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
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.

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 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.

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 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
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. 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 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.

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
HubCity(config-if)# encapsulation frame-relay
Spokane(config)# interface serial 0
Spokane(config-if)# ip address
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 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-05QHDQ TCOM-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-05QHDQ TCOM-002
Internet address is , subnet mask
MTU 1500 bytes, BW 1544 Kbit, DLY 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 This command also displays the status of the PVC
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.

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
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
encapsulation frame-relay
Spokane
ip address
Spokomo
ip address

52. Configuration using Inverse ARP
HubCity# show frame-relay map
Serial0 (up): ip dlci 101, dynamic, broadcast, status defined, active
Serial0 (up): ip dlci 112, dynamic, broadcast, status defined, active
Spokane# show frame-relay map
Serial0 (up): ip dlci 102, dynamic, broadcast, status defined, active
Spokomo# show frame-relay map
Serial0 (up): ip dlci 211, dynamic, broadcast, status defined, active

53. Configuration using Inverse ARP
HubCity# show frame-relay map
Serial0 (up): ip dlci 101, dynamic, broadcast, status defined, active
Serial0 (up): ip dlci 112, dynamic, broadcast, status defined, active
Spokane# show frame-relay map
Serial0 (up): ip dlci 102, dynamic, broadcast, status defined, active
Spokomo# show frame-relay map
Serial0 (up): ip 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. Frame-Relay Map Statements
HubCity
interface Serial0
ip address
encapsulation frame-relay
(Inverse-ARP still works here)
Spokane
ip address
frame-relay map ip
frame-relay map ip
Spokomo
ip address
frame-relay map ip
frame-relay map ip
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
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
encapsulation frame-relay
Spokane
ip address
frame-relay map ip
Spokomo
ip address
frame-relay map ip

59. Mixing Inverse ARP and Frame Relay Map Statements
HubCity# show frame-relay map
Serial0 (up): ip dlci 101, dynamic, broadcast, status defined, active
Serial0 (up): ip dlci 112, dynamic, broadcast, status defined, active
Spokane# show frame-relay map
Serial0 (up): ip dlci 102, dynamic, broadcast, status defined, active
Serial0 (up): ip dlci 102, static, CISCO, status defined, active
Spokomo# show frame-relay map
Serial0 (up): ip dlci 211, dynamic, broadcast, status defined, active
Serial0 (up): ip dlci 211, static, CISCO, status defined, active

60. Mixing Inverse ARP and Frame Relay Map Statements
HubCity# show frame-relay map
Serial0 (up): ip dlci 101, dynamic, broadcast, status defined, active
Serial0 (up): ip dlci 112, dynamic, broadcast, status defined, active
Spokane# show frame-relay map
Serial0 (up): ip dlci 102, dynamic, broadcast, status defined, active
Serial0 (up): ip dlci 102, static, CISCO, status defined, active
Spokomo# show frame-relay map
Serial0 (up): ip dlci 211, dynamic, broadcast, status defined, active
Serial0 (up): ip 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 dlci 101, dynamic, broadcast, status defined, active
Serial0 (up): ip dlci 112, dynamic, broadcast, status defined, active
Spokane# show frame-relay map
Serial0 (up): ip dlci 102, dynamic, broadcast, status defined, active
Serial0 (up): ip dlci 102, static, CISCO, status defined, active
Spokomo# show frame-relay map
Serial0 (up): ip dlci 211, dynamic, broadcast, status defined, active
Serial0 (up): ip 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 dlci 101, dynamic, broadcast, status defined, active
Serial0 (up): ip dlci 112, dynamic, broadcast, status defined, active
Spokane# show frame-relay map
Serial0 (up): ip dlci 102, dynamic, broadcast, status defined, active
Serial0 (up): ip dlci 102, static, CISCO, status defined, active
Spokomo# show frame-relay map
Serial0 (up): ip dlci 211, dynamic, broadcast, status defined, active
Serial0 (up): ip 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 dlci 101, dynamic, broadcast, status defined, active
Serial0 (up): ip dlci 112, dynamic, broadcast, status defined, active
Spokane# show frame-relay map
NOW MISSING: Serial0 (up): ip dlci 102, dynamic, broadcast, status defined, active
Serial0 (up): ip dlci 102, static, CISCO, status defined, active
Spokomo# show frame-relay map
NOW MISSING: Serial0 (up): ip dlci 211, dynamic, broadcast, status defined, active
Serial0 (up): ip 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 dlci 101, dynamic, broadcast, status defined, active
Serial0 (up): ip dlci 112, dynamic, broadcast, status defined, active
Spokane# show frame-relay map
Serial0 (up): ip dlci 102, static, CISCO, status defined, active
Spokomo# show frame-relay map
Serial0 (up): ip 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!

65. Frame-Relay Map Statements
HubCity
interface Serial0
ip address
encapsulation frame-relay
(Inverse-ARP still works here)
Spokane
ip address
frame-relay map ip
frame-relay map ip
Spokomo
ip address
frame-relay map ip
frame-relay map ip
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(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.

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)#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
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 What is missing???
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???

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.
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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. Two subnets Point-to-Point Subinterfaces at the Hub and Spokes
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
frame-relay interface dlci 301
interface Serial0.2 point-to-point
ip address
frame-relay interface dlci 302
Spokane
ip address
frame-relay interface dlci 103
Spokomo
ip address
frame-relay interface dlci 203
Point-to-Point Subinterfaces at the Hub and Spokes
Two subnets

83. Mod. 5 – Frame Relay