2. Chapter 6
Frame Relay is a connection oriented, standard NBMA layer
2 WAN protocol
Frame relay is a Multi-access WAN connection service
Frame relay allows one router to access more than one router
using a single physical link (access link)
Connection in frame relay are provided by virtual circuits.
Virtual circuits are multiple logical connection on same
physical connection.
4. Chapter 6
Frame Relay virtual connection types
PVC
SVC
PVC(Permanent virtual connection)
Permanent connection is used.
When constant data has to be sent to a particular
destination.
Always use the same path.
SVC (switched virtual connection)
Virtual connection is dynamically built when data has to
be send and torn down after use.
5. Chapter 6
Using a virtual connection (virtual circuit) to each site, the
router will be able to communicate with sites over a single
link
A virtual connection to a site has an address known as Data
Link Connection Identifier (DLCI)
When the router sends to a site, it tags packets with the
proper DLCI address which represents the connection to
this site
7. Chapter 6
Frame Relay Operation
The connection between a DTE device and a DCE device
consists of both a physical layer component and a link layer
component:
The physical component defines the mechanical, electrical,
functional between the devices.
The link layer component defines the protocol that establishes
the connection between the DTE device (router), and the DCE
device (switch).
9. Chapter 6
Packet Switching Across Frame Relay Network
Frame relay switches at the service provider network are
configured to forward received frames with certain DLCIs to
the appropriate output interface with its DLCIs
10. Chapter 6
Frame Relay Encapsulation
Frame Relay takes data packets from a network layer
protocol, such as IP or IPX, encapsulates them as the data
portion of a Frame Relay frame, and then passes the frame
to the physical layer for delivery on the wire.
11. Chapter 6
There are two types of frame relay encapsulations
CISCO (default and cisco proprietary)
IETF (when different vendor routers are used)
RouterA (config-if)#encapsulation frame-relay ?
IETF
<cr>
12. Chapter 6
DLCI (data link connection identifier):
Address of virtual connections
every VC there is one DLCI number.
Inverse ARP (address resolution protocol) is used to map
local DLCI to a remote IP
Unique identifier <16-1007>
Router(config-if)#frame –relay interface –dlci 18
14. Chapter 6
Local Management Interface (LMI)
LMI allows DTE(router) to send status messages to DCE(frame
relay switch) to exchange status information about the virtual
circuits devices.
LMI is used between the router and its frame relay switch to
discover each other using keepalive messages
16. Chapter 6
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
Static Mapping
To map between a next hop protocol address and DLCI
destination address
You want to control broadcast traffic across a PVC
You want to have different Frame Relay encapsulations across
18. Chapter 6
Dynamic mapping
Frame relay map is created dynamically using Inverse-
arp protocol
Inverse-arp operates after receiving LMI PVC status
response messages that explain PVCs, their status, and
DLCIs
When the router receives a PVC active with its DLCI, it
creates an Inverse-arp message contains its IP address to
be sent over this PVC
The router can then map the IP address in the message
with its local DLCI
20. Chapter 6
Split horizon can cause problems in NBMA environments.
Solution: subinterfaces
A single physical interface simulates multiple logical interfaces
21. Chapter 6
Configuring Frame Relay Subinterfaces
Point-to-point
Subinterfaces act like leased lines.
Each point-to-point subinterface requires its own subnet.
Point-to-point is applicable to hub-and-spoke topologies.
Multipoint
Subinterfaces act like NBMA networks, so they do not
resolve the split-horizon issues.
Multipoint can save address space because it uses a single
subnet.
Multipoint is applicable to partial-mesh and full-mesh
topologies
24. Chapter 6
Committed information rate(CIR)
Is the line speed
CIR: Lower speed guaranteed for a certain time.
Congestion :
indicates traffic problem in the path when more packets are
transmitted in one direction
congestion notifications
1.FECN 2.BECN 3.DE
DE
The descent speed of receiving and sending to CIR
25. Chapter 6
FECN
Indicates congestion as frame goes from source to destination
Used this value inside frame relay frame header in forward
direction
FECN=0 indicates no congestion
BECN
Used by the destination(and send to source) to indicate that there
is congestion
Used this value inside frame relay frame header in backward
direction
BECN=0 indicates no congestion