mod 5 – frame relay. overview frame relay has replaced x.25 as the packet-switching technology of...

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.


• 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.


• 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

• 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.

UNINNI

• 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 operationAccess 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 operationAccess 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.

Access circuits

Frame Relay operation - VC

• 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.

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.

• 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.

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.

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

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

• 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

10230

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

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

• 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.

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

Inverse ARP

• 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.

12

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)

• cisco - Default.

– Use this if connecting to another Cisco router.

• Ietf - Select this if connecting to a non-Cisco router.

– RFC 1490

Router(config-if)#encapsulation frame-relay {cisco | ietf}

Frame Relay Encapsulation

Frame Relay LMI

• 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.

Router(config-if)#frame-relay lmi-type {ansi | cisco | q933a}

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

Frame RelayNetw ork

HeadquartersHub City

Satellite Office 1Spokane

172.16.1.1172.16.1.2

DLCI 101 DLCI 102

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!

Minimum Frame Relay Configuration

Frame RelayNetw ork



172.16.1.1172.16.1.2

DLCI 101 DLCI 102

HubCity# show frame-relay map

Serial0 (up): ip 172.16.1.1 dlci 101, dynamic, broadcast, status defined, active

Frame RelayNetw ork



172.16.1.1172.16.1.2

DLCI 101 DLCI 102

Inverse ARP

• 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

• 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.

Frame RelayNetw ork



172.16.1.1172.16.1.2

DLCI 101 DLCI 102

Configuring Frame Relay maps

• 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)

Router(config-if)#frame-relay map protocol protocol-address dlci [broadcast] [ietf | cisco]

Frame Relay Maps

Remote IP Address

Local DLCIUses cisco encapsulation for this DLCI (not needed, default)

By default, cisco is the default encapsulation

More on Frame Relay Encapsulation

• 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.

Applies to all DLCIs unless configured otherwise

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

• To simplify the WAN management, use the description command at the interface level to record the circuit number.

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

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

• Use the debug frame-relay lmi command to determine whether the router and the Frame Relay switch are sending and receiving LMI packets properly.

Enquiry

Response

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

• 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/

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).

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

• 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.

Full Mesh Topology

Number of Number of

Connections PVCs

----------------- --------------

2 1

4 6

6 15

8 28

10 45

A Frame-Relay Configuration Supporting Multiple Sites

Frame RelayNetw ork



Satellite Office 2Spokomo

172.16.1.1 172.16.1.3

172.16.1.2

DLCI 101

DLCI 102

DLCI 112

DLCI 211

• 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).

Hub Router

Spoke Routers

HubCityinterface Serial0ip address 172.16.1.2 255.255.255.0encapsulation frame-relay

Spokaneinterface Serial0ip address 172.16.1.1 255.255.255.0encapsulation frame-relay

Spokomointerface Serial0ip address 172.16.1.3 255.255.255.0encapsulation frame-relay

Frame RelayNetw ork




172.16.1.1 172.16.1.3

172.16.1.2

DLCI 101

DLCI 102

DLCI 112

DLCI 211

Configuration using Inverse ARP

HubCity# show frame-relay mapSerial0 (up): ip 172.16.1.1 dlci 101, dynamic, broadcast,

status defined, activeSerial0 (up): ip 172.16.1.3 dlci 112, dynamic, broadcast,

status defined, active

Spokane# show frame-relay mapSerial0 (up): ip 172.16.1.2 dlci 102, dynamic, broadcast,


Spokomo# show frame-relay mapSerial0 (up): ip 172.16.1.2 dlci 211, dynamic, broadcast,


Frame RelayNetw ork




172.16.1.1 172.16.1.3

172.16.1.2

DLCI 101

DLCI 102

DLCI 112

DLCI 211


• 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?

HubCity# show frame-relay mapSerial0 (up): ip 172.16.1.1 dlci 101, dynamic, broadcast,

status defined, activeSerial0 (up): ip 172.16.1.3 dlci 112, dynamic, broadcast,


Spokane# show frame-relay mapSerial0 (up): ip 172.16.1.2 dlci 102, dynamic, broadcast,


Spokomo# show frame-relay mapSerial0 (up): ip 172.16.1.2 dlci 211, dynamic, broadcast,



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 ARP1. Add an additional PVC between Spokane and Spokomo (Full Mesh)2. Configure Frame-Relay Map Statements3. Configure Point-to-Point Subinterfaces.

Frame RelayNetw ork




172.16.1.1 172.16.1.3

172.16.1.2

DLCI 101

DLCI 102

DLCI 112

DLCI 211

Frame Relay Map Statements

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

Router(config-if)#frame-relay map protocol protocol-address dlci [broadcast] [ietf | cisco]

Frame RelayNetwork




172.16.1.1 172.16.1.3

172.16.1.2

DLCI 101

DLCI 102

DLCI 112

DLCI 211

HubCityinterface Serial0ip address 172.16.1.2 255.255.255.0encapsulation frame-relay(Inverse-ARP still works here)

Spokaneinterface Serial0ip address 172.16.1.1 255.255.255.0encapsulation frame-relayframe-relay map ip 172.16.1.3 102frame-relay map ip 172.16.1.2 102

Spokomointerface Serial0ip address 172.16.1.3 255.255.255.0encapsulation frame-relayframe-relay map ip 172.16.1.1 211frame-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.

• 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!

Frame RelayNetwork




172.16.1.1 172.16.1.3

172.16.1.2

DLCI 101

DLCI 102

DLCI 112

DLCI 211

Inverse ARP

Mixing Inverse ARP and Frame Relay Map Statements

Frame Relay maps

HubCityinterface Serial0ip address 172.16.1.2 255.255.255.0encapsulation frame-relay

Spokaneinterface Serial0ip address 172.16.1.1 255.255.255.0encapsulation frame-relayframe-relay map ip 172.16.1.3 102

Spokomointerface Serial0ip address 172.16.1.3 255.255.255.0encapsulation frame-relayframe-relay map ip 172.16.1.1 211

Frame RelayNetwork




172.16.1.1 172.16.1.3

172.16.1.2

DLCI 101

DLCI 102

DLCI 112

DLCI 211


HubCity# show frame-relay mapSerial0 (up): ip 172.16.1.1 dlci 101, dynamic,

broadcast, status defined, activeSerial0 (up): ip 172.16.1.3 dlci 112, dynamic,

broadcast, status defined, activeSpokane# show frame-relay mapSerial0 (up): ip 172.16.1.2 dlci 102, dynamic,

broadcast, status defined, activeSerial0 (up): ip 172.16.1.3 dlci 102, static, CISCO,

status defined, active Spokomo# show frame-relay mapSerial0 (up): ip 172.16.1.2 dlci 211, dynamic,

broadcast, status defined, activeSerial0 (up): ip 172.16.1.1 dlci 211, static, CISCO,



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 StatementsHubCity# show frame-relay mapSerial0 (up): ip 172.16.1.1 dlci 101, dynamic, broadcast, status defined, activeSerial0 (up): ip 172.16.1.3 dlci 112, dynamic, broadcast, status defined, active

Spokane# show frame-relay mapSerial0 (up): ip 172.16.1.2 dlci 102, dynamic, broadcast, status defined, activeSerial0 (up): ip 172.16.1.3 dlci 102, static, CISCO, status defined, active

Spokomo# show frame-relay mapSerial0 (up): ip 172.16.1.2 dlci 211, dynamic, broadcast, status defined, activeSerial0 (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 StatementsHubCity# show frame-relay mapSerial0 (up): ip 172.16.1.1 dlci 101, dynamic, broadcast, status defined, activeSerial0 (up): ip 172.16.1.3 dlci 112, dynamic, broadcast, status defined, active

Spokane# show frame-relay mapSerial0 (up): ip 172.16.1.2 dlci 102, dynamic, broadcast, status defined, activeSerial0 (up): ip 172.16.1.3 dlci 102, static, CISCO, status defined, active

Spokomo# show frame-relay mapSerial0 (up): ip 172.16.1.2 dlci 211, dynamic, broadcast, status defined, activeSerial0 (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.

HubCity# show frame-relay mapSerial0 (up): ip 172.16.1.1 dlci 101, dynamic, broadcast, status defined, activeSerial0 (up): ip 172.16.1.3 dlci 112, dynamic, broadcast, status defined, activeSpokane# show frame-relay mapSerial0 (up): ip 172.16.1.2 dlci 102, dynamic, broadcast, status defined, activeSerial0 (up): ip 172.16.1.3 dlci 102, static, CISCO, status defined, active Spokomo# show frame-relay mapSerial0 (up): ip 172.16.1.2 dlci 211, dynamic, broadcast, status defined, activeSerial0 (up): ip 172.16.1.1 dlci 211, static, CISCO, status defined, active


HubCity# show frame-relay map (after reload)



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



HubCity# show frame-relay map (after reload)



Spokane# show frame-relay map


Spokomo# show frame-relay map


Spokane and Spokomo can no longer ping HubCity because they do not have a dlci-to-IP mapping for the other’s IP address!


Frame RelayNetwork




172.16.1.1 172.16.1.3

172.16.1.2

DLCI 101

DLCI 102

DLCI 112

DLCI 211

HubCityinterface Serial0ip address 172.16.1.2 255.255.255.0encapsulation frame-relay(Inverse-ARP still works here)

Spokaneinterface Serial0ip address 172.16.1.1 255.255.255.0encapsulation frame-relayframe-relay map ip 172.16.1.3 102frame-relay map ip 172.16.1.2 102

Spokomointerface Serial0ip address 172.16.1.3 255.255.255.0encapsulation frame-relayframe-relay map ip 172.16.1.1 211frame-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.

Reachability issues with routing updates

• 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.

Frame Relay is an NBMA Network


• 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.

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.


Split Horizon prohibits routing updates received on an interface from exiting that same interface.

One Solution: Disable 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.

Router(config-if)#no ip split-horizon

Router(config-if)#ip split-horizon

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.

• 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.

Mulitpoint

Point-to-point

Configuring Frame Relay subinterfaces

• 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.

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

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 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


Mulitpoint

Point-to-point

172.30.1.0/24

172.30.2.0/24

172.30.3.0/24

Each subinterface is on a separate network or subnet with a single remote router at the other end of the PVC.


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

S0 S1 S2

Site A Site B Site C

172.30.1.1/24 172.30.2.1/24 172.30.3.1/24

172.30.1.2/24 172.30.2.2/24 172.30.3.2/24

• 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 subinterface configuration, minimum of two commands:

Router(config)# interface Serial0.1 point-to-pointRouter(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


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-relayno ip address

HubCityinterface Serial0.1 point-to-pointip address 172.16.1.1 255.255.255.0encapsulation frame-relayframe-relay interface dlci 301

interface Serial0.2 point-to-pointip address 172.16.2.1 255.255.255.0encapsulation frame-relayframe-relay interface dlci 302

Spokaneinterface Serial0.1 point-to-pointip address 172.16.1.2 255.255.255.0frame-relay interface dlci 103

Spokomointerface Serial0.1 point-to-pointip address 172.16.2.2 255.255.255.0frame-relay interface dlci 203

Frame RelayNetw ork




Serial 0.1172.16.1.2/24

Serial 0.1172.16.2.2/24

Serial 0.1172.16.1.1/24

DLCI 301

DLCI 103

DLCI 302

DLCI 203

Serial 0.2172.16.2.1/24

Two subnets

Mod. 5 – Frame Relay

mod 5 – frame relay. overview frame relay has replaced x.25 as the packet-switching technology of...

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