Ch. 5 – Frame Relay
CCNA 4 version 3.0
Rick Graziani
Cabrillo College
Note to instructors
• If you have downloaded this presentation from the Cisco Networking
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• For the latest PowerPoints for all my CCNA, CCNP, and Wireless
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• The username is cisco and the password is perlman for all of
my materials.
• If you have any questions on any of my materials or the curriculum,
please feel free to email me at graziani@cabrillo.edu (I really don’t
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• I will add “(Updated – date)” next to each presentation on my web site
that has been updated since these have been uploaded to the FTP
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Thanks! Rick
Rick Graziani graziani@cabrillo.edu
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Note
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Much of the information in this presentation comes from
the CCNP 2 version 3.0 module on Frame Relay.
I find a lot of the information in CCNA 4 module 5 Frame
Relay not very well written or not well explained.
CCNP 3 does a much better job of presenting and
explaining these concepts.
Rick Graziani graziani@cabrillo.edu
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Overview
• Identify the components of a Frame Relay network
• Explain the scope and purpose of Frame Relay
• Discuss the technology of Frame Relay
• Compare point-to-point and point-to-multipoint topologies
• Examine the topology of a Frame Relay network
• Configure a Frame Relay Permanent Virtual Circuit (PVC)
• Create a Frame Relay Map on a remote network
• Explain the issues of a non-broadcast multi-access
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network
Describe the need for subinterfaces and how to configure
them
Verify and troubleshoot a Frame Relay connection
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Introducing Frame Relay
• Frame Relay is a packet-switched, connection-oriented, WAN service.
It operates 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.
– It does not define the way the data is transmitted within the service
provider’s Frame Relay cloud.
– This is ATM in many cases!
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Frame Relay vs. X.25
• Frame Relay does not have the sequencing, windowing, and
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retransmission mechanisms that are used by X.25.
Without the overhead, the streamlined operation of Frame Relay
outperforms X.25.
Typical speeds range from 56 kbps up to 2 Mbps, although higher
speeds are possible. (Up to 45 Mbps)
The network providing the Frame Relay service can be either a carrierprovided public network or a privately owned network.
Because it was designed to operate on high-quality digital lines, Frame
Relay provides no error recovery mechanism.
If there is an error in a frame it is discarded without notification.
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Introducing Frame Relay
Access circuits
• A Frame Relay network may be privately owned, but it is more
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commonly provided as a service by a public carrier.
It typically consists of many geographically scattered Frame Relay
switches interconnected by trunk lines.
Frame Relay is often used to interconnect LANs. When this is the
case, a router on each LAN will be the DTE.
A serial connection, such as a T1/E1 leased line, will connect the router
to a Frame Relay switch of the carrier at the nearest point-of-presence
for the carrier. (access circuit)
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DTE – Data Terminal Equipment
• DTEs generally are considered to be terminating equipment for a
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specific network and typically are located on the premises of the
customer.
The customer may also own this equipment.
Examples of DTE devices are routers and Frame Relay Access
Devices (FRADs).
A FRAD is a specialized device designed to provide a connection
between a LAN and a Frame Relay WAN.
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DCE – Data Communications Equipment
UNI
NNI
• DCEs are carrier-owned internetworking devices.
• The purpose of DCE equipment is to provide clocking and switching
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services in a network.
In most cases, these are packet switches, which are the devices that
actually transmit data through the WAN.
The connection between the customer and the service provider is
known as the User-to-Network Interface (UNI).
The Network-to-Network Interface (NNI) is used to describe how
Frame Relay networks from different providers connect to each other.
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Frame Relay terminology
An SVC between the same two
DTEs may change.
Path may change.
A PVC between the same two
DTEs will always be the same.
Always same Path.
• The connection through the Frame Relay network between two DTEs is
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called a virtual circuit (VC).
Switched Virtual Circuits (SVCs) are Virtual circuits may be established
dynamically by sending signaling messages to the network.
However, SVCs are not very common.
Permanent Virtual Circuits (PVCs) are more common.
PVC are VCs that have been preconfigured by the carrier are used.
The switching information for a VC is stored in the memory of the switch.
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Access Circuits and Cost Savings
• The FRAD or router connected to the Frame Relay network may have
multiple virtual circuits connecting it to various end points.
• This makes it a very cost-effective replacement for a full mesh of
access lines.
• Each end point needs only a single access line and interface.
• More savings arise as the capacity of the access line is based on the
average bandwidth requirement of the virtual circuits, rather than on
the maximum bandwidth requirement.
• Note: Also do not have to pay for leased line between two sites even
when no traffic is being sent.
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IETF Frame Relay Frame
• Cisco routers support two types of Frame Relay headers.
•
– Cisco, which is a 4-byte header.
– IETF, which is a 2-byte header that conforms to the IETF
standards.
The Cisco proprietary 4-byte header is the default and cannot be used
if the router is connected to another vendor's equipment across a
Frame Relay network.
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IETF Frame
Relay Frame
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IETF Frame Relay Frame
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DLCI
• A data-link connection identifier (DLCI) identifies the logical VC
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between the CPE and the Frame Relay switch.
The Frame Relay switch maps the DLCIs between each pair of routers
to create a PVC.
DLCIs have local significance, although there some implementations
that use global DLCIs.
DLCIs 0 to 15 and 1008 to 1023 are reserved for special purposes.
Service providers assign DLCIs in the range of 16 to 1007.
– DLCI 1019, 1020: Multicasts
– DLCI 1023: Cisco LMI
– DLCI 0: ANSI LMI
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DLCI
• Your Frame Relay provider sets up the DLCI numbers to be used by
the routers for establishing PVCs.
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Frame Relay bandwidth and flow control
Note:
• I am going to use information from CCNA version 2.0 and CCNP 2
version 3.0 to explain this topic.
• I do not like how this section (5.1.4) was written as I do not think it
explains the topic very well at all.
Rick Graziani graziani@cabrillo.edu
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Frame Relay bandwidth
and flow control
The first thing we need to do is
become familiar with some of
the terminology.
• Local access rate – This is the clock speed or port speed of the
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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 rate measurement interval (Tc).
– In general, the duration of Tc is proportional to the "burstiness" of
the traffic.
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Frame Relay bandwidth and flow control
per VC
• Oversubscription – Oversubscription is when the sum of the CIRs on
all the VCs exceeds the access line speed.
– Oversubscription can also occur when the access line can support
the sum of CIRs purchased, but not of the CIRs plus the bursting
capacities of the VCs.
– Oversubscription increases the likelihood that packets will be
dropped.
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Frame Relay bandwidth and flow control
Tc = 2 seconds
Bc = 64 kbps
CIR = 32 kbps
• Committed burst (Bc) – The maximum number of bits that the switch
•
agrees to transfer during any Tc.
– The higher the Bc-to-CIR ratio, the longer the switch can handle a
sustained burst.
– For example, if the Tc is 2 seconds and the CIR is 32 kbps, the Bc
is 64 kbps.
– The Tc calculation is Tc = Bc/CIR.
Committed Time Interval (Tc) – Tc is not a recurrent time interval. It is
used strictly to measure inbound data, during which time it acts like a
sliding window. Inbound data triggers the Tc interval.
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Frame Relay bandwidth
and flow control
• Excess burst (Be) – This is the maximum number of uncommitted bits
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that the Frame Relay switch attempts to transfer beyond the CIR.
– Excessive Burst (Be) is dependent on the service offerings
available from your vendor, but it is typically limited to the port
speed of the local access loop.
Excess Information Rate (EIR) – This defines the maximum
bandwidth available to the customer, which is the CIR plus the Be.
– Typically, the EIR is set to the local access rate.
– In the event the provider sets the EIR to be lower than the local
access rate, all frames beyond that maximum can be discarded
automatically, even if there is no congestion.
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Frame Relay bandwidth
and flow control
• Forward Explicit Congestion Notification (FECN) – When a Frame
•
Relay switch recognizes congestion in the network, it sends an FECN
packet to the destination device.
– This indicates 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.
– This instructs the router to reduce the rate at which it is sending
packets.
– With Cisco IOS Release 11.2 or later, Cisco routers can respond to
BECN notifications.
– This topic is discussed later in this module.
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Frame Relay bandwidth
and flow control
• Discard eligibility (DE) bit – When the router or switch detects
network congestion, it can mark the packet "Discard Eligible".
– The DE bit is set on the traffic that was received after the CIR was
met.
– These packets are normally delivered. However, in periods of
congestion, the Frame Relay switch will drop packets with the DE
bit set first.
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Frame Relay bandwidth
• Several factors determine the rate at which a customer can send data
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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 a T1, no more than 1.544 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 a T1 connection to the packet-switched
network (PSN) may agree to a CIR of 768 Kbps.
This means that the provider guarantees 768 Kbps of bandwidth to the
customer’s link at all times.
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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
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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 T1 or 1.544 Mbps, and the CIR is 768 Kbps,
half of the potential bandwidth (as determined by the local access rate)
remains available.
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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
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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.
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Frame Relay bandwidth
• Either a router or a Frame Relay switch tags each frame that is
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transmitted beyond the CIR as eligible to be discarded.
When a frame is tagged DE, a single bit in the Frame Relay frame is
set to 1.
This bit is known as the discard eligible (DE) bit.
The Frame Relay specification also includes a protocol for congestion
notification.
This mechanism relies on the FECN/ BECN bits in the Q.922 header of
the frame.
The provider’s switches or the customer’s routers can selectively set
the DE bit in frames.
These frames will be the first to be dropped when congestion occurs.
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LMI – Local Management Interface
• LMI is a signaling standard between
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the DTE and the Frame Relay switch.
LMI is responsible for managing the connection and maintaining
the status between devices.
LMI includes:
– A keepalive mechanism, which verifies that data is flowing
– A multicast mechanism, which provides the network server
(router) with its local DLCI.
– The 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
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LMI
LMI
• In order to deliver the first LMI services to customers as soon as
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possible, vendors and standards committees worked separately to
develop and deploy LMI in early Frame Relay implementations.
The result is that 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 ITU-T each released their own versions.
The LMI type must match between the provider Frame Relay switch
and the customer DTE device.
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LMI
LMI
• In Cisco IOS releases prior to 11.2, the Frame Relay interface must be
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manually configured to use the correct LMI type, which is furnished by
the service provider.
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.
No matter which LMI type is used, when LMI autosense is active, it
sends out a full status request to the provider switch.
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LMI
• Frame Relay devices can now listen in on both DLCI 1023 or Cisco
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LMI and DLCI 0 or ANSI and ITU-T simultaneously.
The order is ansi, q933a, cisco and is done in rapid succession to
accommodate intelligent switches that can handle multiple formats
simultaneously.
The Frame Relay switch uses LMI to report the status of configured
PVCs.
The three possible PVC states are as follows:
– Active state – Indicates that the connection is active and that
routers can exchange data.
– Inactive state – Indicates that the local connection to the Frame
Relay switch is working, but the remote router connection to the
Frame Relay switch is not working.
– Deleted state – Indicates that no LMI is being received from the
Frame Relay switch, or that there is no service between the CPE
router and Frame Relay switch.
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DLCI Mapping to Network Address
• Manual
•
– Manual: Administrators use a frame relay map statement.
Dynamic
– Inverse Address Resolution Protocol (I-ARP) provides a given
DLCI and requests next-hop protocol addresses for a specific
connection.
– The router then updates its mapping table and uses the information
in the table to forward packets on the correct route.
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Inverse ARP
2
1
• Once the router learns from the switch about available PVCs and their
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corresponding DLCIs, the router can send an Inverse ARP request to
the other end of the PVC. (unless statically mapped – later)
For each supported and configured protocol on the interface, the router
sends an Inverse ARP request for each DLCI. (unless statically
mapped)
In effect, the Inverse ARP request asks the remote station for its Layer
3 address.
At the same time, it provides the remote system with the Layer 3
address of the local system.
The return information from the Inverse ARP is then used to build the
Frame Relay map.
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Inverse ARP
• Inverse Address Resolution Protocol (Inverse ARP) was developed to
provide a mechanism for dynamic DLCI to Layer 3 address maps.
• Inverse ARP works much the same way Address Resolution Protocol
(ARP) works on a LAN.
• However, with ARP, the device knows the Layer 3 IP address and
needs to know the remote data link MAC address.
• With Inverse ARP, the router knows the Layer 2 address which is the
DLCI, but needs to know the remote Layer 3 IP address.
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Frame Relay Encapsulation
Router(config-if)#encapsulation frame-relay {cisco | ietf}
•
cisco - Default.
– Use this if connecting to another Cisco router.
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Ietf - Select this if connecting to a non-Cisco router.
– RFC 1490
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Frame Relay LMI
Router(config-if)#frame-relay lmi-type {ansi | cisco | q933a}
• It is important to remember that the Frame Relay service provider
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maps the virtual circuit within the Frame Relay network connecting the
two remote customer premises equipment (CPE) devices that are
typically routers.
Once the CPE device, or router, and the Frame Relay switch are
exchanging LMI information, the Frame Relay network has everything
it needs to create the virtual circuit with the other remote router.
The Frame Relay network is not like the Internet where any two
devices connected to the Internet can communicate.
In a Frame Relay network, before two routers can exchange
information, a virtual circuit between them must be set up ahead of
time by the Frame Relay service provider.
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Minimum Frame Relay Configuration
172.16.1.2
Headquarters
Hub City
DLCI 101
Frame Relay
Network
172.16.1.1
DLCI 102
Satellite Office 1
Spokane
HubCity(config)# interface serial 0
HubCity(config-if)# ip address 172.16.1.2 255.255.255.0
HubCity(config-if)# encapsulation frame-relay
Spokane(config)# interface serial 0
Spokane(config-if)# ip address 172.16.1.1 255.255.255.0
Spokane(config-if)# encapsulation frame-relay
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Minimum Frame Relay Configuration
172.16.1.2
Headquarters
Hub City
DLCI 101
Frame Relay
Network
172.16.1.1
DLCI 102
Satellite Office 1
Spokane
• Cisco Router is now ready to act as a Frame-Relay DTE device.
The following process occurs:
1. The interface is enabled.
2. The Frame-Relay switch announces the configured DLCI(s) to the
router.
3. Inverse ARP is performed to map remote network layer addresses to
the local DLCI(s).
The routers can now ping each other!
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Inverse ARP
172.16.1.2
Headquarters
Hub City
DLCI 101
Frame Relay
Network
172.16.1.1
DLCI 102
Satellite Office 1
Spokane
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
• The DLCI 101 is configured on the Frame Relay Switch by the
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provider.
We will see this in a moment.
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Inverse ARP Limitations
172.16.1.2
Headquarters
Hub City
DLCI 101
Frame Relay
Network
172.16.1.1
DLCI 102
Satellite Office 1
Spokane
• Inverse ARP only resolves network addresses of remote Frame-Relay
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connections that are directly connected.
Inverse ARP does not work with Hub-and-Spoke connections. (We will
see this in a moment.)
When using dynamic address mapping, Inverse ARP requests a nexthop protocol address for each active PVC.
Once the requesting router receives an Inverse ARP response, it
updates its DLCI-to-Layer 3 address mapping table.
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.
Therefore, no static address mapping is required.
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Configuring Frame Relay maps
Router(config-if)#frame-relay map protocol protocol-address
dlci [broadcast] [ietf | cisco]
• If the environment does not support LMI autosensing and Inverse ARP,
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a Frame Relay map must be manually configured.
Use the frame-relay map command to configure static address
mapping.
Once a static map for a given DLCI is configured, Inverse ARP is
disabled on that DLCI.
The broadcast keyword is commonly used with the frame-relay
map command.
The broadcast keyword provides two functions.
– Forwards broadcasts when multicasting is not enabled.
– Simplifies the configuration of OSPF for nonbroadcast networks
that use Frame Relay. (coming)
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Frame
Relay Maps
By default,
cisco is the
default
encapsulation
Uses cisco
encapsulation for
this DLCI (not
needed, default)
Rick Graziani graziani@cabrillo.edu
Remote IP
Address
Local DLCI
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More on Frame Relay Encapsulation
Applies to all DLCIs unless
configured otherwise
• If the Cisco encapsulation is configured on a serial interface, then by
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default, that encapsulation applies to all VCs on that serial interface.
If the equipment at the destination is Cisco and non-Cisco, configure
the Cisco encapsulation on the interface and selectively configure IETF
encapsulation per DLCI, or vice versa.
These commands configure the Cisco Frame Relay encapsulation for
all PVCs on the serial interface.
Except for the PVC corresponding to DLCI 49, which is explicitly
configured to use the IETF encapsulation.
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Verifying Frame Relay interface
configuration
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The show interfaces serial command displays
information regarding the encapsulation and the status of
Layer 1 and Layer 2.
It also displays information about the multicast DLCI, the
DLCIs used on the Frame Relay-configured serial
interface, and the DLCI used for the LMI signaling.
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show interfaces serial
Atlanta(config)#interface serial 0/0
Atlanta(config-if)#description Circuit-05QHDQ101545-080TCOM-002
Atlanta(config-if)#^z
Atlanta#show interfaces serial 0/0
Serial 0/0 is up, line protocol is up Hardware is MCI Serial
Description Circuit-05QHDQ101545-080TCOM-002
Internet address is 150.136.190.203, subnet mask 255.255.255.0
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 uses, rely 255/255, load 1/255
• To simplify the WAN management, use the description command at
the interface level to record the circuit number.
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show frame-relay pvc
• The show frame-relay pvc command displays the status of each
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configured connection, as well as traffic statistics.
This command is also useful for viewing the number of Backward
Explicit Congestion Notification (BECN) and Forward Explicit
Congestion Notification (FECN) packets received by the router.
The command show frame-relay pvc shows the status of all
PVCs configured on the router.
If a single PVC is specified, only the status of that PVC is shown.
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show frame-relay map
• The show frame-relay map command displays the current map
entries and information about the connections.
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show frame-relay lmi
• The show frame-relay lmi command displays LMI traffic statistics
showing the number of status messages exchanged between the local
router and the Frame Relay switch.
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clear frame-relay-inarp
• To clear dynamically created Frame Relay maps, which are created
using Inverse ARP, use the clear frame-relay-inarp command.
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Troubleshooting the Frame Relay
configuration
•
Use the debug frame-relay lmi command to
determine whether the router and the Frame Relay switch
are sending and receiving LMI packets properly.
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debug frame-relay lmi (continued)
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The possible values of the status field are as follows:
0x0 – Added/inactive means that the switch has this DLCI programmed but for
some reason it is not usable. The reason could possibly be the other end of the
PVC is down.
0x2 – Added/active means the Frame Relay switch has the DLCI and
everything is operational.
0x4 – Deleted means that the Frame Relay switch does not have this DLCI
programmed for the router, but that it was programmed at some point in the
past. This could also be caused by the DLCIs being reversed on the router, or
by the PVC being deleted by the service provider in the Frame Relay cloud.
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Frame Relay Topologies
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NBMA – Non Broadcast
Multiple Access
Frames between two routers are only seen
by those two devices (non broadcast).
Similar to a LAN, multiple computers have
access to the same network and
potentially to each other (multiple access).
• An NBMA network is the opposite of a broadcast network.
• On a broadcast network, multiple computers and devices are
•
•
attached to a shared network cable or other medium. When one
computer transmits frames, all nodes on the network "listen" to the
frames, but only the node to which the frames are addressed actually
receives the frames. Thus, the frames are broadcast.
A nonbroadcast multiple access network is a network to which
multiple computers and devices are attached, but data is transmitted
directly from one computer to another over a virtual circuit or across a
switching fabric. The most common examples of nonbroadcast network
media include ATM (Asynchronous Transfer Mode), frame relay, and
X.25.
http://www.linktionary.com/
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Star Topology
• A star topology, also known as a hub and spoke configuration, is the
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most popular Frame Relay network topology because it is the most
cost-effective.
In this topology, remote sites are connected to a central site that
generally provides a service or application.
This is the least expensive topology because it requires the fewest
PVCs.
In this example, the central router provides a multipoint connection,
because it is typically using a single interface to interconnect multiple
PVCs.
Rick Graziani graziani@cabrillo.edu
54
Full Mesh
Full Mesh Topology
Number of
Number of
Connections
PVCs
-----------------------------2
1
4
6
6
15
8
28
10
45
• In a full mesh topology, all routers have PVCs to all other destinations.
• This method, although more costly than hub and spoke, provides direct
•
•
•
connections from each site to all other sites and allows for redundancy.
For example, when one link goes down, a router at site A can reroute
traffic through site C.
As the number of nodes in the full mesh topology increases, the
topology becomes increasingly more expensive.
The formula to calculate the total number of PVCs with a fully meshed
WAN is [n(n - 1)]/2, where n is the number of nodes.
Rick Graziani graziani@cabrillo.edu
55
A Frame-Relay Configuration Supporting Multiple Sites
Headquarters
Hub City
Hub Router
• This is known
as a Hub and
Spoke
Topology,
where the Hub
router relays
information
between the
Spoke routers.
• Limits the
number of PVCs
needed as in a
full-mesh
topology
(coming).
Rick Graziani graziani@cabrillo.edu
DLCI 101
DLCI 112
172.16.1.2
Frame Relay
Network
DLCI 102
DLCI 211
172.16.1.1
Satellite Office 1
Spokane
172.16.1.3
Spoke
Routers
Satellite Office 2
Spokomo
56
Headquarters
Hub City
Configuration using Inverse
ARP
HubCity
interface Serial0
ip address 172.16.1.2 255.255.255.0
encapsulation frame-relay
Spokane
interface Serial0
ip address 172.16.1.1 255.255.255.0
encapsulation frame-relay
DLCI 101
DLCI 112
172.16.1.2
Frame Relay
Network
DLCI 102
172.16.1.1
Satellite Office 1
Spokane
DLCI 211
172.16.1.3
Satellite Office 2
Spokomo
Spokomo
interface Serial0
ip address 172.16.1.3 255.255.255.0
encapsulation frame-relay
Rick Graziani graziani@cabrillo.edu
57
Configuration using Inverse ARP:
Rick Graziani graziani@cabrillo.edu
58
Headquarters
Hub City
Configuration using Inverse
ARP
DLCI 101
DLCI 112
172.16.1.2
Frame Relay
Network
DLCI 102
172.16.1.1
Satellite Office 1
Spokane
DLCI 211
172.16.1.3
Satellite Office 2
Spokomo
HubCity# show frame-relay map
Serial0 (up): ip 172.16.1.1 dlci 101, dynamic, broadcast,
status defined, active
Serial0 (up): ip 172.16.1.3 dlci 112, dynamic, broadcast,
status defined, active
Spokane# show frame-relay map
Serial0 (up): ip 172.16.1.2 dlci 102, dynamic, broadcast,
status defined, active
Spokomo# show frame-relay map
Serial0 (up): ip 172.16.1.2 dlci 211, dynamic, broadcast,
status defined, active
Rick Graziani graziani@cabrillo.edu
59
Configuration using Inverse ARP
HubCity# show frame-relay map
Serial0 (up): ip 172.16.1.1 dlci 101, dynamic, broadcast,
status defined, active
Serial0 (up): ip 172.16.1.3 dlci 112, dynamic, broadcast,
status defined, active
Spokane# show frame-relay map
Serial0 (up): ip 172.16.1.2 dlci 102, dynamic, broadcast,
status defined, active
Spokomo# show frame-relay map
Serial0 (up): ip 172.16.1.2 dlci 211, dynamic, broadcast,
status defined, active
• Inverse ARP resolved the ip addresses for HubCity for both
•
•
•
Spokane and Spokomo
Inverse ARP resolved the ip addresses for Spokane for HubCity
Inverse ARP resolved the ip addresses for Spokomo for HubCity
What about between Spokane and Spokomo?
Rick Graziani graziani@cabrillo.edu
60
Headquarters
Hub City
Inverse ARP Limitations
DLCI 101
DLCI 112
172.16.1.2
Frame Relay
Network
DLCI 102
DLCI 211
172.16.1.1
Satellite Office 1
Spokane
172.16.1.3
Satellite Office 2
Spokomo
• Can HubCity ping both Spokane and Spokomo? Yes!
• Can Spokane and Spokomo ping HubCity? Yes!
• Can Spokane and Spokomo ping each other? No! The Spoke
routers’ serial interfaces (Spokane and Spokomo) drop the ICMP
packets because there is no DLCI-to-IP address mapping for the
destination address.
Solutions to the limitations of Inverse ARP
1. Add an additional PVC between Spokane and Spokomo (Full Mesh)
2. Configure Frame-Relay Map Statements
3. Configure Point-to-Point Subinterfaces.
Rick Graziani graziani@cabrillo.edu
61
Frame Relay Map Statements
Router(config-if)#frame-relay map protocol protocol-address
dlci [broadcast] [ietf | cisco]
Instead of using additional PVCs, Frame-Relay map statements can be
used to:
• Statically map local DLCIs to an unknown remote network layer
addresses.
• Also used when the remote router does not support Inverse ARP
Rick Graziani graziani@cabrillo.edu
62
HubCity
interface Serial0
ip address 172.16.1.2
255.255.255.0
encapsulation frame-relay
(Inverse-ARP still works here)
Frame-Relay Map Statements
Headquarters
Hub City
Spokane
interface Serial0
ip address 172.16.1.1
255.255.255.0
encapsulation frame-relay
frame-relay map ip 172.16.1.3 102
frame-relay map ip 172.16.1.2 102
Spokomo
interface Serial0
ip address 172.16.1.3
255.255.255.0
encapsulation frame-relay
frame-relay map ip 172.16.1.1 211
frame-relay map ip 172.16.1.2 211
DLCI 101
DLCI 112
172.16.1.2
Frame Relay
Network
DLCI 102
172.16.1.1
Satellite Office 1
Spokane
DLCI 211
172.16.1.3
Satellite Office 2
Spokomo
Notice that the routers are configured to use either IARP or Frame Relay
maps. Using both on the same interface will cause problems.
Rick Graziani graziani@cabrillo.edu
63
Mixing Inverse ARP and
Frame Relay Map Statements
Headquarters
Hub City
DLCI 101
DLCI 112
172.16.1.2
Inverse ARP
Frame Relay
Network
Frame Relay
maps
DLCI 102
172.16.1.1
Satellite Office 1
Spokane
DLCI 211
172.16.1.3
Satellite Office 2
Spokomo
• The previous configuration works fine and all routers can ping each
•
•
other.
What if we were to use I-ARP between the spoke routers and the hub,
and frame relay map statements between the two spokes?
There would be a problem!
Rick Graziani graziani@cabrillo.edu
64
Mixing Inverse ARP and Frame Relay Map
Statements
Headquarters
Hub City
HubCity
interface Serial0
ip address 172.16.1.2
255.255.255.0
encapsulation frame-relay
DLCI 101
172.16.1.2
Spokane
interface Serial0
ip address 172.16.1.1
255.255.255.0
encapsulation frame-relay
frame-relay map ip 172.16.1.3 102
Spokomo
interface Serial0
ip address 172.16.1.3
255.255.255.0
encapsulation frame-relay
frame-relay map ip 172.16.1.1 211
Rick Graziani graziani@cabrillo.edu
DLCI 112
Frame Relay
Network
DLCI 102
172.16.1.1
Satellite Office 1
Spokane
DLCI 211
172.16.1.3
Satellite Office 2
Spokomo
65
Mixing Inverse ARP and Frame Relay Map
Statements
HubCity# show frame-relay map
Serial0 (up): ip 172.16.1.1 dlci 101,
broadcast, status defined, active
Serial0 (up): ip 172.16.1.3 dlci 112,
broadcast, status defined, active
Spokane# show frame-relay map
Serial0 (up): ip 172.16.1.2 dlci 102,
broadcast, status defined, active
Serial0 (up): ip 172.16.1.3 dlci 102,
status defined, active
Spokomo# show frame-relay map
Serial0 (up): ip 172.16.1.2 dlci 211,
broadcast, status defined, active
Serial0 (up): ip 172.16.1.1 dlci 211,
status defined, active
Rick Graziani graziani@cabrillo.edu
dynamic,
dynamic,
dynamic,
static, CISCO,
dynamic,
static, CISCO,
66
Mixing Inverse ARP and Frame Relay Map
Statements
HubCity# show frame-relay map
Serial0 (up): ip 172.16.1.1 dlci 101, dynamic, broadcast, status defined, active
Serial0 (up): ip 172.16.1.3 dlci 112, dynamic, broadcast, status defined, active
Spokane# show frame-relay map
Serial0 (up): ip 172.16.1.2 dlci 102, dynamic, broadcast, status defined, active
Serial0 (up): ip 172.16.1.3 dlci 102, static, CISCO, status defined, active
Spokomo# show frame-relay map
Serial0 (up): ip 172.16.1.2 dlci 211, dynamic, broadcast, status defined, active
Serial0 (up): ip 172.16.1.1 dlci 211, static, CISCO, status defined, active
Good News:
• Everything looks fine!
• Now all routers can ping each other!
Bad News:
• Problem when using Frame-Relay map statements AND Inverse ARP.
• This will only work until the router is reloaded, here is why...
Rick Graziani graziani@cabrillo.edu
67
Mixing Inverse ARP and Frame Relay Map
Statements
HubCity# show frame-relay map
Serial0 (up): ip 172.16.1.1 dlci 101, dynamic, broadcast, status defined, active
Serial0 (up): ip 172.16.1.3 dlci 112, dynamic, broadcast, status defined, active
Spokane# show frame-relay map
Serial0 (up): ip 172.16.1.2 dlci 102, dynamic, broadcast, status defined, active
Serial0 (up): ip 172.16.1.3 dlci 102, static, CISCO, status defined, active
Spokomo# show frame-relay map
Serial0 (up): ip 172.16.1.2 dlci 211, dynamic, broadcast, status defined, active
Serial0 (up): ip 172.16.1.1 dlci 211, static, CISCO, status defined, active
Frame-Relay Map Statement Rule:
• When a Frame-Relay map statement is configured for a particular
protocol (IP, IPX, …) Inverse-ARP will be disabled for that specific
protocol, only for the DLCI referenced in the Frame-Relay map
statement.
Rick Graziani graziani@cabrillo.edu
68
Mixing Inverse ARP and Frame Relay Map
Statements
HubCity# show
Serial0 (up):
Serial0 (up):
Spokane# show
Serial0 (up):
Serial0 (up):
Spokomo# show
Serial0 (up):
Serial0 (up):
•
•
•
•
frame-relay map
ip 172.16.1.1 dlci
ip 172.16.1.3 dlci
frame-relay map
ip 172.16.1.2 dlci
ip 172.16.1.3 dlci
frame-relay map
ip 172.16.1.2 dlci
ip 172.16.1.1 dlci
101, dynamic, broadcast, status defined, active
112, dynamic, broadcast, status defined, active
102, dynamic, broadcast, status defined, active
102, static, CISCO, status defined, active
211, dynamic, broadcast, status defined, active
211, static, CISCO, status defined, active
The previous solution worked only because the Inverse ARP had taken
place between Spokane and HubCity, and between Spokomo and HubCity,
before the Frame-Relay map statements were added. (The Frame-Relay
map statement was added after the Inverse ARP took place.)
Both the Inverse-ARP and Frame-Relay map statements are in effect.
Once the router is reloaded (rebooted) the Inverse-ARP will never occur
because of the configured Frame-Relay map statement. (assuming the
running-config is copied to the startup-config)
Rule: Inverse-ARP will be disabled for that specific protocol, for the
DLCI referenced in the Frame-Relay map statement.
Rick Graziani graziani@cabrillo.edu
69
Mixing Inverse ARP and Frame Relay Map
Statements
HubCity# show frame-relay map (after reload)
Serial0 (up): ip 172.16.1.1 dlci 101, dynamic, broadcast, status
defined, active
Serial0 (up): ip 172.16.1.3 dlci 112, dynamic, broadcast, status
defined, active
Spokane# show frame-relay map
NOW MISSING: Serial0 (up): ip 172.16.1.2 dlci 102, dynamic,
broadcast, status defined, active
Serial0 (up): ip 172.16.1.3 dlci 102, static, CISCO, status
defined, active
Spokomo# show frame-relay map
NOW MISSING: Serial0 (up): ip 172.16.1.2 dlci 211, dynamic,
broadcast, status defined, active
Serial0 (up): ip 172.16.1.1 dlci 211, static, CISCO, status
defined, active
Rick Graziani graziani@cabrillo.edu
70
Mixing Inverse ARP and Frame Relay Map
Statements
HubCity# show frame-relay map
(after reload)
Serial0 (up): ip 172.16.1.1 dlci 101, dynamic, broadcast, status
defined, active
Serial0 (up): ip 172.16.1.3 dlci 112, dynamic, broadcast, status
defined, active
Spokane# show frame-relay map
Serial0 (up): ip 172.16.1.3 dlci 102, static, CISCO, status
defined, active
Spokomo# show frame-relay map
Serial0 (up): ip 172.16.1.1 dlci 211, static, CISCO, status
defined, active
Spokane and Spokomo can no longer ping HubCity because they do not
have a dlci-to-IP mapping for the other’s IP address!
Rick Graziani graziani@cabrillo.edu
71
HubCity
interface Serial0
ip address 172.16.1.2
255.255.255.0
encapsulation frame-relay
(Inverse-ARP still works here)
Frame-Relay Map Statements
Headquarters
Hub City
Spokane
interface Serial0
ip address 172.16.1.1
255.255.255.0
encapsulation frame-relay
frame-relay map ip 172.16.1.3 102
frame-relay map ip 172.16.1.2 102
Spokomo
interface Serial0
ip address 172.16.1.3
255.255.255.0
encapsulation frame-relay
frame-relay map ip 172.16.1.1 211
frame-relay map ip 172.16.1.2 211
DLCI 101
DLCI 112
172.16.1.2
Frame Relay
Network
DLCI 102
172.16.1.1
Satellite Office 1
Spokane
DLCI 211
172.16.1.3
Satellite Office 2
Spokomo
Solution: Do not mix IARP with Frame Relay maps statements. If need
be use Frame-Relay map statements instead of IARP.
Rick Graziani graziani@cabrillo.edu
72
Reachability issues
with routing updates
Frame Relay is an NBMA Network
• An NBMA network is a multiaccess network, which means more than
•
•
•
•
two nodes can connect to the network.
Ethernet is another example of a multiaccess architecture.
In an Ethernet LAN, all nodes see all broadcast and multicast frames.
However, in a nonbroadcast network such as Frame Relay, 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.
Rick Graziani graziani@cabrillo.edu
73
Reachability issues
with routing updates
Split Horizon prohibits routing
updates received on an interface
from exiting that same interface.
• The Central router must receive the broadcast from Branch A and then
•
•
send its own broadcast to Branch B.
In this example, there are problems with routing protocols because of
the split horizon rule.
A full mesh topology with virtual circuits between every site would solve
this problem, but having additional virtual circuits is more costly and
does not scale well.
Rick Graziani graziani@cabrillo.edu
74
Reachability issues
with routing updates
Split Horizon prohibits routing
updates received on an interface
from exiting that same interface.
• Using a hub and spoke topology, the split horizon rule reduces the
•
•
•
•
chance of a routing loop with distance vector routing protocols.
It prevents a routing update received on an interface from being
forwarded through the same interface.
If the Central router learns about Network X from Branch A, that update
is learned via S0/0.
According to the split horizon rule, Central could not update Branch B
or Branch C about Network X.
This is because that update would be sent out the S0/0 interface,
which is the same interface that received the update.
Rick Graziani graziani@cabrillo.edu
75
One Solution: Disable Split Horizon
Router(config-if)#no ip split-horizon
Router(config-if)#ip split-horizon
• To remedy this situation, turn off split horizon for IP.
• When configuring a serial interface for Frame Relay encapsulation,
•
•
•
split horizon for IP is automatically turned off.
Of course, with split horizon disabled, the protection it affords against
routing loops is lost.
Split horizon is only an issue with distance vector routing protocols like
RIP, IGRP and EIGRP.
It has no effect on link state routing protocols like OSPF and IS-IS.
Rick Graziani graziani@cabrillo.edu
76
Another Solution for split horizon issue:
subinterfaces
• To enable the forwarding of broadcast routing updates in a Frame
•
•
•
•
•
Relay network, configure the router with subinterfaces.
Subinterfaces are logical subdivisions of a physical interface.
In split-horizon routing environments, routing updates received on one
subinterface can be sent out on another subinterface.
With subinterface configuration, each PVC can be configured as a
point-to-point connection.
This allows each subinterface to act similar to a leased line.
This is because each point-to-point subinterface is treated as a
separate physical interface.
Rick Graziani graziani@cabrillo.edu
77
Mulitpoint
Point-to-point
• A key reason for using subinterfaces is to allow distance vector routing
•
protocols to perform properly in an environment in which split horizon is
activated.
There are two types of Frame Relay subinterfaces.
– Point-to-point
– multipoint
Rick Graziani graziani@cabrillo.edu
78
Mulitpoint
Point-to-point
• Physical interfaces: With a hub and spoke topology Split Horizon will
•
•
prevent the hub router from propagating routes learned from one spoke
router to another spoke router.
Point-to-point subinterfaces: Each subinterface is on its own subnet.
Broadcasts and Split Horizon not a problem because each point-topoint connection is its own subnet.
Multipoint subinterfaces: All participating subinterfaces would be in
the same subnet. Broadcasts and routing updates are also subject to
the Split Horizon Rule and may pose a problem.
Rick Graziani graziani@cabrillo.edu
79
Configuring Frame Relay subinterfaces
RTA(config)#interface s0/0
RTA(config-if)#encapsulation frame-relay ietf
Router(config-if)#interface serial number subinterface-number
{multipoint | point-to-point}
Router(config-subif)# frame-relay interface-dlci dlci-number
• Subinterface can be configured after the physical interface has been
•
•
•
•
configured for Frame Relay encapsulation
Subinterface numbers can be specified in interface configuration mode
or global configuration mode.
subinterface number can be between 1 and 4294967295.
At this point in the subinterface configuration, either configure a static
Frame Relay map or use the frame-relay interface-dlci
command.
The frame-relay interface-dlci command associates the
selected subinterface with a DLCI.
Rick Graziani graziani@cabrillo.edu
80
Configuring Frame Relay subinterfaces
• The frame-relay interface-dlci command is required for all
•
•
•
point-to-point subinterfaces.
It is also required for multipoint subinterfaces for which inverse ARP is
enabled.
It is not required for multipoint subinterfaces that are configured with
static route maps.
It can not be used on physical interfaces.
Rick Graziani graziani@cabrillo.edu
81
Show frame-relay map
Point-to-point subinterfaces are listed as a “point-to-point dlci”
Router#show frame-relay map
Serial0.1 (up): point-to-point dlci, dlci 301 (0xCB, 0x30B0),
broadcast status defined, active
With multipoint subinterfaces, they are listed as an inverse ARP entry,
“dynamic”
Router#show frame-relay map
Serial0 (up): ip 172.30.2.1 dlci, 301 (0x12D, 0x48D0),
dynamic,, broadcast status defined, active
Rick Graziani graziani@cabrillo.edu
82
Point-to-point Subinterfaces
Mulitpoint
Point-to-point
Point-to-point subinterfaces are like conventional point-to-point interfaces
(PPP, …) and have no concept of (do not need):
• Inverse-ARP
• mapping of local DLCI address to remote network address (frame-relay
map statements)
Frame-Relay service supplies multiple PVCs over a single physical
interface and point-to-point subinterfaces subdivide each PVC as if it
were a physical point-to-point interface.
Point-to-point subinterfaces completely bypass the local DLCI to
remote network address mapping issue.
Rick Graziani graziani@cabrillo.edu
83
Point-to-point Subinterfaces
Mulitpoint
Point-to-point
With point-to-point subinterfaces you:
• Cannot have multiple DLCIs associated with a single point-to-point
subinterface
• Cannot use frame-relay map statements
• Cannot use Inverse-ARP
• Can use the frame-relay interface dlci statement (for both point-topoint and multipoint)
Rick Graziani graziani@cabrillo.edu
84
Point-to-point Subinterfaces
Each subinterface is on a separate
network or subnet with a single remote
router at the other end of the PVC.
172.30.1.0/24
172.30.2.0/24
172.30.3.0/24
Rick Graziani graziani@cabrillo.edu
85
172.30.1.1/24
172.30.2.1/24
172.30.3.1/24
S0
S1
S2
172.30.1.2/24
172.30.2.2/24
172.30.3.2/24
Site A
Site B
Site C
• Point-to-point subinterfaces are equivalent to using multiple physical
“point to point” interfaces.
Rick Graziani graziani@cabrillo.edu
86
Point-to-point Subinterfaces
• A single subinterface is used to establish one PVC connection to
another physical or subinterface on a remote router.
• In this case, the interfaces would be:
– In the same subnet and
– Each interface would have a single DLCI
• Each point-to-point connection is its own subnet.
• In this environment, broadcasts are not a problem because the
routers are point-to-point and act like a leased line.
Rick Graziani graziani@cabrillo.edu
87
Point-to-point Subinterfaces
Point-to-point subinterface configuration, minimum of two
commands:
Router(config)# interface Serial0.1 point-to-point
Router(config-subif)# frame-relay interface-dlci dlci
Rules:
1. No Frame-Relay map statements can be used with point-to-point
subinterfaces.
2. One and only one DLCI can be associated with a single point-to-point
subinterface
By the way, encapsulation is done only at the physical interface:
interface Serial0
no ip address
encapsulation frame-relay
Rick Graziani graziani@cabrillo.edu
88
Each subinterface on Hub router requires a
separate subnet (or network)
• Each subinterface on Hub router is treated
like a regular physical point-to-point
interface, so split horizon does not need to
be disabled.
Point-to-Point Subinterfaces at the Hub
and Spokes
Headquarters
Hub City
Interface Serial0 (for all routers)
encapsulation frame-relay
no ip address
DLCI 301
HubCity
interface Serial0.1 point-to-point
ip address 172.16.1.1 255.255.255.0
encapsulation frame-relay
frame-relay interface dlci 301
Serial 0.1
172.16.1.1/24
DLCI 302
Serial 0.2
172.16.2.1/24
Frame Relay
Network
interface Serial0.2 point-to-point
ip address 172.16.2.1 255.255.255.0
encapsulation frame-relay
frame-relay interface dlci 302
DLCI 103
Spokane
interface Serial0.1 point-to-point
ip address 172.16.1.2 255.255.255.0
frame-relay interface dlci 103
Spokomo
interface Serial0.1 point-to-point
ip address 172.16.2.2 255.255.255.0
frame-relay interface dlci 203
Rick Graziani graziani@cabrillo.edu
Serial 0.1
172.16.1.2/24
DLCI 203
Two subnets
Satellite Office 1
Spokane
Serial 0.1
172.16.2.2/24
Satellite Office 2
Spokomo
89
Multipoint Subinterfaces
Mulitpoint
Point-to-point
Share many of the same characteristics as a physical Frame-Relay interface
With multipoint subinterface you can have:
• can have multiple DLCIs assigned to it.
• can use frame-relay map & interface dlci statements
• can use Inverse-ARP
Remember, with point-to-point subinterfaces you:
• cannot have multiple DLCIs associated with a single point-to-point
subinterface
• cannot use frame-relay map statements
• cannot use Inverse-ARP
• (can use the frame-relay interface dlci statement for both point-to-point and
multipoint)
Rick Graziani graziani@cabrillo.edu
90
Multipoint subinterfaces
Each subinterface is on a separate
network or subnet but may have
multiple connections, with a different
DLCI for each connection.
172.30.1.0/24
172.30.2.0/24
172.30.3.0/24
Split horizon still an issue on each Multipoint
subinterface.
Rick Graziani graziani@cabrillo.edu
91
172.30.1.1/24
172.30.2.1/24
172.30.3.1/24
S0
S1
S2
172.30.1.2/24
172.30.3.3/24
Site A1
Site C2
172.30.3.2/24
172.30.1.3/24
Site A2
172.30.2.2/24
172.30.2.3/24
Site C1
Site B1
Site B2
• Multipoint subinterfaces are equivalent to using multiple physical “hub
to spoke” interfaces.
Rick Graziani graziani@cabrillo.edu
92
Notes
• Highly scalable solution
Multipoint subinterface at the Hub and
Point-to-Point Subinterfaces at the
Spokes
• Disable Split Horizon on Hub router when
running a distance vector routing protocol
Headquarters
Hub City
Interface Serial0 (for all routers)
encapsulation frame-relay
no ip address
DLCI 301
HubCity
interface Serial0.1 mulitpoint
ip address 172.16.3.3 255.255.255.0
frame-relay interface-dlci 301
frame-relay interface-dlci 302
no ip split-horizon
DLCI 302
Serial 0
172.16.3.3
Frame Relay
Network
Spokane
interface Serial0.1 point-to-point
ip address 172.16.3.1 255.255.255.0
frame-relay interface-dlci 103
DLCI 103
Spokomo
interface Serial0.1 point-to-point
ip address 172.16.3.2 255.255.255.0
frame-relay interface-dlci 203
Serial 0
172.16.3.1
Satellite Office 1
Spokane
Rick Graziani graziani@cabrillo.edu
DLCI 203
One subnet
Serial 0
172.16.3.2
Satellite Office 2
Spokomo
93
Ch. 5 – Frame Relay
CCNA 4 version 3.0
Rick Graziani
Cabrillo College