Mod 5

1
Mod 5 Frame Relay
2
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.

3
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.

4
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.

5
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.

6
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.

7
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

8
Frame Relay devices UNI and NNI
NNI
UNI

• It is quite common to find ATM as the technology
  used within the service providers 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.

9
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 providers
  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.

10
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.

11
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.

12
Frame Relay operation - PVC
An SVC between the same two DTEs may change.
A PVC between the same two 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 switch.

13
Frame Relay operation - SVC
An SVC between the same two DTEs may change.
A PVC between the same two 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.

14
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.

15
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.

16
IETF Frame Relay Frame
17
IETF Frame Relay Frame
18
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.

19
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

20
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.

21
DLCI

• Your Frame Relay provider sets up the DLCI
  numbers to be used by the routers for
  establishing PVCs.

22
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

23
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.

24
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.

25
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.

26
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.

27
Inverse ARP
1
2

• 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.

28
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)

29
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

30
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.

31
Minimum Frame Relay Configuration

• 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

32
Minimum Frame Relay Configuration

• 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!

33
Inverse ARP

• 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.

34
Inverse ARP Limitations

• 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.

35
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)

36
Frame Relay Maps
By default, cisco is the default encapsulation
Local DLCI
Remote IP Address
Uses cisco encapsulation for this DLCI (not
needed, default)
37
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.

38
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.

39
show interfaces serial
Atlanta(config)interface serial
0/0 Atlanta(config-if)description
Circuit-05QHDQ101545-080TCOM-002 Atlanta(config-if
)z Atlantashow interfaces serial 0/0 Serial
0/0 is up, line protocol is up Hardware is MCI
Serial Description Circuit-05QHDQ101545-080TCOM-00
2 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.

40
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.

41
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
42
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.

43
clear frame-relay-inarp

• To clear dynamically created Frame Relay maps,
  which are created using Inverse ARP, use the
  clear frame-relay-inarp command.

44
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.

45
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.

46
Frame Relay Topologies
47
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/

48
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.

49
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.

50

• A Frame-Relay Configuration Supporting Multiple
  Sites

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).

Spoke Routers
51
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
• Spokomo
• interface Serial0
• ip address 172.16.1.3 255.255.255.0
• encapsulation frame-relay

52
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

53
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?

54
Inverse ARP Limitations

• 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.

55
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

56
HubCity interface Serial0 ip address 172.16.1.2
255.255.255.0 encapsulation frame-relay (Inverse-A
RP still works here) Spokane interface
Serial0 ip address 172.16.1.1 255.255.255.0 encap
sulation 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
Frame-Relay Map Statements
Notice that the routers are configured to use
either IARP or Frame Relay maps. Using both on
the same interface will cause problems.
57
Mixing Inverse ARP and Frame Relay Map Statements
Inverse ARP
Frame Relay maps

• 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!

58
Mixing Inverse ARP and Frame Relay Map Statements
HubCity interface Serial0 ip address 172.16.1.2
255.255.255.0 encapsulation frame-relay Spokane i
nterface Serial0 ip address 172.16.1.1
255.255.255.0 encapsulation frame-relay frame-rela
y map ip 172.16.1.3 102 Spokomo interface
Serial0 ip address 172.16.1.3 255.255.255.0 encap
sulation frame-relay frame-relay map ip
172.16.1.1 211
59
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

60
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...

61
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.

62
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.

63
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

64
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 others IP address!
65
HubCity interface Serial0 ip address 172.16.1.2
255.255.255.0 encapsulation frame-relay (Inverse-A
RP still works here) Spokane interface
Serial0 ip address 172.16.1.1 255.255.255.0 encap
sulation 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
Frame-Relay Map Statements
Solution Do not mix IARP with Frame Relay maps
statements. If need be use Frame-Relay map
statements instead of IARP.
66
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.

67
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.

68
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.

69
One Solution Disable Split Horizon
Router(config-if)no ip split-horizon Router(confi
g-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.

70
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.

71
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

72
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.

73
Configuring Frame Relay subinterfaces
RTA(config)interface s0/0 RTA(config-if)encapsul
ation frame-relay ietf Router(config-if)interfa
ce 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.

74
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.

75
Show frame-relay map

• Point-to-point subinterfaces are listed as a
  point-to-point dlci
• Routershow frame-relay map
• Serial0.1 (up) point-to-point dlci, dlci 301
  (0xCB, 0x30B0), broadcast status defined, active
• What is missing???

76
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.

77
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

78
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
79

• Point-to-point subinterfaces are equivalent to
  using multiple physical point to point
  interfaces.

80
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.

81
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

82

• 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.
• Interface Serial0 (for all routers)
• encapsulation frame-relay
• no ip address
• 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
• interface Serial0.2 point-to-point
• ip address 172.16.2.1 255.255.255.0
• encapsulation frame-relay
• frame-relay interface dlci 302
• Spokane
• interface Serial0.1 point-to-point

• Point-to-Point Subinterfaces at the Hub and Spokes

Two subnets
83
Mod. 5 Frame Relay