CCNA4
– Chapter 3
* Cost Effectiveness of Frame Relay
 First, with Frame Relay, customers only pay for the local loop, and for the bandwidth they purchase from the network provider.

Distance between nodes is not important.
 with dedicated lines, customers pay for an end-to-end connection. That includes the local loop and the network link.

The second reason for Frame Relay's cost effectiveness is that it shares bandwidth across a larger base of customers.
 Typically, a network provider can service 40 or more 56 kb/s customers over one T1 circuit.
* Frame Relay has lower overhead than X.25 because
 Frame Relay does not provide error correction,

Frame Relay node simply drops packets without notification when it detects errors.
 Any necessary error correction, such as retransmission of data, is left to the endpoints.
 relies on the upper layers--such as TCP/IP--to perform error checking
 Frame Relay handles transmission errors through a standard Cyclic Redundancy
Check.

If a CRC (bit-level error checking) error is found, the packet is dropped
* DLCI

DLCI values typically are assigned by the Frame Relay service provider.
 Frame Relay DLCIs have local significance, which means that the values themselves are not unique in the Frame Relay WAN.

A DLCI identifies a VC to the equipment at an endpoint.
 A DLCI has no significance beyond the single link.
* Virtual circuit (VC)
 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.


With VCs, any single site can communicate with any other single site without using multiple dedicated physical lines.
* Before a router is able to transmit data over Frame Relay, it needs to know which local DLCI maps to the Layer 3 address of the remote destination.
 This address-to-DLCI mapping can be accomplished either by static or dynamic mapping

Dynamic Mapping (Inverse ARP)
 The Inverse Address Resolution Protocol (ARP) obtains Layer 3 addresses of other stations from Layer 2 addresses, such as the DLCI in
Frame Relay networks.
 LMI status messages combined with Inverse ARP messages allow a router to associate network layer and data link layer addresses.
 Static Mapping (Inverse ARP)

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.

You cannot use Inverse ARP and a map statement for the same DLCI and protocol.
* Frame Relay Address Mapping
 Before a router is able to transmit data over Frame Relay, it needs to know which local DLCI maps to the Layer 3 address of the remote destination.
 This address-to-DLCI mapping can be accomplished either by static or dynamic mapping.
* Configuring Basic Frame Relay: Configuring Encapsulation
 The encapsulation frame-relay interface configuration command enables Frame
Relay.
 The encapsulation command encapsulation frame-relay [cisco | ietf] command.
 The default encapsulation is Cisco version of HDLC.
 Use the IETF encapsulation type option if connecting to a non-Cisco router.

* Configuring Basic Frame Relay: Setting the LMI Type (optional)
– Cisco routers autosense the LMI type.
– Cisco supports three LMI types: Cisco (default) , ANSI, and Q933-A.
* Configuring Static Frame Relay Maps
 Setting the IP Address on the Interface
 Configuring Encapsulation

Setting the Bandwidth
 To map between a next hop protocol address and a DLCI destination address, use the frame-relay map protocol protocol-address dlci [broadcast] command.

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.
 You cannot use Inverse ARP and a map statement for the same DLCI and protocol.

You can greatly simplify the configuration for the OSPF protocol by adding the optional broadcast keyword when doing this task
* Configuring Static Frame Relay Maps
 If you have multiple locations, you need to list the frame-relay map for all the locations.

* Split Horizon
Split horizon updates reduce routing loops by preventing a routing update received on one interface to be forwarded out the same interface

Frame Relay can partition a physical interface into multiple virtual interfaces called subinterfaces. Frame Relay subinterfaces can be configured:
 Point-to-point: A single point-to-point subinterface establishes one PVC connection to another physical interface or subinterface on a remote router.

Each pair of the point-to-point routers is on its own subnet, and each point-to-point subinterface has a single DLCI.
 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.
* Configuring Frame Relay Subinterfaces

Frame Relay subinterfaces ensures that a physical interface is treated as multiple virtual interfaces to overcome split horizon rules.
 To create a subinterface, Specify the port number, followed by a period (.) and the subinterface number.
 R1(config-if)#interface serial 0/0/0.103 point-to-point
 You must also specify whether the interface is point-to-point or point-tomultipoint using either the multipoint or point-to-point keyword.

The DLCI is also required for multipoint subinterfaces for which Inverse ARP is enabled.
 R1(config-subif)#frame-relay interface-dlci 103.

* Frame Relay Flow Control
 The frame header also contains a Discard Eligibility (DE) bit, which identifies less important traffic that can be dropped during periods of congestion.

When the network is congested, DCE discard the frames with the DE bit set to 1.
 Frame Relay reduces network overhead by implementing congestion-notification mechanisms.
 Forward Explicit Congestion Notification (FECN)
 Backward Explicit Congestion Notification (BECN).
 BECN is a direct notification.
 FECN is an indirect one.
* show frame-relay map

Use the command to display the current map entries and information about the connections.
 The output shows the following information:

10.140.1.1 is the IP address of the remote router, dynamically learned via the Inverse ARP process .
 100 is the decimal value of the local DLCI number. .
* Verify Frame Relay Interfaces: show interfaces command
 The show interfaces command displays how the encapsulation is set up, along with useful Layer 1 and Layer 2 status information, including:
 LMI type
 LMI DLCI

Frame Relay DTE/DCE type

* Verify PVC status
 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 PVC status can be active, inactive, or deleted.