CCIE chapter 15: frame relay:
Resources used:
Cisco press, CCIE Practical Studies, Volume I
Cisco press, BCRAN Exam certification guide, Second edition
CCIE Routing and Switching Official Exam Certification Guide ,Second Edition
PVC, permanent virtual connection, logical end-to-end connection, pvc end point addressed with a DLCI
Data-link connection identifier (DLCI) – logical number 16 to 1007, DLCI Is only locally significant. 0 to 15 are reserved, as are 1008 to 1023
DTE devices are generally considered to be terminating equipment for a specific network and are located at the customer premises.
DCE devices are carrier-owned internetworking devices. DCE provides clocking and switching services in a network
Local Management interface (LMI) – signaling standard used between routers and frame relay switch. LMI is used to lean PVC’s and does keepalive of 10 sec apart. There are 3 types of LMI:
CISCO- default, if autonegotiation fails. LMI status information is sent on DLCI 0.
ANSI - LMI type defined by ANSI standard T1.617, most common, LMI status information is sent on DLCI 1023
Q933a - LMI type defined as ITU-T Q.933, or simply Annex A. LMI status
information is sent on DLCI 0.
Frame Relay Headers
The Link Access Procedure/Protocol for Frame Mode Services (Q.922) LAPF header includes all the fields used by Frame Relay switches to deliver frames across the FR cloud, including the DLCI, DE, BECN, and FECN fields.
The Frame Relay encapsulation header follows the LAPF header, holding fields that are important only to the DTEs on the ends of a VC
Committed burst (Bc)— The number of bits committed to accept and transmit at
the CIR.
Committed Information Rate (CIR) The maximum permitted traffic level per PVC.
When this is exceeded, the Discard Eligible (DE) bit is set. The DE bit can be used as
an indication to the frame carrier that it can drop that frame if capacity between
Frame Relay switches is reached. This value is expressed in bits per second.
Excess burst (Be)— The number of bits to transmit after the committed burst value
is reached.
MinCIR (Minimum CIR) The minimum amount of data to send during periods of
congestion. This is usually what you get from the telco. MinCIR defaults to one-half of CIR.
PIR (Peak Information Rate) The highest possible rate of transmission on a given
interface.
MIR (Minimum Information Rate) The slowest rate of transmission on a given interface.
Interval Bc / CIR. The maximum is 125 ms, or 1/8 second.
When a Frame Relay link becomes active on a Cisco DTE device, it sends three LMI messages in rapid succession. The order is ANSI, ITU, and then Cisco. The router listens on DLCI 1023 for Cisco LMI and DLCI 0 for ANSI and ITU.
The Frame switch responds with the LMI with which it is configured, received. If multiple types of LMI are received, the last one received is used.
On every sixth LMI status request(keepalive), the DTE device sends a full status request. This request serves as another keepalive and requests the frame switch to respond with a
list of all DLCIs that have been defined for that link.
For each active DLCI, the router sends an Inverse ARP request per Layer 3 protocol
configured on that interface. If IP and IPX are configured, for example, the router
sends two Inverse ARP requests. If IARP isn’t supported the DLCI to ip address info must be manually added.
show frame-relay pvc command:
- ACTIVE, Indicates that the PVC is active and that information can be
Exchanged.
- INACTIVE, Indicates that the local connection to the Frame switch is
working, but the remote router's connection to the frame switch is not working.
- DELETED, Indicates that no LMI is being received from the frame switch or
that the physical layer is still not established.
Encapsulation:
cisco is the default encapsulation type and should be used when connecting to another
Cisco device or an RFC 1490–compliant device.
ietf should be used when connecting to non-Cisco devices.
Configuring Frame Relay
router(config-if)#encapsulation frame-relay [cisco | ietf]
Router(config-if)#frame-relay lmi-type [ansi | cisco | q933i]
if point-to-point interface
Router(config-if)#frame-relay interface-dlci dlci_number
If point to multipoint interface
frame-relay map ip next_hop_address dlci [broadcast] [ietf | cisco]
the ietf cisco allows you to specify a encapsulation type on a per PVC basis.
So if point to multipoint is configured destinations must have frame-relay map ip for the other end points on the frame PVC.
frame-relay map ip 172.16.1.5 111 broadcast
frame-relay map ip 172.16.1.1 111 broadcast
Router(config-if)keepalive keepalive_interval_in_seconds— Sets the default 10-
second keepalive interval to another value. Be sure to use this command on both
sides of the link.
FECN and BECN
If a Frame Relay switch(carrier device) senses congestion in the network, it sets the FECN bit to 1 in the Frame Relay header for traffic moving toward the destination device.
the destination device sets the BECN bit in return traffic to the source. This
informs the source device of the congestion in the network and that it should reduce the transmission rate.
If the router receives BECNs during the current time interval, it decreases the transmission rate by 25 percent. The rate for every BECN drops until the min CIR is reach. If no BECN heard for 16 Tc then transmition rate increases every TC by 1/16th.
routers can be configured to watch for received frames with FECN set, reacting by returning a Q.922 test frame over that VC with the BECN bit set. This feature, sometimes called FECN reflection, is configured with the shape fecn-adapt (CB Shaping) or traffic-shape fecn-adapt (FRTS) command
Frame relay Traffic shaping:
marlin(config)#int serial 0
marlin(config-if)#frame-relay traffic-shaping Enable FRTS
marlin(config-if)#int s0.1
marlin(config-subif)#frame-relay class 64kb Set map class
marlin(config-subif)#exit
marlin(config)#int s0.2
marlin(config-subif)#frame-relay class t1 Set other map class
marlin(config)#map-class frame-relay 64kb
marlin(config-map-class)#frame-relay adaptive-shaping becn Enable BENC response
mode (drop transmit rate 25%)
marlin(config-map-class)#frame-relay cir 1544000 Set to physical port speed
marlin(config-map-class)#frame-relay bc 8000 set to remote port speed/8
marlin(config-map-class)#frame-relay be 64000 Initial burst
marlin(config-map-class)#frame-relay mincir 32000 Carrier enforced CIR
marlin(config-map-class)#exit
marlin(config)#map-class frame-relay t1
marlin(config-map-class)#frame-relay adaptive-shaping becn Enable BENC response
mode
marlin(config-map-class)#frame-relay cir 1544000 Set to physical port speed
marlin(config-map-class)#frame-relay bc 8000 set to remote port speed/8 (bytes)
marlin(config-map-class)#frame-relay be 64000 Initial burst
marlin(config-map-class)#frame-relay mincir 512000 Carrier enforced CIR
marlin#show traffic-shape
Best debug commands
show frame-relay pvc [dlci | interface]
show frame-relay lmi
show frame-relay map
debug frame-relay lmi
Frame Relay Payload Compression
Cisco IOS software supports three options for payload compression on Frame Relay VCs: packetby- packet, data-stream, and Frame Relay Forum Implementation Agreement 9 (FRF.9). FRF.9 is the only standardized protocol of the three options.
All three FR compression options use LZS as the compression algorithm, difference is in the use of compression dictionaries.
Does substitution of data for a small place holder ( the list of placeholders is called the dictionary)
FR payload compression configuration is configured per VC
On point-to-point subinterfaces, the frame-relay payload-compress type
frame-relay map ip 10.1.123.3 103 broadcast payload-compress data-stream stac
Frame Relay Fragmentation
All packets can be fragmented, but Cisco suggests choosing a fragment size so that the packets typically placed into the LLQ PQ will not be fragmented. Only packets from the shaping LLQ are placed into the Dual FIFO interface high queue, and only those
packets are interleaved.