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Frame Relay


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Frame Relay overview

Frame Relay is an International Telecommunications Union (ITU-T)and American National Standards Institute (ANSI) standard thatdefines 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.


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Generally, the greater the distance covered by a leased line, themore expensive the service.

Maintaining a full mesh of leased lines to remote sites proves tooexpensive for many organizations.

On the other hand, packet-switched networks provide a means for

multiplexing several logical data conversations over a single physicaltransmission link.

A single connection to a providers packet-switched network will beless expensive than separate leased lines between the customer andeach remote site.

Packet-switched networks use virtual circuits to deliver packets fromend to end over a shared infrastructure.

Frame Relay opera

tionAccess circuits


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In order for any two Frame Relay sites to communicate, the serviceprovider must set up a virtual circuit between these sites within theFrame 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-meshtopologies are needed.

As discussed later, many enterprises use a hub and spoke topologyusing only virtual circuits between a central site and each of the branchoffices.

For two branch offices to reach each other, the traffic must passthrough the central site.

Access circuits

Frame Relay operation - VC


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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 whenthere is frequent and consistent data transfer between DTE devicesacross a Frame Relay network.

PVC are VCs that have been preconfigured by the carrier areused.

The switching information for a VC is stored in the memory of theswitch.

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.


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SVCs are temporary connections that are only used when there issporadic data transfer between DTE devices across the Frame Relaynetwork.

Because they are temporary, SVC connections require call setup andtermination for each connection supported by Cisco IOS Release 11.2or later.

Before implementing these temporary connections, determine whetherthe 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.


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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 DLCInumber to reach that destination.


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DLCI

A data-link connection identifier (DLCI)identifies the logical VCbetween the CPE and the Frame Relay switch.

The Frame Relay switch maps the DLCIs between each pair of routersto create a PVC.

DLCIs have local significance, although there some

implementations that use global DLCIs. DLCIs 0 to 15 and 1008 to 1023 are reserved for special purposes. Service providers assign DLCIs in the range of 16 to 1007.

DLCI 1019, 1020: Multicasts

DLCI 1023: Cisco LMI

DLCI 0: ANSI LMI


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DLCI 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 specificconnection.

The router then updates its mapping table and uses the informationin the table to forward packets on the correct route.


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Inverse ARP

Once the router learns from the switch about available PVCs andtheir corresponding DLCIs, the router can send an Inverse ARPrequest to the other end of the PVC. (unless statically mapped later)

For each supported and configured protocol on the interface, the routersends an Inverse ARP request for each DLCI. (unless statically mapped)

In effect, the Inverse ARP requestasks the remote station for itsLayer 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 theFrame Relay map.

12


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Inverse ARP

Inverse Address Resolution Protocol (Inverse ARP) wasdeveloped to provide a mechanism for dynamic DLCI to Layer 3address maps.

Inverse ARP works much the same way Address Resolution Protocol(ARP) works on a LAN.

However, with ARP, the device knows the Layer 3 IP address andneeds to know the remote data link MAC address.

With Inverse ARP, the router knows the Layer 2 address which is theDLCI, but needs to know the remote Layer 3 IP address.


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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 0Spokane(config-if)# ip address 172.16.1.1 255.255.255.0

Spokane(config-if)# encapsulation frame-relay

Frame RelayNetwork

HeadquartersHub City

Satellite Office 1Spokane

172.16.1.1172.16.1.2

DLCI 101 DLCI 102

Minimum Frame Relay Configuration


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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 therouter.

3. Inverse ARP is performed to map remote network layer addresses tothe local DLCI(s).

The routers can now ping each other!

Minimum Frame Relay Configuration

Frame RelayNetwork



172.16.1.1172.16.1.2

DLCI 101 DLCI 102


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HubCity# show frame-relay map

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

status defined, active

Frame Relay

NetworkHeadquarters

Hub CitySatellite Office 1

Spokane

172.16.1.1172.16.1.2

DLCI 101 DLCI 102

Inverse ARP

dynamicrefers 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.


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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. (Wewill 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, itupdates its DLCI-to-Layer 3 address mapping table.

Dynamic address mapping is enabled by default for all protocolsenabled on a physical interface.

If the Frame Relay environment supports LMI autosensing and InverseARP, dynamic address mapping takes place automatically.

Therefore, no static address mapping is required.

Frame Relay

NetworkHeadquarters

Hub City

Satellite Office 1

Spokane

172.16.1.1172.16.1.2

DLCI 101 DLCI 102


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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 addressmapping.

Once a static map for a given DLCI is configured, Inverse ARP isdisabled on that DLCI.

Thebroadcast keyword is commonly used with the frame-relaymap command.

Thebroadcast 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 mapprotocol protocol-addressdlci [broadcast] [ietf | cisco]

F


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FrameRelay Maps

Remote IPAddress

Local DLCIUses ciscoencapsulation forthis DLCI (not

needed, default)

By default,cisco is the

defaultencapsulation


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More on Frame Relay Encapsulation

If the Cisco encapsulation is configured on a serial interface, then bydefault, that encapsulation applies to all VCs on that serial interface.

If the equipment at the destination is Cisco and non-Cisco, configurethe Cisco encapsulation on the interface and selectively configure IETFencapsulation per DLCI, or vice versa.

These commands configure the Cisco Frame Relay encapsulation forall 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 unlessconfigured otherwise

Verifying Frame Relay interface


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Verifying Frame Relay interfaceconfiguration

The show interfaces serial command displays

information regarding the encapsulation and the status ofLayer 1 and Layer 2.

It also displays information about the multicast DLCI, theDLCIs used on the Frame Relay-configured serial

interface, and the DLCI used for the LMI signaling.


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show frame-relay pvc

The show frame-relay pvc command displays the status of eachconfigured connection, as well as traffic statistics.

This command is also useful for viewing the number of BackwardExplicit Congestion Notification (BECN) and Forward ExplicitCongestion Notification (FECN) packets received by the router.

The command show frame-relay pvc shows the status of allPVCs configured on the router.

If a single PVC is specified, only the status of that PVC is shown.


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show frame-relay map

The show frame-relay map command displays the current mapentries and information about the connections.


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Frame Relay Topologies


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Star Topology

A star topology, also known as a hub and spoke configuration, is themost popular Frame Relay network topology because it is the mostcost-effective.

In this topology, remote sites are connected to a central site thatgenerally provides a service or application.

This is the least expensive topology because it requires the fewestPVCs.

In this example, the central router provides a multipoint connection,because it is typically using a single interface to interconnect multiple

PVCs.


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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 reroutetraffic through site C.

As the number of nodes in the full mesh topology increases, thetopology 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 2810 45


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A Frame-Relay Configuration Supporting Multiple Sites

Frame RelayNetwork



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 knownas a Hub andSpokeTopology,where the Hubrouter relays

informationbetween theSpoke routers.

Limits thenumber of PVCsneeded as in afull-meshtopology(coming).

Hub Router

Spoke

Routers

HeadquartersHub CityConfiguration using Inverse


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HubCityinterface Serial0

ip address 172.16.1.2 255.255.255.0

encapsulation frame-relay

Spokaneinterface Serial0

ip address 172.16.1.1 255.255.255.0


Spokomo

interface Serial0

ip address 172.16.1.3 255.255.255.0


Frame Relay

Network

Hub City

Satellite Office 1

Spokane

Satellite Office 2

Spokomo

172.16.1.1 172.16.1.3

172.16.1.2

DLCI 101

DLCI 102

DLCI 112

DLCI 211

Configuration using InverseARP

HeadquartersHub CityConfiguration using Inverse


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Serial0 (up): ip 172.16.1.3 dlci 112, dynamic, broadcast,status defined, active

Spokane# show frame-relay map


Spokomo# show frame-relay map



Frame Relay

Network

Hub City

Satellite Office 1

Spokane

Satellite Office 2

Spokomo

172.16.1.1 172.16.1.3

172.16.1.2

DLCI 101

DLCI 102

DLCI 112

DLCI 211

Configuration using InverseARP


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Inverse ARP resolved the ip addresses for HubCity for bothSpokane 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?




Spokane# show frame-relay map


Spokomo# show frame-relay map



Configuration using Inverse ARP



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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 ICMPpackets because there is no DLCI-to-IP address mapping for thedestination address.

Solutions to the limitations of Inverse ARP

1. Add an additional PVC between Spokane and Spokomo (Full Mesh)

2. Configure Frame-Relay Map Statements

3. Configure Point-to-Point Subinterfaces.

Frame Relay

Network

y

Satellite Office 1

Spokane

Satellite Office 2

Spokomo

172.16.1.1 172.16.1.3

172.16.1.2

DLCI 101

DLCI 102

DLCI 112

DLCI 211


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Frame Relay Map Statements

Instead of using additional PVCs, Frame-Relay map statements can beused to:

Statically map local DLCIs to an unknown remote network layeraddresses. Also used when the remote router does not support Inverse ARP

Router(config-if)#frame-relay mapprotocol protocol-addressdlci [broadcast] [ietf | cisco]

b i


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Frame Relay

Network

Headquarters

Hub City

Satellite Office 1

Spokane

Satellite Office 2

Spokomo

172.16.1.1 172.16.1.3

172.16.1.2

DLCI 101

DLCI 102

DLCI 112

DLCI 211

HubCity

interface Serial0

ip address 172.16.1.2

255.255.255.0


(Inverse-ARP still works here)

Spokane

interface Serial0


255.255.255.0


frame-relay map ip 172.16.1.3 102frame-relay map ip 172.16.1.2 102

Spokomo

interface Serial0


255.255.255.0


frame-relay map ip 172.16.1.1 211


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.

HeadquartersH b CitMixing Inverse ARP and


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The previous configuration works fine and all routers can ping eachother.

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 Relay

Network

Hub City

Satellite Office 1

Spokane

Satellite Office 2

Spokomo

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 andFrame Relay Map Statements

Frame Relaymaps

Mixing Inverse ARP and Frame Relay Map


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HubCity

interface Serial0ip address 172.16.1.2

255.255.255.0


Spokane

interface Serial0


255.255.255.0



Spokomo

interface Serial0ip address 172.16.1.3

255.255.255.0



Frame Relay

Network

Headquarters

Hub City

Satellite Office 1

Spokane

Satellite Office 2

Spokomo

172.16.1.1 172.16.1.3

172.16.1.2

DLCI 101

DLCI 102

DLCI 112

DLCI 211

Mixing Inverse ARP and Frame Relay MapStatements



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HubCity#show frame-relay map

Serial0 (up): ip 172.16.1.1 dlci 101, 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




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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 MapStatementsHubCity#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 102, static, CISCO, status defined, active






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Frame-Relay Map Statement Rule:

When a Frame-Relay map statement is configured for a particularprotocol (IP, IPX, ) Inverse-ARP will be disabled for that specificprotocol, only for the DLCI referenced in the Frame-Relay mapstatement.

Mixing Inverse ARP and Frame Relay MapStatementsHubCity#show frame-relay map










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HubCity#show frame-relay map (after reload)

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

Serial0 (up): ip 172.16.1.3 dlci 112, dynamic, broadcast, status

defined, active


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


NOW MISSING: Serial0 (up): ip 172.16.1.2 dlci 211, dynamic,broadcast, status defined, active


defined, active




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HubCity#show frame-relay map (after reload)

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

Serial0 (up): ip 172.16.1.3 dlci 112, dynamic, broadcast, status

defined, active


Serial0 (up): ip 172.16.1.3 dlci 102, static, CISCO, statusdefined, active



defined, active

Spokane and Spokomo can no longer ping HubCity because they do nothave a dlci-to-IP mapping for the others IP address!


HubCityS


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Frame Relay

Network

Headquarters

Hub City

Satellite Office 1

Spokane

Satellite Office 2

Spokomo

172.16.1.1 172.16.1.3

172.16.1.2

DLCI 101

DLCI 102

DLCI 112

DLCI 211

HubCity

interface Serial0


255.255.255.0


(Inverse-ARP still works here)

Spokane

interface Serial0


255.255.255.0


frame-relay map ip 172.16.1.3 102frame-relay map ip 172.16.1.2 102

Spokomo

interface Serial0


255.255.255.0




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


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Reachability issueswith routing updates

An NBMA network is a multiaccess network, which means more thantwo 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 directlyconnected by a virtual circuit.

This means that Branch A cannot directly see the broadcasts fromBranch B, because they are connected using a hub and spoketopology.

Frame Relay is an NBMA Network

Reachability issues


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The Central router must receive the broadcast from Branch A and thensend 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 anddoes not scale well.

Split Horizon prohibits routingupdates received on an interfacefrom exiting that same interface.

Reachability issues


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Using a hub and spoke topology, the split horizon rule reduces thechance of a routing loop with distance vector routing protocols.

It prevents a routing update received on an interface from beingforwarded through the same interface.

If the Central router learns about Network X from Branch A, that updateis 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 routingupdates received on an interfacefrom exiting that same interface.

O S l ti Di bl S lit H i


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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-horizonRouter(config-if)#ip split-horizon

Another Solution for split horizon issue:


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Another Solution for split horizon issue:subinterfaces

To enable the forwarding of broadcast routing updates in a FrameRelay 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 apoint-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.


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A key reason for using subinterfaces is to allow distance vector routing

protocols to perform properly in an environment in which split horizon isactivated.

There are two types of Frame Relay subinterfaces. Point-to-point

multipoint

Mulitpoint

Point-to-point


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Physical interfaces: With a hub and spoke topology Split Horizon willprevent the hub router from propagating routes learned from one spokerouter to another spoke router.

Point-to-point subinterfaces: Each subinterface is on its own subnet.Broadcasts and Split Horizon not a problem because each point-to-point connection is its own subnet.

Multipoint subinterfaces: All participating subinterfaces would be inthe same subnet. Broadcasts and routing updates are also subject to

the Split Horizon Rule and may pose a problem.

Mulitpoint

Point-to-point

Configuring Frame Relay subinterfaces


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Subinterface can be configured after the physical interface has beenconfigured for Frame Relay encapsulation Subinterface numbers can be specified in interface configuration mode

or global configuration mode.

subinterface number can be between 1 and 4294967295.

At this point in the subinterface configuration, either configure astatic Frame Relay map oruse the frame-relay interface-dlci command.

The frame-relay interface-dlci command associates theselected 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



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The frame-relay interface-dlci command is required for allpoint-to-point subinterfaces.

It is also required for multipoint subinterfaces for which inverse

ARP is enabled.

It is not required for multipoint subinterfaces that are configuredwith static route maps.

It can not be used on physical interfaces.

Show frame relay map


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Show frame-relay map

Point-to-point subinterfaces are listed as a point-to-point dlci

Router#show frame-relay map

Serial0.1 (up): point-to-point dlci, dlci 301 (0xCB, 0x30B0),

broadcast status defined, active

With multipoint subinterfaces, they are listed as an inverse ARP entry,dynamic

Router#show frame-relay map

Serial0 (up): ip 172.30.2.1 dlci, 301 (0x12D, 0x48D0),

dynamic,, broadcast status defined, active

P i t

t i t S bi t f


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Point-to-point subinterfaces are like conventional point-to-point interfaces(PPP, ) and have no conceptof (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 physicalinterface and point-to-point subinterfaces subdivide each PVC as if itwere 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

P i t

t i t S bi t f


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With point-to-point subinterfaces you:

Cannot have multiple DLCIs associated with a single point-to-pointsubinterface

Cannot use frame-relay map statements Cannot use Inverse-ARP

Canuse theframe-relay interface dlcistatement (for bothpoint-to-pointandmultipoint)


Mulitpoint

Point-to-point

Point to point Subinterfaces


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172.30.1.0/24

172.30.2.0/24

172.30.3.0/24

Each subinterface is on a separatenetwork or subnet with a single remoterouter at the other end of the PVC.


S0 S1 S2

172.30.1.1/24 172.30.2.1/24 172.30.3.1/24


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Point-to-point subinterfaces are equivalent to using multiple physicalpoint to point interfaces.

S0 S1 S2

Site A Site B Site C

172.30.1.2/24 172.30.2.2/24 172.30.3.2/24

Point to point Subinterfaces


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A single subinterfaceis used to establish one PVC connection toanother 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 therouters are point-to-point and act like a leased line.




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Point-to-point subinterface configuration, minimum of two

commands:

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

Router(config-subif)# frame-relay interface-dlci dlci

Rules:1. No Frame-Relay map statements can be usedwith point-to-point

subinterfaces.

2. One and only one DLCIcan be associated with a single point-to-pointsubinterface

By the way, encapsulation is done only at the physical interface:interface Serial0

no ip address



Point-to-Point Subinterfaces at the Hubd S k

Each subinterface on Hub router requires aseparate subnet (or network)


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and Spokesseparate subnet (or network) Each subinterface on Hub router is treatedlike a regular physical point-to-pointinterface, so split horizon does not need tobe disabled.Interface Serial0 (for all routers)


no ip address

HubCity

interface Serial0.1point-to-point

ip address 172.16.1.1 255.255.255.0


frame-relay interface dlci 301


ip address 172.16.2.1 255.255.255.0



Spokane


ip address 172.16.1.2 255.255.255.0


Spokomo


ip address 172.16.2.2 255.255.255.0


Frame Relay

Network

Headquarters

Hub City

Satellite Office 1

Spokane

Satellite Office 2

Spokomo

Serial 0.1172.16.1.2/24

Serial 0.1172.16.2.2/24

Serial 0.1

172.16.1.1/24

DLCI 301

DLCI 103

DLCI 302

DLCI 203

Serial 0.2

172.16.2.1/24

Two subnets

Multipoint Subinterfaces


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Share many of the same characteristics as a physical Frame-Relay interface

With multipoint subinterface you can have:

can have multiple DLCIs assigned to it. can use frame-relay map & interface dlci statements can use Inverse-ARP

Remember, with point-to-point subinterfaces you: cannot have multiple DLCIs associated with a single point-to-point

subinterface

cannot use frame-relay map statements cannot use Inverse-ARP

(canuse theframe-relay interface dlcistatement for bothpoint-to-pointandmultipoint)

Mulitpoint

Point-to-point

Multipoint Subinterfaces

Multipoint subinterfaces


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Multipoint subinterfaces

172.30.1.0/24

172.30.2.0/24

172.30.3.0/24

Each subinterface is on a separatenetwork or subnet but may havemultiple connections, with a differentDLCI for each connection.

Split horizon still an issue on each Multipointsubinterface.

172 30 1 1/24 172 30 2 1/24 172 30 3 1/24


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Multipoint subinterfaces are equivalent to using multiple physical hubto spoke interfaces.

S0 S1 S2

Site A1

Site B1

Site C2

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

172.30.3.3/24

Site A2

172.30.1.3/24

Site B2

172.30.2.3/24

Site C1

172.30.3.2/24

NotesMultipoint subinterface at the Hub and

Point-to-Point Subinterfaces at the


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Frame RelayNetwork

Headquarters

Hub City


Satellite Office 2Spokomo

Serial 0

172.16.3.1

Serial 0

172.16.3.2

Serial 0

172.16.3.3

DLCI 301

DLCI 103

DLCI 302

DLCI 203

Highly scalable solution Disable Split Horizon on Hub router whenrunning a distance vector routing protocol

Interface Serial0 (for all routers)


no ip address

HubCity

interface Serial0.1mulitpoint

ip address 172.16.3.3 255.255.255.0

frame-relay interface-dlci 301

frame-relay interface-dlci 302no ip split-horizon

Spokane


ip address 172.16.3.1 255.255.255.0


Spokomo


ip address 172.16.3.2 255.255.255.0


Spokes

One subnet

FrameRelay 1

Documents

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