notes: Frame Relay Operation

Frame-relay is a packet-switching technology commonly implemented as an encapsulation technique, used between LANs over a wide area network (WAN).

- Virtual Circuit (VC) logical communication path betwe 2 or more routers, VC can be permanent (PVC) or switched (SVC).


DLCI (DataLink Connection Identifiers)

- identifies the PVC going to peer.
- DLCI's are used as a frame-relay address, which identifies the VC over which frames should travel in a frame-relay cloud.
- It is contained within a 10-bit field inside the frame-relay header.
- DLCI's are locally significant to a link and can change as it passes through the network.
- L2 addressing

4 DLCI status:

1. Active - operational. both sides are communicating.

2. Inactive - something on the other end is not working ( but your local connection to FR is ok.)

3. Deleted - during LMI enquiry. i.e you have dlci 222 but in the FR cloud or switch that dlci didnt exists.

4. static - Indicates that LMI was turned off with the "no keepalives".

LMI (Local Management Interface)

- LMI communication is dependent on keepalive.
- LMI messages manage the communication between the DCE (frame-relay switch) and the DTE (a router).
- A DTE sends LMI status inquiry messages to the DCE.
- The DCE responds with LMI status messages to inform the DTE (router) about the DLCIs and status of each VC.
- These inquiry/status messages functions as, and are referred to as LMI keepalives too.
- 10 seconds default keepalive
- LMI can enabled/disabled by using the keepalive/no keepalive commands.
- LMI holdtime is 3x keepalives. LMI holtime cannot be adjusted directly, but only by changing the keepalive interval times three.
- If 3 keepalives (default) are missed an interface will be considered down.
- There are three LMI types: Cisco/ANSI/q933a.
- LMI autosense is enabled by default, which determines the LMI type to be used.
- LMI messages/keepalives will inform the router of all of the DLCIs in use, but will not give any information as to what DLCI is associated with what interfaces/sub-interface.
- The command "encapsulation frame-relay" enables LMI automatically.


CCIE LAB SCENARIO: configure routers such that the LMI status enquiries are sent every 5 seconds and full status lmi request are sent every 3 cycles instead of 6

config-set:

int s0/0
keepalive 5
frame-relay lmi-n391 dte 3







LMI Keepalives and Full Status Update

- default, LMI keepalives are sent every 10 seconds.
- Keepalives must match, to prevent flapping interfaces.
- If LMI autosense is unsuccessful, an intelligent retry scheme is built in.
- Every N391 interval (default is 60 seconds, which is 6 keepalives at 10 seconds each), LMI autosense will attempt to ascertain the LMI type and request a complete status info about each VC. This is also known as full status update.
- If required to change the full status update timers, change the N391 interval to how often a full update should be requested.

Lab Scenario: If a router should request a full update once every 180 sec, (180sec / 10 sec keepalive = 18), thus only request an update every 18th keepalive.

interface command:

frame lmi-n391dte 18


2 types of FR encapsulation:

1. Cisco - enabled by default, is used if both DTE are cisco devices

2. IETF - is for multivendor environment.

config-set encapsulation per interface and per dlci

interface s1/0
encapsulation frame-relay ietf
frame-relay map ip 131.108.123.2 48 broadcast
frame-relay map ip 131.108.123.3 49 broadcast cisco
!
interface s1/1
encapsulation frame-relay
frame-relay map ip 131.108.143.2 58 broadcast ietf
frame-relay map ip 131.108.143.3 59 broadcast


- FECN, BECN and DE

- FECN (Forward Explicit Congestion Notification) and BECN (Backward Explicit Congestion Notification) are set in the LAPF header to signal congestion on a particular PVC.
- When a frame-relay switch notices congestion on a PVC, the switch will set the FECN bit indicating congestion in that direction.
- A router or switch noticing the FECN, will set the BECN bit on traffic returning to the source, to indicate congestion and possible instruct the source to slow down transmission.
- The DE (Discard Eligibility) is used to indicate traffic that are in violation of the conformed rate, might be subject to discarding during periods of congestion. Frames marked with DE bit will be dropped

Broadcast Queue

- With large frame-relay networks huge amounts of DLCI updates can consume bandwidth, interface buffers and even cause packet loss.
- To avoid such problems, you can create a special broadcast queue on an interface, to use its own queue and buffers.

frame broadcast-queue {Q-size} {Bps} {packet-rate}

- CDP is enabled by default on all supported interfaces (except for frame-relay multipoint sub-interfaces)