Basic Concept of Frame Relay

Frame Relay is a scalable WAN solution that is often used as an alternative to leased lines when leased lines prove to be cost unaffordable. With Frame Relay, you can have a single serial interface on a router connecting into multiple remote sites through virtual circuits.

Basic concept of Frame Relay

For exam prospective You should be familiar with terms

Virtual Circuits (VCs)

A VC is a logical connection between two devices; therefore, many of these VCs can exist on the same physical connection. The advantage that VCs have over leased lines is that they can provide full connectivity at a much lower price. VCs are also full-duplex: you can simultaneously send and receive on the same VC.

There are two types of VCs: permanent VCs (PVCs) and switched or semipermanent VCs (SVCs).

PVC is similar to a leased line: it is configured up front by the carrier and remains up as long as there is a physical circuit path from the source to the destination.

SVC are similar to telephone circuit-switched connections: whenever you need to send data to a connection, an SVC is dynamically built and then torn down once your data has been sent.

Disadvantage of PVCs is that they require a lot of manual configuration up front to establish the VC. Another disadvantage is that they aren't very flexible: if the PVC fails, there is no dynamic rebuilding of the PVC around the failure.

LMI

Three different standards are defined for LMI:-

1. ANSI's Annex D standard, T1.617
2. ITU-T's Q.933 Annex A standard
3. The Gang of Four

Because LMI is locally significant, each Frame Relay DTE in your network does not have to use the same LMI type The main function of LMI is to allow the Frame Relay DTE and DCE to exchange status information about the VCs and themselves Cisco has default timers for their status enquiry and full status update messages. Status enquiry messages are sent every ten seconds, by default. Every sixth message is a full status update message.

The three possible states that your PVC can be in are

• Active— Active is good. Active means that everything is up and operational.
• Inactive— Inactive is bad. Inactive means that you are connected to your Frame Relay provider, but there is a problem with the far-end connection. The problem is most likely between the far-end router and its connection to the Frame Relay provider. You should contact your provider to troubleshoot the issue.
• Deleted— Deleted is also bad. Deleted means that there is a problem between your router and the Frame Relay provider's equipment. You should contact your provider to troubleshoot this issue.

DLCI

Each VC has a unique local address, called a DLCI. Circuits are identified by data-link connection identifiers (DLCI). DLCIs are assigned by your provider and are used between your router and the Frame Relay provider. In other words, DLCIs are locally significant. This means that as a VC traverses various segments in a WAN, the DLCI numbers can be different for each segment. DLCIs are locally significant. The carrier’s switches take care of mapping DLCI numbers for a VC between DTEs and DCEs.

Nonbroadcast Multiaccess

Nonbroadcast multiaccess (NBMA) is a term used to describe WAN networks that use VCs for connectivity Frame Relay is a nonbroadcast multi-access (NBMA) medium, which means that broadcast traffic is not allowed to traverse Frame Relay traffic.

Split Horizon Issues

The main problem of NBMA environments arises when the network is partially meshed for a subnet. This can create problems with routing protocols that support split horizon.

Solutions to Split Horizon Problems

Given the preceding problem with routing protocols that use split horizon, there are solutions that you can use to overcome this issue:

• Use static routes instead of dynamic routing protocols. This is not a scalable solution.
• Disable split horizon with the no ip split-horizon command.This could create a loop, If you are not careful
• Have a fully meshed topology where every router has a PVC to every other router. This can get expensive.
• Use subinterfaces. This is your best option.

Subinterfaces

A subinterface is a subset of an existing physical interface. As far as the router is concerned, the subinterface is a separate interface. By creating subinterfaces, each circuit can be on its own subnet. There are two types of subinterfaces:

• Point-to-point— This maps a single IP subnet to a single subinterface and DLCI.
• Multipoint— This maps a single IP subnet to multiple DLCIs on a subinterface.

Inverse-Arp

Frame Relay needs a mechanism to map Layer 3 addresses withLayer 2 Frame Relay DLCIs. This can be done through a static map command (shown later in the configuration section) or through inverse-arp. Just like Ethernet ARP, inverse-arp is used to map a Layer 3 address to a Layer 2 address. However, Ethernet ARP maps an IP address to a MAC address and inverse-arp works to map an IP address (or other protocol) to a DLCI.

FECN (Forward explicit congestion notification)

This value in the Frame Relay frame header is set by the carrier switch (typically) to indicate congestion inside the carrier network to the destination device at the end of the VC; the carrier may be doing this to your traffic as it is on its way to its destination.

BECN (backward explicit congestion notification)

This value is set by the destination DTE (Frame Relay device) in the header of the Frame Relay frame to indicate congestion (from the source to the destination) to the source of the Frame Relay frames (the source DTE, the router). Sometimes the carrier switches can generate BECN frames in the backward direction to the source to speed up the congestion notification process. The source can then adapt its rate on the VC appropriately.

Access rate

This is the speed of the physical connection (such as a T1) between your router and the Frame Relay switch.

CIR (committed information rate)

This is the average data rate, measured over a fixed period of time, that the carrier guarantees for a VC.

BC (committed burst rate)

This is the average data rate (over a period of a smaller fixed time than CIR) that a provider guarantees for a VC; in other words, it implies a smaller time period but a higher average than the CIR to allow for small bursts in traffic.

BE (excessive burst rate)

This is the fastest data rate at which the provider will ever service the VC. Some carriers allow you to set this value to match the access rate.

DE (discard eligibility)

This is used to mark a frame as low priority. You can do this manually, or the carrier will do this for a frame that is nonconforming to your traffic contract (exceeding CIR/BC values).

Oversubscription

When you add up all of the CIRs of your VCs on an interface, they exceed the access rate of the interface: you are betting that all of your VCs will not run, simultaneously, at their traffic-contracted rates.