Frame Relay reduces network costs by using less equipment, less complexity, and an easier implementation. Moreover, Frame Relay provides greater bandwidth, reliability, and resiliency than private or leased lines. With increasing globalization and the growth of one-to-many branch office topologies, Frame Relay offers simpler network architecture and lower cost of ownership.
primarily because it is inexpensive compared to dedicated lines. In addition, configuring user equipment in a Frame Relay network is very simple. Frame Relay connections are created by configuring CPE routers or other devices to communicate with a service provider Frame Relay switch.
Virtual Circuits
The connection through a Frame Relay network between two DTEs is called a virtual circuit (VC). The circuits are virtual because there is no direct electrical connection from end to end. The connection is logical, and data moves from end to end, without a direct electrical circuit.
There are two ways to establish VCs:
• SVCs, switched virtual circuits, are established dynamically by sending signaling messages to the network (CALL SETUP, DATA TRANSFER, IDLE, CALL TERMINATION).
• PVCs, permanent virtual circuits, are preconfigured by the carrier, and after they are set up, only operate in DATA TRANSFER and IDLE modes. Note that some publications refer to PVCs as private VCs.
Multiple VCs
Frame Relay is statistically multiplexed, meaning that it transmits only one frame at a time, but that many logical connections can co-exist on a single physical line. The Frame Relay Access Device (FRAD) or router connected to the Frame Relay network may have multiple VCs connecting it to various endpoints. Multiple VCs on a single physical line are distinguished because each VC has its own DLCI.
Star topology
In a star topology, the location of the hub is usually chosen by the lowest leased-line cost.
FR Star
The lines going out from the cloud represent the connections from the Frame Relay service provider and terminate at the customer premises.
Full Mesh Topology
Full mesh topology using dedicated lines. A full mesh topology suits a situation in which the services to be accessed are geographically dispersed and highly reliable access to them is required. A full mesh topology connects every site to every other site. Using leased-line interconnections, additional serial interfaces and lines add costs.
FR Full Mesh
Using Frame Relay, a network designer can build multiple connections simply by configuring additional VCs on each existing link. This software upgrade grows the star topology to a full mesh topology without the expense of additional hardware or dedicated lines. Multiple VCs on an access link generally make better use of Frame Relay than single VCs.
Inverse ARP
The Inverse Address Resolution Protocol, also called Inverse ARP, obtains Layer 3 addresses of other stations from Layer 2 addresses, such as the DLCI in Frame Relay networks. It is primarily used in Frame Relay and ATM networks, where Layer 2 addresses of VCs are sometimes obtained from Layer 2 signaling, and the corresponding Layer 3 addresses must be available before these VCs can be used.
Dynamic Mapping
Dynamic address mapping relies on Inverse ARP to resolve a next hop network protocol address to a local DLCI value. The Frame Relay router sends out Inverse ARP requests on its PVC to discover the protocol address of the remote device connected to the Frame Relay network.
Inverse ARP is enabled by default for all protocols enabled on the physical interface. Inverse ARP packets are not sent out for protocols that are not enabled on the interface.
Dynamic Inverse ARP relies on the presence of a direct point-to-point connection between two ends.
Static Mapping
The user can choose to override dynamic Inverse ARP mapping by supplying a manual static mapping for the next hop protocol address to a local DLCI. A static map works similarly to dynamic Inverse ARP by associating a specified next hop protocol address to a local Frame Relay DLCI
Local Management Interface (LMI)
The LMI is a keepalive mechanism that provides status information about Frame Relay connections between the router (DTE) and the Frame Relay switch (DCE). Every 10 seconds or so, the end device polls the network, either requesting a dumb sequenced response or channel status information. If the network does not respond with the requested information, the user device may consider the connection to be down.
Three types of LMIs are supported by Cisco routers:
• Cisco – Original LMI extension
• Ansi – Corresponding to the ANSI standard T1.617 Annex D
• q933a – Corresponding to the ITU standard Q933 Annex A
Using the Broadcast Keyword
Frame Relay, ATM, and X.25 are nonbroadcast multiaccess (NBMA) networks. NBMA networks allow only data transfer from one computer to another over a VC or across a switching device. NBMA networks do not support multicast or broadcast traffic, so a single packet cannot reach all destinations.
Because NBMA does not support broadcast traffic, using the broadcast keyword is a simplified way to forward routing updates. The broadcast keyword allows broadcasts and multicasts over the PVC and, in effect, turns the broadcast into a unicast so that the other node gets the routing updates.
Split Horizon
By default, a Frame Relay network provides NBMA connectivity between remote sites. NBMA clouds usually use a hub-and-spoke topology.
split horizon is a technique used to prevent a routing loop in networks using distance vector routing protocols. Split horizon updates reduce routing loops by preventing a routing update received on one interface to be forwarded out the same interface.
Problem: Broadcast traffic must be replicated for each active connection.
Frame Relay Subinterfaces
Frame Relay can partition a physical interface into multiple virtual interfaces called subinterfaces. A subinterface is simply a logical interface that is directly associated with a physical interface.
A partially meshed network can be divided into a number of smaller, fully meshed, point-to-point networks.
Frame Relay subinterfaces can be configured in either point-to-point or multipoint mode:
• Point-to-point – A single point-to-point subinterface establishes one PVC connection to another physical interface or subinterface on a remote router. In this case, each pair of the point-to-point routers is on its own subnet, and each point-to-point subinterface has a single DLCI. In a point-to-point environment, each subinterface is acting like a point-to-point interface. Typically, there is a separate subnet for each point-to-point VC. Therefore, routing update traffic is not subject to the split horizon rule.
• Multipoint – A single multipoint subinterface establishes multiple PVC connections to multiple physical interfaces or subinterfaces on remote routers. All the participating interfaces are in the same subnet. The subinterface acts like an NBMA Frame Relay interface, so routing update traffic is subject to the split horizon rule. Typically, all multipoint VCs belong to the same subnet.
In a subinterface configuration, each VC can be configured as a point-to-point connection. This allows each sub interface to act similarly to a leased line.
A great advantage of Frame Relay is that any network capacity that is being unused is made available or shared with all customers, usually at no extra charge.
Bursting
A great advantage of Frame Relay is that any network capacity that is being unused is made available or shared with all customers, usually at no extra charge.
Bursting allows devices that temporarily need additional bandwidth to borrow it at no extra cost from other devices not using it.
Various terms are used to describe burst rates including the Committed Burst Information Rate (CBIR) and Excess Burst (BE) size.
The CBIR is a negotiated rate above the CIR which the customer can use to transmit for short burst. It allows traffic to burst to higher speeds, as available network bandwidth permits.
The BE is the term used to describe the bandwidth available above the CBIR up to the access rate of the link. Unlike the CBIR, it is not negotiated. Frames may be transmitted at this level but will most likely be dropped.
Frame Relay reduces network overhead by implementing simple congestion-notification mechanisms rather than explicit, per-VC flow control. These congestion-notification mechanisms are the Forward Explicit Congestion Notification (FECN) and the Backward Explicit Congestion Notification (BECN).
FECN and BECN are each controlled by a single bit contained in the frame header. They let the router know that there is congestion and that the router should stop transmission until the condition is reversed.
BECN is a direct notification. FECN is an indirect one.
The frame header also contains a Discard Eligibility (DE) bit, which identifies less important traffic that can be dropped during periods of congestion.
In periods of congestion, the provider’s Frame Relay switch applies the following logic rules to each incoming frame based on whether the CIR is exceeded:
• If the incoming frame does not exceed the CIR, the frame is passed.
• If an incoming frame exceeds the CIR, it is marked DE.
• If an incoming frame exceeds the CIR plus the BE, it is discarded.
show frame-relay lmi command. In the output, look for any non-zero “Invalid” items. This helps isolate the problem to a Frame Relay communications issue between the carrier’s switch and your router.
Use the show frame-relay pvc [interfaceinterface] [dlci] command to view PVC and traffic statistics. This command is also useful for viewing the number of BECN and FECN packets received by the router.
The show frame-relay pvc command displays the status of all the PVCs configured on the router. You can also specify a particular PVC.
Use the debug frame-relay lmi command to determine whether the router and the Frame Relay switch are sending and receiving LMI packets properly.
These states are active state, inactive state, and deleted state
• ACTIVE States indicates a successful end-to-end (DTE to DTE) circuit.
• INACTIVE State indicates a successful connection to the switch (DTE to DCE) without a DTE detected on the other end of the PVC. This can occur due to residual or incorrect configuration on the switch.
• DELETED State indicates that the DTE is configured for a DLCI the switch does not recognize as valid for that interface.
short defined
.DLCI – Data Link Connection Identifier
VCs are identified by DLCIs, and the DLCI values are assigned by the Frame Relay service provider.
Frame Relay DLCIs have local significance and no significance beyond the single link.
A DLCI identifies a VC to the equipment at an endpoint.
.LMI – Local Management Interface
LMI is a keepalive mechanism that provides status information about Frame Relay connections between the router (DTE) and the Frame Relay switch (DCE).
.Three types of LMIs are supported by Cisco routers: Cisco, ANSI, and q933a.
.Inverse ARP
Inverse Address Resolution Protocol (ARP) obtains Layer 3 addresses of other stations from Layer 2 addresses, such as the DLCI in Frame Relay networks (which is the reverse of what ARP does).
It is primarily used in Frame Relay and ATM networks, where Layer 2 addresses of VCs are sometimes obtained from Layer 2 signaling, and the corresponding Layer 3 addresses must be available before these VCs can be used.
.Access rate (or port speed)
The capacity of the local loop.
This line is charged based on the port speed between the DTE to the DCE (customer to service provider).
.CIR – Committed Information Rate
The capacity through the local loop guaranteed by the provider.
Customers normally choose a CIR lower than the access rate to allow them to take advantage of bursts.
.CBIR – Committed Burst Information Rate
Negotiated maximum which a frame is allowed to burst above the CIR.
.Frames are marked as discard eligible (DE).
It cannot exceed the access rate of the link.
.BE – Excess Burst
Amount of data above the CBIR up to the access rate which frames may use to burst with no guarantee.
Frames are also marked as discard eligible (DE) and cannot exceed the access rate of the link.
