Frame Relay Written Notes

These are my ‘crib notes’ that I’ve made to serve as a last minute refresher. Please forgive the grammer / spelling as I did not develop these notes with publishing in mind.

Frame Relay

*** Theory ***

  • Nutshell – evolved from X25 and allows bandwidth to be shared
  • PVC (Permanent Virtual Circuit) – makes routers think they are directly connected.
  • DLCI (Data Link Connection Identifier) – path to use to get to the ‘directly’ connected router. It leaves via the source DLCI rather than the destination DLCI. DLCI are locally significant
  • LMI (Local Management Interface) – is the language used for DLCI.  Local Management Interface is a signaling standard used between routers and frame relay switches. Communication takes place between a router and the first frame relay switch it’s connected to. Information about keepalives, global addressing, IP Multicast and the status of virtual circuits is commonly exchanged using LMI.
  • NNI (Network-to-network interface) –
  • Frame relay traffic shaping – A interface cant send slower than it physical speed. Like a water bucket and a hole analogy
  • TC (time Intervals) – how often to release traffic or hand over the hole in bucket. Usually 1/8 second
  • CIR (Committed Information Rate) – How much the SP guarantee you.
  • BC (Committed Bursts) – CIR / tc. How much that can be sent per TC. So if we give CIR of 64Kbps then BC is 8000bps per TC.
  • BE (Excess Burst) – if in one TC, we send less than the BC e.g. 4000bp, it means we didn’t use all that we could and therefore can bank we have not used. SP will put a cap on how much can be banked.
  • DE (Discard Eligible) – If the SP experiences congestion, then everything in the BE is considered DE and will be dropped.
  • BECN (Backwards Explicit Congestion Notification) – Sends sos to source router to tell them to slow down. Does this by appending to the destination reply packet.
  • FECN (Forward Explicit Congestion Notification) – Sends a request to the destination router to send a generic packet to the source router so that it can append a BECN to it.
  • Router needs to be configured to understand BECN and FECN. Do this via
    • Class MAP and define parameters
      • Conf# map-class frame-relay WORD
      • Conf-conf-map-class# frame –relay adaptive-shaping becn
      • Conf-conf-map-class# frame –relay mincir 24000 (incase i don’t trust SP, this will guarantee no matter how many becn i get, it wont go lower than mincir)
      • Set BC BE and CIR here / can also do by traffic-rate wizard command
  • Interface configuration ‘Frame Relay Traffic Shaping’
    • Conf-if#frame-relay traffic-shaping
    • Conf-subif# frame-relay class WORD
  • Apply to interface or PVC
  • Split horizon – stops routing updates going out on interfaces it learned /source from. To overcome in Frame relay, you make interface point to multipoint instead of P2P? If using P2P then you must use no ip split-horizon eigrp 90 on the hub p2m sub interface
  • Inverse ARP – dynamically learns all the DLCI and IP mapping, however for CCIE lab it better to turn this off as you will have DLCI for routers that you shouldn’t have.
    • Conf-if# no frame-relay inverse arp
    • Conf-if# no arp frame-relay (stops other routers learning of you)
    • Broadcast – use this to allow broadcast and multicast traffic such as routing updates as this is disabled by default in Frame relay

*** Commands ***

show frame-relay map

  • The output of the show frame-relay map will show the mapping information learned by Inverse ARP on the multipoint subinterfaces.
  • The output will show Layer 3 information because there are more than one DLCI associated with a multipoint interface therefore the router needs the mapping information to match the next-hop IP address to the correct DLCI.
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