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Mod 5 – Frame Relay
Overview
• Frame Relay has replaced X.25 as the packet-switching technology of
choice in many nations, particularly the United States.
• First standardized in 1990, Frame Relay streamlines Layer 2 functions
and provides only basic error checking rather than error correction.
• This low-overhead approach to switching packets increases
performance and efficiency.
• Modern fiber optic links and digital transmission facilities offer much
lower error rates than their copper predecessors.
• For that reason, the use of X.25 reliability mechanisms at Layer 2 and
Layer 3 is now generally regarded as unnecessary overhead.
• This module presents Frame Relay technology, including its benefits
and requirements.
Frame Relay overview
• Frame Relay is an International Telecommunications Union (ITU-T)
and American National Standards Institute (ANSI) standard that
defines the process for sending data over a packet-switched network.
• It is a connection-oriented data-link technology that is optimized to
provide high performance and efficiency.
Frame Relay overview
• Modern telecommunications networks are characterized by relatively
error-free digital transmission and highly reliable fiber infrastructures.
• Frame Relay takes advantage of these technologies by relying almost
entirely on upper-layer protocols to detect and recover from errors.
• Frame Relay does not have the sequencing, windowing, and
retransmission mechanisms that are used by X.25.
• Without the overhead associated with comprehensive error detection,
the streamlined operation of Frame Relay outperforms X.25.
• Typical speeds range from 56 kbps up to 2 Mbps, although higher
speeds are possible. (45 Mbps)
• The network providing the Frame Relay service can be either a
carrier-provided public network or a privately owned network.
Frame Relay overview
• Like X.25, Frame Relay defines the interconnection process between
the customer's data terminal equipment (DTE), such as the router, and
the service provider's data communication equipment (DCE).
• Frame Relay does not define the way the data is transmitted within the
service provider's network once the traffic reaches the provider's
switch.
• Therefore, a Frame Relay provider could use a variety of technologies,
such as Asynchronous Transfer Mode (ATM) or Point-to-Point Protocol
(PPP), to move data from one end of its network to another.
Frame Relay devices - DTE
• DTEs generally are considered to be terminating equipment for a
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
– Frame Relay Access Devices (FRADs).
• A FRAD is a specialized device designed to provide a connection
between a LAN and a Frame Relay WAN.
Frame Relay devices - DCE
• DCEs are carrier-owned internetworking devices.
• The purpose of DCE equipment is to provide clocking and switching
services in a network.
• In most cases, these are packet switches, which are the devices that
actually transmit data through the WAN
Frame Relay devices – UNI and NNI
NNI
UNI
• It is quite common to find ATM as the technology used within the
service provider’s Frame Relay network or cloud.
• Regardless of the technology used inside the cloud, the connection
between the customer and the Frame Relay service provider is still
Frame Relay.
• 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.
Frame Relay operation
Access circuits
• Generally, the greater the distance covered by a leased line, the
more expensive the service.
• Maintaining a full mesh of leased lines to remote sites proves too
expensive for many organizations.
• On the other hand, packet-switched networks provide a means for
multiplexing several logical data conversations over a single physical
transmission link.
• A single connection to a provider’s packet-switched network will be
less expensive than separate leased lines between the customer and
each remote site.
• Packet-switched networks use virtual circuits to deliver packets from
end to end over a shared infrastructure.
Frame Relay operation
Access circuits
• A packet-switched service such as Frame Relay requires that a
customer maintain only one circuit, typically a T1, to the provider's
Central Office (CO). (Access Circuit)
• Frame Relay provides tremendous cost-effectiveness, since one site
can connect many geographically distant sites using a single T1 and
single channel service unit/data service unit (CSU/DSU) to the local
CO.
Frame Relay operation - VC
Access circuits
• In order for any two Frame Relay sites to communicate, the service
provider must set up a virtual circuit between these sites within the
Frame Relay network.
• Service providers will typically charge for each virtual circuit.
• However, the charge for each virtual circuit is typically very low.
• This makes Frame Relay an ideal technology when full-mesh
topologies are needed.
• As discussed later, many enterprises use a hub and spoke topology
using only virtual circuits between a central site and each of the branch
offices.
• For two branch offices to reach each other, the traffic must pass
through the central site.
Frame Relay operation - PVC
An SVC between the same two A PVC between the same two
DTEs may change. DTEs will always be the same.
Path may change. Always same Path.
• Frame Relay and X.25 networks support both permanent virtual circuits
(PVCs) and switched virtual circuits (SVCs).
• A PVC is the most common type of Frame Relay virtual circuit.
• PVCs are permanently established connections that are used when
there is frequent and consistent data transfer between DTE devices
across a Frame Relay network.
• 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
Frame Relay operation - SVC
An SVC between the same two A PVC between the same two
DTEs may change. DTEs will always be the same.
Path may change. Always same Path.
• SVCs are temporary connections that are only used when there is
sporadic data transfer between DTE devices across the Frame Relay
network.
• Because they are temporary, SVC connections require call setup and
termination for each connection supported by Cisco IOS Release 11.2
or later.
• Before implementing these temporary connections, determine whether
the service carrier supports SVCs since many Frame Relay providers
only support PVCs.
DLCI
• RTA can use only one of three configured PVCs to reach RTB.
• In order for router RTA to know which PVC to use, Layer 3 addresses
must be mapped to DLCI numbers.
• RTA must map Layer 3 addresses to the available DLCIs.
• RTA maps the RTB IP address 1.1.1.3 to DLCI 17.
• Once RTA knows which DLCI to use, it can encapsulate the IP packet
with a Frame Relay frame, which contains the appropriate DLCI
number to reach that destination.
DLCI
• Cisco routers support two types of Frame Relay headers,
encapsulation.
• One type is cisco, which is a 4-byte header.
• The second is itef, 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.
IETF Frame
Relay Frame
IETF Frame Relay Frame
DLCI
• By including a DLCI number in the Frame Relay header, RTA can
communicate with both RTB and RTC over the same physical circuit.
• This technique of allowing multiple logical channels to transmit across
a single physical circuit is called statistical multiplexing.
• Statistical multiplexing dynamically allocates bandwidth to active
channels.
• If RTA has no packets to send RTB, RTA can use all the available
bandwidth to communicate with RTC.
• Statistical multiplexing contrasts with time-division multiplexing
(TDM), which is typically used over dedicated circuits or leased lines.
• Unfortunately, TDM allocates bandwidth to each channel regardless of
whether the station has data to transmit.
DLCI
• A data-link connection identifier (DLCI) identifies the logical VC
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
– Remember that DLCI is a 10-bit field
DLCI
• In order to build a map of DLCIs to Layer 3 addresses, the router must
first know what VCs are available.
• Typically, the process of learning about available VCs and their
DLCI values is handled by the LMI signaling standard.
• LMI is discussed in the next section.
• Once the DLCIs for available VCs are known, the router must learn
which Layer 3 addresses map to which DLCIs.
• The address mapping can be either configured manually or
dynamically.
• Whether the mapping of a DLCI to remote IP address happens
manually or dynamically, the DLCI that is used does not have to be the
same number at both ends of the PVC.
DLCI
• Your Frame Relay provider sets up the DLCI numbers to be used by
the routers for establishing PVCs.
LMI – Local Management Interface
1023
0
• 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.
• LMI includes:
– A keepalive mechanism, which verifies that data is flowing
– A multicast mechanism, which provides the network server
(router) with its local DLCI.
– A status mechanism, which provides an ongoing status on the
DLCIs known to the switch
LMI
LMI
• The three types of LMI are not compatible
with each others.
• The LMI type must match between the
provider Frame Relay switch and the
customer DTE device.
LMI
LMI
• In Cisco IOS releases prior to 11.2, the Frame Relay interface must be
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.
LMI
• Frame Relay devices can now listen in on both DLCI 1023 (Cisco LMI)
and DLCI 0 (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.
DLCI Mapping to Network Address
RTA will know how to reach RTB from the
routing information; however, it will need
to use a statically or dynamically configure
frame map to encapsulate the frame at
layer 2 with the correct DLCI
• 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.
Inverse ARP
2 1
• Once the router learns from the switch about available PVCs and
their corresponding DLCIs, the router can send an Inverse ARP
request to the other end of the PVC. (unless statically mapped – later)
• 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.
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, Layer 3 address (IP) is used to learn layer 2
address (MAC).
• With Inverse Layer 2 address (DLCI) is used to learn Layer 3 address
(IP)
Frame Relay Encapsulation
Router(config-if)#encapsulation frame-relay {cisco | ietf}
• cisco - Default.
– Use this if connecting to another Cisco router.
• Ietf - Select this if connecting to a non-Cisco router.
– RFC 1490
Frame Relay LMI
Router(config-if)#frame-relay lmi-type {ansi | cisco | q933a}
• It is important to remember that the Frame Relay service provider
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.
Minimum Frame Relay Configuration
172.16.1.2 172.16.1.1
Frame Relay
DLCI 101 Network DLCI 102
Headquarters Satellite Office 1
Hub City 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
Minimum Frame Relay Configuration
172.16.1.2 172.16.1.1
Frame Relay
DLCI 101 Network DLCI 102
Headquarters Satellite Office 1
Hub City 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!
Inverse ARP
172.16.1.2 172.16.1.1
Frame Relay
DLCI 101 Network DLCI 102
Headquarters Satellite Office 1
Hub City 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
provider.
• We will see this in a moment.
Inverse ARP Limitations
172.16.1.2 172.16.1.1
Frame Relay
DLCI 101 Network DLCI 102
Headquarters Satellite Office 1
Hub City Spokane
• Inverse ARP only resolves network addresses of remote Frame-
Relay 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 next-
hop 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.
• If the Frame Relay environment supports LMI autosensing and Inverse
ARP, dynamic address mapping takes place automatically.
• Therefore, no static address mapping is required.
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,
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. (Not on the entire interface. Inverse ARP
could be still working for other DLCIs on the same interface).
• 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)
Frame
Relay Maps
By default,
cisco is the
default
encapsulation
Remote IP Local DLCI
Uses cisco
encapsulation for Address
this DLCI (not
needed, default)
More on Frame Relay Encapsulation
Applies to all DLCIs unless
configured otherwise
• If the Cisco encapsulation is configured on a serial interface, then by
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.
Verifying Frame Relay interface
configuration
• 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.
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.
show frame-relay pvc
• The show frame-relay pvc command displays the status of each
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.
show frame-relay map
• The show frame-relay map command displays the current map
entries and information about the connections.
This command also
displays the status of the
PVC
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.
clear frame-relay-inarp
• To clear dynamically created Frame Relay maps, which are created
using Inverse ARP, use the clear frame-relay-inarp command.
Troubleshooting the Frame Relay
configuration
Enquiry
Response
• Use the debug frame-relay lmi command to
determine whether the router and the Frame Relay switch
are sending and receiving LMI packets properly.
debug frame-relay lmi (continued)
• 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.
Frame Relay Topologies
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/
Star Topology
• A star topology, also known as a hub and spoke configuration, is the
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.
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.
A Frame-Relay Configuration Supporting Multiple Sites
Headquarters
Hub City
• This is known Hub Router
as a Hub and DLCI 101 DLCI 112
Spoke
172.16.1.2
Topology,
where the Hub
router relays
information
between the Frame Relay
Network
Spoke routers.
• Limits the
number of PVCs DLCI 102 DLCI 211
needed as in a
full-mesh 172.16.1.1 172.16.1.3
topology Spoke
(coming). Satellite Office 1 Routers Satellite Office 2
Spokane Spokomo
Headquarters
Configuration using Inverse Hub City
ARP DLCI 101
172.16.1.2
DLCI 112
Frame Relay
HubCity Network
interface Serial0
ip address 172.16.1.2 255.255.255.0 DLCI 102 DLCI 211
encapsulation frame-relay 172.16.1.1 172.16.1.3
Satellite Office 1 Satellite Office 2
Spokane Spokomo
Spokane
interface Serial0
ip address 172.16.1.1 255.255.255.0
encapsulation frame-relay
Spokomo
interface Serial0
ip address 172.16.1.3 255.255.255.0
encapsulation frame-relay
Headquarters
Configuration using Inverse Hub City
DLCI 101 DLCI 112
ARP 172.16.1.2
Frame Relay
Network
DLCI 102 DLCI 211
172.16.1.1 172.16.1.3
Satellite Office 1 Satellite Office 2
Spokane 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
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?
Headquarters
Hub City
Inverse ARP Limitations DLCI 101
172.16.1.2
DLCI 112
Frame Relay
Network
DLCI 102 DLCI 211
172.16.1.1 172.16.1.3
Satellite Office 1 Satellite Office 2
Spokane 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.
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
HubCity
interface Serial0
Frame-Relay Map Statements
ip address 172.16.1.2
255.255.255.0
encapsulation frame-relay Headquarters
(Inverse-ARP still works here) Hub City
Spokane DLCI 101 DLCI 112
interface Serial0
172.16.1.2
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 Frame Relay
Network
Spokomo
interface Serial0
ip address 172.16.1.3 DLCI 102 DLCI 211
255.255.255.0
encapsulation frame-relay 172.16.1.1 172.16.1.3
frame-relay map ip 172.16.1.1 211
frame-relay map ip 172.16.1.2 211 Satellite Office 1 Satellite Office 2
Spokane Spokomo
Notice that the routers are configured to use either IARP or Frame Relay
maps. Using both on the same interface will cause problems.
Headquarters
Mixing Inverse ARP and Hub City
Frame Relay Map Statements DLCI 101 DLCI 112
172.16.1.2
Inverse ARP
Frame Relay
Network
Frame Relay DLCI 102 DLCI 211
maps
172.16.1.1 172.16.1.3
Satellite Office 1 Satellite Office 2
Spokane 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!
Mixing Inverse ARP and Frame Relay Map
Statements
Headquarters
HubCity Hub City
interface Serial0
ip address 172.16.1.2
255.255.255.0 DLCI 101 DLCI 112
encapsulation frame-relay 172.16.1.2
Spokane
interface Serial0
ip address 172.16.1.1
Frame Relay
255.255.255.0 Network
encapsulation frame-relay
frame-relay map ip 172.16.1.3 102
DLCI 102 DLCI 211
Spokomo
interface Serial0
172.16.1.1 172.16.1.3
ip address 172.16.1.3
255.255.255.0
encapsulation frame-relay Satellite Office 1 Satellite Office 2
frame-relay map ip 172.16.1.1 211 Spokane Spokomo
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
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...
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.
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
• 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.
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
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!
HubCity
interface Serial0
Frame-Relay Map Statements
ip address 172.16.1.2
255.255.255.0
encapsulation frame-relay Headquarters
(Inverse-ARP still works here) Hub City
Spokane DLCI 101 DLCI 112
interface Serial0
172.16.1.2
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 Frame Relay
Network
Spokomo
interface Serial0
ip address 172.16.1.3 DLCI 102 DLCI 211
255.255.255.0
encapsulation frame-relay 172.16.1.1 172.16.1.3
frame-relay map ip 172.16.1.1 211
frame-relay map ip 172.16.1.2 211 Satellite Office 1 Satellite Office 2
Spokane Spokomo
Solution: Do not mix IARP with Frame Relay maps statements. If need
be use Frame-Relay map statements instead of IARP.
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.
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.
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.
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.
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.
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
Mulitpoint
Point-to-point
• Point-to-point subinterfaces: Each subinterface is on its
own subnet. Broadcasts and Split Horizon not a problem
because each point-to-point connection is its own subnet.
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, use the
frame-relay interface-dlci command.
• The frame-relay interface-dlci command
associates the selected subinterface with a DLCI.
Configuring Frame Relay subinterfaces
• The frame-relay interface-dlci command is required for all
point-to-point subinterfaces.
• Each point-to-pint subinterface can be associated with one PVC
only
• It can not be used on physical interfaces.
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
What is missing???
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.
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 (disabled by default on a point-
to-point subinterface)
• Must use the frame-relay interface dlci statement
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
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.
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.
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
Each subinterface on Hub router requires a
separate subnet (or network) Point-to-Point Subinterfaces at the Hub
• Each subinterface on Hub router is treated and Spokes
like a regular physical point-to-point
interface, so split horizon does not need to
be disabled.
Interface Serial0 (for all routers) Headquarters
Hub City
encapsulation frame-relay
no ip address
DLCI 301 DLCI 302
HubCity
Serial 0.1 Serial 0.2
interface Serial0.1 point-to-point 172.16.1.1/24 172.16.2.1/24
ip address 172.16.1.1 255.255.255.0
encapsulation frame-relay
frame-relay interface dlci 301
Frame Relay
interface Serial0.2 point-to-point Network
ip address 172.16.2.1 255.255.255.0
encapsulation frame-relay
frame-relay interface dlci 302
DLCI 103 DLCI 203
Spokane Serial 0.1 Two subnets Serial 0.1
interface Serial0.1 point-to-point 172.16.1.2/24 172.16.2.2/24
ip address 172.16.1.2 255.255.255.0
frame-relay interface dlci 103
Satellite Office 1 Satellite Office 2
Spokane Spokomo
Spokomo
interface Serial0.1 point-to-point
ip address 172.16.2.2 255.255.255.0
frame-relay interface dlci 203
Mod. 5 – Frame Relay
Related docs
