulation frame-relay ietf
Router(config-if)# interface Serial0.123 multipoint
Router(config-if)# ip address 172.30.1.1 255.255.255.0
Router(config-subif)# frame-relay interface-dlci 100
Router(config-subif)# frame-relay interface-dlci 200
Router(config-subif)# frame-relay interface-dlci 300
46
172.30.1.2/24
S0
DLCI 101
172.30.1.3/24
S0
DLCI 201
Point-to-Point
172.30.1.4/24
S0
DLCI 301
Point-to-Point
Chapter 3
Multipoint Sub-interfaces
Share many of the same characteristics as a physical
Frame-Relay interface.
With multipoint sub-interface you can have:
•
•
•
Multiple DLCIs assigned to it.
Frame-relay map & interface dlci statements
Inverse-ARP
Which is the opposite of point-to-point interfaces.
47
Chapter 3
Mixing Inverse ARP and Frame Relay
Map Statements
Frame-Relay Map Statement Rule:
When a Frame-Relay map statement is
configured for a particular protocol (e.g. IP or
IPX) Inverse-ARP will be disabled for that
specific protocol, not only for the DLCI
referenced in the Frame-Relay map statement.
48
Chapter 3
Frame
Relay
Network
Interface
S0
S0.1
172.30.1.1/24
Multipoint
172.30.1.2/24
S0
DLCI 101
Point-to-Point
DLCI 100
DLCI 200
DLCI 300
172.30.1.3/24
S0
DLCI 201
Point-to-Point
172.30.1.4/24
S0
DLCI 301
Point-to-Point
Solution: Do not mix IARP with Frame Relay maps statements. If
need be use Frame-Relay map statements instead of IARP.
49
Chapter 3
Frame Relay Sub-interfaces
Summary
Point-to-Point
•Sub-interfaces act as a leased line
•Each point-to-point sub-interface requires its own subnet
•Applicable to hub and spoke topologies
Multipoint
•Sub-interfaces act as NBMA network, so they do not resolve
the split horizon issue
•Can save address space because it uses a single subnet
•Applicable to partial-mesh and full-mesh topology
50
Chapter 3
Verify Frame Relay
51
Chapter 3
Verify Frame Relay
•Shows the number of status messages exchanged
between the local router and the local Frame Relay
switch.
52
Chapter 3
Verify Frame Relay
•This command is also useful for viewing the number of
BECN and FECN packets received by the router. The PVC
status can be active, inactive, or deleted.
53
Chapter 3
Verify Frame Relay
To clear dynamically created Frame Relay maps, which are created
using Inverse ARP, use the clear frame-relay-inarp command.
54
Chapter 3
Debug frame-relay lmi
Command
•0x0 - Added/inactive – Not usable
•0x2 - Added/active - Operational
•0x4 - Deleted means that the Frame Relay switch does not
have this DLCI programmed for the router
55
Chapter 3
Chap 3 – Frame Relay
Learning Objectives
•
•
•
•
Describe the fundamental concepts of Frame Relay technology in terms of
Enterprise WAN services including Frame Relay operation, Frame Relay
implementation requirements, Frame Relay maps, and LMI operation.
Configure a basic Frame Relay PVC including configuring and troubleshooting
Frame Relay on a router serial interface and configuring a static Frame Relay
map.
Describe advanced concepts of Frame Relay technology in terms of
Enterprise WAN services including Frame Relay sub-interfaces, Frame Relay
bandwidth and flow control.
Configure an advanced Frame Relay PVC including solving reachability issues,
configuring Frame Relay sub-interfaces, verifying and troubleshooting Frame
Relay configuration.
56
Chapter 3
Any
Questions?
57
Chapter 3
Chapter 3.1 – Frame Relay
Map Config
Lab Topology
Computer
Fa0/0
.1
S0/0
192.168.4.1 / 24
S0/0
192.168.4.2 / 24
Computer
PC1
192.168.2.10 / 24
Fa0/0
.1
58
R1
R2
PC2
192.168.1.10 / 24
S0/1
192.168.5.1 / 24
S0/1
192.168.5.2 / 24
R3
Fa0/0
.1
Computer
PC3
192.168.3.10 / 24
Chapter 3
Chapter 3.2 – Frame Relay
Point-to-Point Config
Lab Topology
Computer
Fa0/0
.1
S0/0.102
192.168.4.1 / 24
S0/0.201
192.168.4.2 / 24
Computer
PC1
192.168.2.10 / 24
Fa0/0
.1
R1
S0/0.203
192.168.6.1 / 24
59
R2
PC2
192.168.1.10 / 24
S0/0.103
192.168.5.1 / 24
S0/0.301
192.168.5.2 / 24
DLCI 203
R3
S0/0.302
192.168.6.2 / 24
Fa0/0
.1
Computer
PC3
192.168.3.10 / 24
Chapter 3
Chapter 3.3 – Frame Relay
Multipoint Config
Lab Topology
Computer
Fa0/0
.1
S0/0.1
192.168.4.1 / 24
S0/0.201
192.168.4.2 / 24
Computer
PC1
192.168.2.10 / 24
Fa0/0
.1
60
R1
PC2
192.168.1.10 / 24
R2
S0/0.301
192.168.4.3 / 24
R3
Fa0/0
.1
Computer
PC3
192.168.3.10 / 24
Chapter 3
Serial 0 (PT4.1)
Port 1 / 1
(Adtran)
DLCI
103
DLCI
301
Serial 1 (PT4.1)
DLCI
102
DLCI
201
DLCI
104
DLCI
203
DLCI
302
DLCI
401
Port 1 / 2
(Adtran)
Port 2 / 1
(Adtran)
Port 2 / 2
(Adtran)
Serial 2 (PT4.1)
Serial 3 (PT4.1)
61
Chapter 3