(a) Local Management Interface (LMI)
(b) Traffic shaping
(c) Topologies
(e) Discard eligible (DE)
(f) Static versus Dynamic L2 to L3 Resolution
(g) Frame-Relay Interface-DLCI
(h) Broadcast Queue
(i) Frame End to End Keepalives
(j) Load Interval
(k) PING Local Interface
LMI
So, once upon a time, Cisco, Stratacom (who got bought out by Cisco), Northern Telecom (Nortel) and Digital Equipment Corporation (HP) got together to come up with a standard for signalling things like PVC status, addressing and multicast. Frame Relay was proving a popular technology in the WAN space, so ANSI and the ITU also came up with their take on how to make LMI behave which is each slightly different and incompatible (not really an issue as such as routers can autosense the appropriate LMI type)
A Frame Relay router (DTE) can send an LMI Status Enquiry message to a Frame Relay switch (DCE) and the switch will reply with a LMI status message to get information about the VCs configured on the interface, with their DLCIs and status. LMI messages are sent every 10 seconds, with every sixth message containing a full Status message (more detailed VC info)
The LMI Status Enquiry/LMI Status messages act as a keep alive function, if there is no activity on LMI after the typical 3 intervals (30 seconds), the interface is considered down at layer 2.
LMI is enabled/disabled with "keepalive/no keepalive" interface commands on the FR interface. It is possible to have back to back routers using FR with both interfaces with a Frame-relay interface-type DTE (default) and keepalive turned off to allow communications to occur at Layer 2.
| LMI Type | Standards Reference | frame-relay lmi-type | Valid DLCI Range | LMI DLCI |
| Cisco | Proprietary | cisco | 16-1007 | 1023 |
| ANSI | ANSI T1.617 Annex D | ansi | 16-991 | 0 |
| ITU | ITU Q.933 Annex A | q933a | 16-991 | 0 |
Traffic Shaping
Although traffic shaping is QoS topic and will be documented further in that topic area;FR has mechanisms that require mentioning on their own.
Traffic shaping is used on router to control the traffic flow towards the WAN to ensure that the outgoing traffic is within the purchased traffic contract (a 128kbps circuit
may be bought on an interface that is physically capable of delivering 1984kbps traffic) Traffic shaping allows the user to take action on the traffic, for example smooth
some of the burstiness of the traffic at the cost of slightly delaying it, since the WAN provider will most likely drop out of contract traffic immediately on ingress into
their network or mark out of contract traffic as eligible to be dropped if their is network congestion further downstream.
Traffic shaping works on the concept of credits. Each packet to be sent requires a credit for each bit to be transmitted. If there is enough credit for the packet, it is
deducted from the interface credit balance and the packet enters the queue for transmission.
Cisco Terminology for Traffic Shaping
| Abbreviation | Term | Definition |
| AR | Available Rate | This is the port speed defined by the DCE clock (for example 1984kbps) |
| CIR | Committed Information Rate | This is the target traffic rate - this is the traffic rate the route will attempt to send traffic out at |
| MinCIR | Minimum CIR | This is the guaranteed data rate the WAN provider will support, with all excess traffic being marked as DE (discard eligible) |
| Bc | Committed Burst | This is number of committed bits (credits) that can be sent during an interval |
| Be | Excess Burst | This is the number of excess burst bits (credits) above Bc a router can use in an interval (Tc) Be credits accumulate from unused Bc credits in previous intervals |
| Tc | Committed Rate Measurement Interval | This is the time interval for which Bc or Bc + Be can be transmitted. Min 10ms, Max 25ms |
Traffic Shaping Commands:
frame-relay traffic-shaping
This interface level command enables traffic shaping to occur for PVCs on the interface. Note: All PVCs will receive a default CIR of 56kbps requiring specific configuration to change this.
map-class frame-relay
This global level command defines various FR parameters related to QoS and keepalives.
Map-class configuration command used to define the target transmit rate.
Map-class configuration command used to define the guaranteed transmit rate - the default value is half cir and is only used for adaptive shaping
frame-relay bc
Map-class configuration command used to define the committed burst size. Tc is indirectly defined and calculated from Bc/CIR
frame-relay be
Map-class configuration command used to define the excess burst size.
frame-relay adaptive-shaping
Map-class configuration used to enable adaptive traffic shaping. The method can be becn (described below), foresight (Stratacom proprietary), or interface-congestion with a listed queue depth (allow egress limiting based on available output queue)
frame-relay fecn-adapt
Map-class configuration used to enable a router to reflect FECNs (Forward congestion notification is detected by the WAN switches and attached to Frames destined to the router, which then replies with frames that have BECN (Backward congestion notification) which informs the transmitting router that it needs to reduce its transmission rate
frame-relay class
Interface/sub-interface configuration used to bind the map-class to the interface.
class
frame-relay interface-dlci
FR Topologies
Frame Relay enables a rich variety of logical topologies enabling direct connection, mesh and partial mesh configurations using point-to-point and point-to-multipoint configurations. FR sub-interface configurations enable the possibility to have multiple point-to-point configurations enabling a L3 subnet per link, as opposted to the point-to-multipoint configurations where multiple links share the same L3 domain. It is possible to combine these configuration types on the same physical interface.
Configuring Discard Eligiblity
The Router itself to mark time-sensitive traffic with the DE bit enabling an extra level of QoS at layer 2. Refer to this note on CCO
Static versus Dynamic L2 to L3 Resolution
Dynamic Mapping occurs using Inverse ARP and is enabled by default. Inverse ARP matches looks at each active PVC and looks for the Layer 3 address associated with it (which is the reverse of ARP on ethernet).
Static mapping bypasses the use of Inverse ARP where a specific PVC is assigned reachability for one (or more) PVCs (make sure to include the broadcast capability if you need to support multicast/broadcast protocols).
show frame-relay map is used to show static or dynamic L2/L3 mappings
Frame-relay interface-dlci
The paragraph below is summarised from http://blog.ine.com/2008/08/14/that-pesky-frame-relay-interface-dlci-command/
This command is used:
On point-to-point subinterfaces specifying the DLCI (the remote side does not have to have an IP mapped to the DLCI since it is implied that non-local traffic on the subnet goes down the PVC) On multipoint subinterfaces when Inverse ARP is active Not used on physical interfaces because all DLCIs appear there by default (until a subinterface claims it)
Broadcast Queue
For large frame-relay hub and spoke networks, performance issues can manifest themselves particularly if there is a lot of broadcast traffic generated that requires replication across the PVCs such as routing protocol updates. Cisco supports a dedicated interface level broadcast queue with its own traffic parameters limiting the maximum rate and is prioritised ahead of regular traffic when the throughput is below the limited rated.
frame-relay broadcast-queue
Where
Frame-Relay End to End Keepalives
Using LMI, keepalives are used to determine remote host availability and logical circuit continuity .
Four methods are possible:
Bidirectonal mode - where each side sends out and waits for keepalive requests
Request mode - where only one side sends out and waits for keepalive responses
Reply mode - where only waits for and replies to keepalive requests
Passive-reply mode - which is like reply mode but doesn't use timers nor maintain event state
These are configurable per PVC using the frame-relay map-class configurations
frame-relay end-to-end keepalive event-window
frame-relay end-to-end keepalive mode
frame-relay end-to-end success-events
frame-relay end-to-end timer
Load-Interval
When you use the "show interface
The configuration can be applied directly to the physical interface, or in the frame-relay interface-dlci
Ping Local Interface
Blog post http://blog.ine.com/2009/12/02/ping-thyself-in-frame-relay/
Summary: By default you cannot ping your local frame-relay IP address unless you include a frame-relay map ip
