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– Local significance

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 InterfaceLMI

• 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 ArpDefault 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