Transcript S6-C7 – Frame Relay

S6-C7 – Frame Relay
Son of X.25
Frame Relay Facts
• Replaced X.25 as the packet-switching technology
of choice
• Frame Relay streamlines Layer 2 functions and
provides only basic error-checking
• Provides efficiency
• ITU-T and ANSI standard that defines the process
for sending data over a public-switched network
• Appropriate for uses that require high throughput,
such as LAN interconnection network (PSN)
• Does not define the way the data are transmitted
Frame Relay Devices
• data terminal equipment (DTE)
– typically are located on the premises of a customer
– routers and Frame Relay Access Devices (FRADs)
• data circuit-terminating equipment (DCE)
– carrier-owned internetworking devices
– provide clocking and switching services in a network
– PACKET switched in most cases
Frame Relay Concepts
• Multiplex several logical data conversations over a
single physical transmission link
• Provides tremendous cost-effectiveness
– one site can connect to many geographically distant
sites using a single T1 (and single CSU/DSU) to the
local CO
• Supports both permanent virtual circuits (PVCs)
and switched virtual circuits (SVCs)
– SVCs require call setup and termination for each
connection.
• Each end of the virtual circuit is assigned a
connection identifier – a DLCI number
DLCI
• Layer 3 addresses must be mapped to DLCI numbers
– Router encapsulates the IP packet with a Frame Relay
frame which contains the appropriate DLCI number to
reach the destination
• Cisco routers support two types of Frame Relay headers:
– a 4-byte header (cisco) -- DEFAULT
– A 2-byte header (ietf) that conforms to the IETF
standard
• Statistical multiplexing
– dynamically allocates bandwidth to active channels.
– Frame Relay does not operate at Layer 3; Multiplexing
is achieved at Layer 2, using a DLCI field
DLCI Numbers
• DLCIs 0 to 15 and 1008 to 1023 are reserved for
special purposes.
• Service providers typically assign DLCIs in the
range of 16 to 1007.
• Multicasts use DLCI 1019 and 1020.
• Local Management Interface (LMI) uses DLCI
1023 or 0.
• Cisco LMI type uses DLCI 1023 and
• ANSI/ITU-T LMI type uses DLCI 0. DLCI 0 is
also used by all Q.933 call control information
transmissions to setup, monitor, and terminate
SVCs.
Local Management Interface
LMI
• Signaling standard between the DTE and the
Frame Relay switch.
• Responsible for managing the connection and
maintaining the status between devices
• Responsible for:
–
–
–
–
Keepalive mechanism
Status mechanism
Multicast mechanism
Global addressing
• >= 11.2 uses autosensing to detect type
PVC States
• Active state - indicates that the connection is
active and that routers can exchange data
• Inactive state - indicates that the local
connection to the Frame Relay switch is working,
but the remote router connection to the Frame
Relay switch is not working
• Deleted state - indicates that no LMI is being
received from the Frame Relay switch, or that
there is no service between the CPE router and
Frame Relay switch
Inverse Arp
Default for Frame Relay
• Developed to provide a mechanism for dynamic
DLCI to Layer 3 address maps
– Static Mapping is Administratively time consuming and
can’t adapt to changes in the topology
– Once the router learns from the switch about available
PVCs and their corresponding DLCIs, the router can
send an Inverse ARP request to the other end of the
PVC
• asks the remote station for its Layer 3 address, while at the
same time providing the remote system with the local system's
Layer 3 address
Frame Relay Configuration
• (config-if)
– Encapsulation frame-relay [cisco|ietf]
– Frame-relay lmi-type [ansi| cisco| q933a]
– Frame-relay map protocol protocol-address dlci
[broadcast] [ietf | cisco]
– Frame-relay map IP 10.1.1.2 broadcast 100 cisco
• Broadcast command provides two functions
– forwards broadcasts when multicasting is not enabled
– simplifies the configuration of OSPF for non-broadcast
networks that will use Frame Relay
• OSPF will automatically use the Frame Relay network as a
broadcast network.
Frame-Relay Encapsulation
• (config-if)
– Frame-relay map IP 10.1.1.3 16 broadcast
– Frame-relay map IP 10.2.2.4 17 broadcast ietf
– Frame-relay map IP 10.2.3.3 18 broadcast
Show Commands
• show interfaces serial
– displays information regarding the encapsulation and
the status of Layer 1 and Layer 2
– displays information about the multicast DLCI
• show frame-relay pvc
– displays the status of each configured connection
– shows traffic statistics
• show frame-relay map
– Displays the current map entries
– Gives information about the connections
• show frame-relay lmi
– displays LMI traffic statistics
Frame Relay Topologies
• star topology aka a hub-and-spoke
– most popular Frame Relay network topology because it
is the most cost-effective
• full-mesh topology
– all routers have PVCs to all other destinations.
– costly, but provides direct connections from each site to
all other sites
– allows for redundancy
• partial-mesh topology
– not all sites have direct access to each other.
• compensates for Frame Relay's Non-broadcast Multiaccess
(NBMA)
• deals with routing issues with split horizon
Frame Relay Routing Issues
• Frame Relay network provides NBMA
(Nonbroadcast Multiaccess) connectivity between
remote sites (DEFAULT)
• In a non-broadcast network such as Frame Relay,
nodes cannot see each other's broadcasts unless
they are directly connected via a virtual circuit
• There are problems in this scenario dealing with
routing protocols because of the split horizon rule
– split horizon for IP is automatically turned off when
encapsulation is frame-relay
– Split horizon for IP is enabled for subinterfaces
More Frame Relay Routing Issues
• When multiple DLCIs terminate in a single
interface, the router must replicate routing updates
and service advertisements for each PV
– updates can consume access-link bandwidth
– Updates can cause significant latency variations in user
traffic
– Updates can consume interface buffers and lead to
higher packet-rate loss for both user data and routing
updates
– Overhead traffic, such as routing updates, can affect the
delivery of critical user data, especially when the local
loop contains low-bandwidth (56-kbps)
Subinterfaces
• Logical subdivisions of a physical interface.
– In split-horizon routing environments, routing updates
received on one subinterface can be sent out on another
subinterface.
– Allow distance-vector routing protocols to perform
properly in an environment in which split horizon is
activated
• Point to point - A single subinterface is used to establish one
PVC connection – its own subnet
• Multipoint - A single subinterface is used to establish multiple
PVC connections – all on same subnet – each subinterface has
its own DLCI