Frame Relay
CCNA4 Chapter 6
Frame Relay Overview
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Layer 2
Connection-oriented
Packet switched
Virtual Circuits:
Permanent (and Switched)
Links two DTE devices through DCE
Originally designed for operation over ISDN
Faster than X.25
Encapsulation can be Cisco or IETF
RouterA(config-if)#encapsulation frame-relay ?
ietf Use RFC1490 encapsulation
Frame Relay Operation
Frame Relay - DLCI
• Data Link Connection Identifier
• IP addresses must be resolved to DLCI
• Logical Virtual Circuits are distinguished by
DLCI
• Only locally significant
• DLCI do not need to match end to end,
though the may do if connected to different
switches
• DLCI are assigned by the service provider.
• FR switch maps DLCI numbers to outgoing
ports
DLCI
A
DLCI=16
B
DLCI=100
DLCI=101
C
DLCI=100
E
DLCI=101
DLCI=102
D
DLCI=100
DLCI=201
DLCI Mapping
• The FR switch uses DLCI numbers to communicate
• Network layer addresses must be mapped
Static mapping:
RouterA(config-if)#frame-relay map ip 172.16.30.19 18
Dynamic mapping uses Inverse ARP:
1. Router learns DLCI from
switch during initial LMI
exchange
2. Router sends IARP
request for each DLCI
DLCI = 100
What’s the next hop address for DLCI 100?
3. The return information is
then used to build the map
My IP address:172.16.1.2
Frame Relay Rates
Network performance at the router level is affected by:
Access Rate
Line speed of connection to the provider
CIR
Guaranteed bandwidth
Bursting
Faster than CIR for a short time period
(Tc) Committed Rate measurement interval
(Bc) Committed Burst
(Be) Excess Burst – maximum transfer attempted
DE = Discard Eligibility bit. Set on oversubscribed traffic
LMI - Local Management Interface
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Signalling between router and frame switch
Determine which PVCs are available and their status
LMI DLCI=1023
Extensions to basic frame relay protocol:
Common: Status of virtual circuits (Keepalives)
Optional: Multicasting (DLCI 1019-1022)
Global Addressing
Simple Flow Control
• Cisco IOS 11.2 and later uses auto-detection
• Cisco routers support three versions of LMI:
RouterA(config-if)#frame-relay lmi-type ?
cisco
ansi
q933a
Frames
Frame Relay Format
10 bit address (0-1023)
8th bit of each byte indicates
address length (currently 2 bytes)
LMI Frame
Forward Explicit Congestion Notification
Backward Explicit Congestion Notification
Discard Eligibility
Mandatory bytes
1023
Status-enquiry message (from user device)
Status message:keepalives and PVC status
Frame Relay Subinterfaces
FR can divide a single physical
WAN interface into multiple
subinterfaces.
Subinterfaces are logical
subdivisions of the physical
interface.
Split-Horizon issues can be
resolved.
RouterA(config)#int s0
RouterA(config-if)#encapsulation frame-relay
RouterA(config-if)#int s0.1 ?
multipoint Treat as a multipoint link
point-to-point Treat as a point-to-point link
Point to Point is used when each PVC is a separate subnet.
Multipoint is used when the PVCs use the same subnet.
Frame Relay Subinterfaces
Configure the local DLCI for the subinterface to distinguish it from
the physical interface:
RouterA(config-if)#frame-relay interface-dlci ?
<16-1007> Define a DLCI as part of the current
subinterface
RouterA(config-if)#frame-relay interface-dlci 16
Example configuration:
RouterA(config)#interface Serial0
RouterA(config-if)# no ip address
RouterA(config-if)# encapsulation frame-relay
RouterA(config-if)# interface Serial0.102 point-to-point
RouterA(config-if)# ip address 10.1.12.1 255.255.255.0
RouterA(config-if)# frame-relay interface-dlci 102
Verifying Frame Relay
Command
show interface s0
Description
Encapsulation, Layer 1 and 2 status,
DLCIs in use on the interface, LMI DLCI
and LMI type.
The status of each configured connection
show frame-relay pvc and traffic statistics,DLCI, BECN, FECN,
DE
Displays the interface, network layer
show frame-relay map
address and DLCI mapping
Statistics for the interface including LMI
show frame-relay lmi type and status enquiries sent and
recieved
Questions
Q- What is the first step in configuring Frame-Relay on an
interface?
A- Setting the encapsulation
Q- What are the possible types of LMI?
A- Cisco, ANSI and Q.933a
Q- What is the solution to the issue of split-horizon when
configuring multiple VCs on a single physical interface?
A- Sub-interfaces
Q- What command can you issue to see your PVCs?
A- show frame-relay pvc
Q- What three bits does frame-relay use for congestion control?
A- FECN, BECN, DE
Q- What DLCI value defines an LMI frame?
A- 1023