Well, I’ve been excited about getting to the Frame Relay section of the book that I spent only one day on PPP chapter of the book and ran on over to this chapter. Don’t worry my bebes, I’lll go back to PPP for a more extensive review. In the meantime, let’s have some fun with Frame Relay.

I learned a little bit about the Frame Relay concepts in ICND1 but not enough to be able to set up a lab to tinker with. This time around I should be able to delve a little bit deeper (CCNA deep, not CCNP deep. At least not yet! ) into frame relay and setup a sweet lab setup. Is it just me or did i just hear an echo?

Notes to take note of, according to the book of Odom:

• In contrast to a point-to-point leased circuit, frame relay allows multiple remote routers to connect to each other using a single physical WAN circuit.
• The model is similar to LANs in which multiple devices can attach to each other like multiple computers can communicate with each other through a switch. But unlike LANs, data-link broadcasts does not happen over frame relay. That is frame relay networks are called nonbroadcast multiaccess (NBMA) networks.
• Routers are connected via leased line (called access link) and connected to the Frame Relay switch.
• Communication between the routers and the frame relay switch is governed by the LMI protocol.
• The routers on the customer side is called the DTE, while the frame relay switch is called the DCE.
• The logical connection between two DCEs is called a VC. The service provider provisions the details of the VC, and predefines the details. The predefined VCs are called permanent virtual circuits (PVC). 
• Each router on the frame relay uses the data-link connection identifier (DLCI) as it’s address; it identifies the VC over which the frame should travel. The frame relay header contains the correct DLCI.
• Important Terms:
  • Virtual Circuit (VC) – a logical concept that delivers data frames between DTEs based on packet switching technology, as opposed to circuit switched such as a leased physical circuit.
  • Permanent Virtual Circuit (PVC) - a predefined virtual circuit. As opposed to a switched virtual circuit, it is alwas connected. Therefore it provides a continuos and dedicated connection between two facilities. It is usually configured by the service provider
  • Switched Virtual Circuit (SVC) – a VC that is set-up on a per-call basis and connection terminates when it is done.
  • Data Terminal Equipment (DTE) – an equipment that usually sits on the customers site and connects to the frame relay service. It converts received signals to useable data. A router or modem would be considered a DTE.
  • Data Communications Equipment (DCE) – the device on the opposite end of the DTE. They normally sit on the service provider’s side and it is the device the usually provides the “clocking” signal where the DTE synchronizes it’s signal with.
  • Access Link – the leased line between a DTE and DCE
  • Access Rate – the clock speed of the access link. the physical line speed of the interface connecting to the frame relay. Example, access rate = 1.544 Mbps
  • Committed Information Rate (CIR) - also called the guaranteed rate, is the average bandwidth that the service provider sets and guarantees for the VC. Example, CIR = 128 kbps
  • Data-link Connection Identifier (DLCI) – the address used to identify a VC.
  • Nonbroadcast Multiaccess (NBMA) – a network where broadcasts do not occur, but more than two devices can be connected
  • LMI – 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. 
    • There are three standards for LMI: ANSI’s Annex D standard, T1.617; ITU-T’s Q.933 Annex A standard; and the “Gang of Four” standard, named for the four companies that developed it: Cisco, DEC, StrataCom and NorTel (Northern Telecom). (source: wikipedia)
  • Link Access Procedure Bearer Service (LAPF) – provides a framing for the Frame Relay header and trailer.
• Full Mesh Frame Relay – each pair of sites is configured with PVCs
• Partial Mesh - not all pairs have connected PVC. Typically when one remore router does not need to connect to another remote router becuase it only needs to connect to the main site.
  • an advantage of partial mesh is it’s cheaper; less VC to pay. The disadvantage is if the two remote routers need to exhance info, they have to go through the main site and have the info forwarded from there.
• LMI vs Encapsulation
  • LMI is between the DTE and DCE. Encapsulation is between DTE and another DTE.
  • Encapsulation defines the headers used by the DTE to communicate with another DTE
  • The switch and DTE care about using the same LMI. The switch does not care about the encapsulation
  • The endpoint routers (DTE) care about the encapsulation
• Important LMI message for CCNA-passing purposes:
  • LMI Status Inquiry Message. The status message perform two key functions:
    • Perfrom keepalive function between the DTE and DCE. No keep alive message means no link good. Might wanna check access link from problems then.
    • Tells you the status of a PVC; whether active or inactive.
• Three LMI protocol options:
  • Cisco
  • ITU
  • ANSI
• Each LMI option is different and incompatible. Just make sure DTE and DCE are using the same LMI standard and LMI is a happy camper and will provide good service with no extra charge.

• Link Access Procedure Frame Bearer Service (LAPF) specification, ITU Q.922-A defines the header and trailer for the Frame Relay encapsulation of Layer 3 packets. It provides error detection with an FCS in the trailer, as well as the DLCI, DE, FECN, and BECN fields in the header. New terms to be defined later.
• LAPF header and trailer does not have a protocol type field needed to define the type of packet contained in the frame. Therefore, DTEs cannot support multiprotocol traffic if the Frame Relay is using only the LAPF header.
• Two solutions to compensate for lack of Protocol Type field:
  • Cisco (and 3 other companies) created an additional header, which comes between the LAPF header and the Layer 3 packet. It includes a 2-byte Protocol Type field, with values similar to Cisco’s HDLC.
  • Multiprotocol Interfonnect over Frame Relay (RFC 1490, obsoleted by RFC 2427) was written to ensure multivendor interoperability between Frame Relay and DTEs. The new header is also placed between the LAPF header and the Layer 3 packet, which includes the Protocol type field including other options.
• Frame Relay switches ignore these two types of encapsulation. In other words, switches don’t care about the encapsulation and the DTEs on each side do have to agree on the encapsulation.

• The DLCI is the Frame Relay address.
• A Frame Relay header has a single DLCI field, in contrast to Ethernet that contains both a Source and Destination fields.

Local vs Global Addressing

• Local Addressing pertains to the fact that DLCIs are locally significant, meaning that the addresses need to be unique only on the local access link. 
  • Said another way: a single access link cannot use the same DLCI to represent multimple VCs on the same access link
• Frame Relay Global Addressing - a way of addressing to make it look like a LAN addressing concept.
  • It resembles addressing used in Layer 3 routers
  • With global addressing, a fixed DLCI is assigned to a specific DTE. A sending router, then, inserts the DLCI value of the destination router into its header (instead of using a DLCI value from the local pool of unused numbers).
  • When the Frame Relay switches receive the header, it changes the value of the DLCI with the DLCI of the sending router.

The sender treats the DLCI field as a destination address, using the destination’s global DLCI in the header

The receiver thinks of the DLCI field as the source address, becuase it contains the global DLCI of the frame’s sender.

Network Layer Concerns with Frame Relay

• Three diferrent options for assigning subnets and IP addresses on Frame Relay interfaces:
  1. One subnet containing all Frame Relay DTEs
     • works well for full mesh networks
     • conserves IP addresses
     • looks like LAN-type addressing that makes it easier to conceptualize
  2. One subnet per VC
     • this is the typical network setup for most organization
     • works better with partially meshed networksA hybrid of the first two options
     • each VC is in its own subnet
     • matches the logic behind a set of point-to-point links.
     • wastes some IP addresses, but overcomes some issues with distance vector routing protocols.
     • Subinterfaces allow a single interface to have multiple IP addresses associated with one physical interface. A router can treat each subinterface, and the VC associated with it, as if it were a point-to-point serial link.
  3. Hybrid of the first two options
     • point-to-point subinterfaces are used when a single VC is considered to be all that is in the group.
     • multipoint subinterfaces are used when more than two routers are considered to be in the same group. These interfaces logically terminate more than one VC.

Layer 3 Broadcast Handling

• Frame Relay DTEs cannot send broadcasts across multiple VCs to multiple destinations. At least not like LAN broadcasts
• Routers do, however need to send broadcasts or multicasts for features such as routing protocol updates.
• Two part solution:
  • If configured, Cisco IOS sends copies of the broadcasts across each VC. This is fine if there’s only a few VCs. It there’s hundreds of VCs terminating in one router, for each broadcast, hundreds of copies could be sent.
  • As a second part of the solution, the routers try to minimize the impact of the first part by placing the copies of the broadcast on a separate output queue than the user traffic.

Controlling Speed and Discards

When a customer with a Frame Relay access rate – the clock rate of the access link – that typically is higher than the CIR (e.g. a T1 access link with CIR of 128-kbps), there is a change that a router can send data that far exceeds the rate that the provider agreed with the customer. When that happens the provider can start discarding frames from the network.

Traffic Shaping allows the router to decrease the overall rate of sending bits to a speed slower than the access rate, and maybe even as low as the CIR of a VC.

• The Frame Relay header includes three single-bit flags that can be used to help control the network:
  1. Forward Explicit Congestion Notification ( FECN) – when this bit is set by the device, it means that there is congestion in the forward direction of the frame.
  2. Backward Explicit Congestion Notification (BECN) – the frame relay switch tells the sending router that a congestion occured in the direction opposite, or backward, of the direction of the frame.
  3. Discard Eligibility (DE) bit – if a provider’s switches need to discard frames because of congestion, the customer can set a DE bit for frames the are less important. The switches can then discard those frames with the DE bit set.