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Frame Relay’s Notes

Frame Relay - Introduction and Concepts

This is the first in a series of lessons which will examine Frame Relay in detail. In this lesson, we will take a high level look at Frame Relay as well as its history, components, and benefits.

The Quick and Dirty

Frame Relay is a high-performance packet-switched WAN protocol which uses statistical multiplexing and variable length packets. Frame Relay operates at Layer 2 (Data Link) of the OSI model. Each customer gets a private line (or leased line) to a frame-relay node called a virtual circuit (VC) through the Frame Relay cloud. These VCs can either be a bandwidth-on-demand Switched Virtual Circuit (SVC) or (more likely) an always-on Permanent Virtual Circuit (PVC).

DTE devices such as a router communicate with a DCE Frame Relay switch. Local Management Interface (LMI) is used to get VC information and as a keepalive between the router and the Frame Relay switch. Each Frame Relay node (DTE) uses a 10-bit data-link connection identifier (DLCI) as a Layer 2 address. These DLCIs are locally significant (although there are extensions that can make them globally significant) and are analogous to MAC addresses in Ethernet.

Frame Relay has many powerful features including built-in congestion notification as well as traffic shaping (QoS) capabilities.

Although Frame Relay is getting long in the tooth (it's been around since the late 1980s) and technologies like MPLS are quickly rising in popularity, it is still a vary popular WAN protocol and once which you are very likely to work with in the field.

Frame Relay Topologies and Designs

In this continuation of the Frame Relay series, we take a look at Frame Relay designs. As we learned in the Frame Relay - Introduction and Concepts lesson, Frame Relay introduces some cost savings benefits over point-to-point leased lines. This lesson will review some of those advantages as well as concentrate on different designs and which of those designs are best suited to take advantage of Frame Relay’s benefits.

The Quick and Dirty

One of the main benefits of Frame Relay over point-to-point leased lines is that you can achieve cost savings by reducing equipment and circuit costs. In order to take advantage of these savings, it’s important to know how to incorporate these benefits into your network design.

While it’s certainly possible to design a full mesh network using Frame Relay, most Frame Relay designs use (at least as their starting point) a hub and spoke design as this design uses Frame Relay’s advantages. Some of the advantages of a hub and spoke Frame Relay network are the ability to easily scale, reduced cost in the form of less network equipment and less physical connections to the provider, and ease of management. There are a few downsides to a hub and spoke design as well such as a single point of failure on the hub physical access circuit and the need for spoke to spoke traffic to traverse the hub. These issues can be minimized in many instances by modifying the hub and spoke design into a partial mesh design.

Frame Relay - Basic Configurations Point to Point

Even though we still have more lessons left to fully cover the major aspects of Frame Relay and its configuration, I wanted to include a couple of short, basic lessons covering Frame Relay configuration to break up the block of theory lectures and to get a look at some of the steps required to configure Frame Relay. The first of these covers a simple point to point Frame Relay configuration using physical serial interfaces...with a little bit of unintended troubleshooting thrown in as a "bonus".

The Quick and Dirty

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While you might not associate point to point connections with Frame Relay, you can certainly configure on over a Frame Relay connection. We’ll revisit Frame Relay point to point connections in more detail in a future lesson, but the steps for configuring a serial interface for a Frame Relay point to point connection are pretty simple:

- Set the interface encapsulation to Frame Relay- Set LMI type, if necessary - Disable Frame Relay Inverse-ARP, if necessary- Configure Layer 3 address on interface- Manually configure Frame Relay IP to DLCI mapping

Frame Relay - Basic Configurations Hub and Spoke

Even though we still have more lessons left to fully cover the major aspects of Frame Relay and its configuration, I wanted to include a couple of short, basic lessons covering Frame Relay configuration to break up the block of theory lectures and to get a look at some of the steps required to configure Frame Relay. The second of these covers a simple hub and spoke Frame Relay configuration using physical serial interfaces.

Note: Sound quality is a bit spotty on these recordings. I was "between microphones" and there are a lot of pops.

The Quick and Dirty

Frame Relay is well-suited for a hub and spoke design. We can take advantage of Frame Relay's ability to assign more than one virtual circuit (VC) to a single physical access circuit and design a hub and spoke network that gives us connectivity between all endpoints with a minimum of circuit and equipment cost.

We'll revisit Frame Relay point to point connections in more detail in a future lesson, but the steps for configuring a serial interface for a Frame Relay point to point connection are pretty simple:

- Set the interface encapsulation to Frame Relay- Set LMI type, if necessary - Disable Frame Relay Inverse-ARP, if necessary- Configure Layer 3 address on interface- Manually configure Frame Relay IP to DLCI mapping including spoke to spoke mappings

OSPF Timers – Hello and Dead Intervals

The OSPF HelloInterval and RouterDeadInterval are two timers that maintain the up/down state of OSPF neighbor relationships. They are among the variables that must match in order for an OSPF neighbor relationship to be established

The Quick and Dirty

The HelloInterval and RouterDeadInterval are the two timers that you can adjust to speed up network convergence in an OSPF network. The HelloInterval determines the interval between sending OSPF Hello messages on an interface, while the RouterDeadInterval is the interval in which a router must receive an OSPF Hello message from a neighbor before it considers that neighbor to be down.

Cisco IOS assigns a default HelloInterval and RouterDeadInterval to OSPF enabled interfaces. Depending on the interface type, the HelloInterval will be either 10 seconds or 30 seconds. The RouterDeadInterval will be four times the HelloInterval (40 or 120 seconds). A Cisco OSPF-enabled device will maintain a count down timer for each neighbor based on the RouterDeadInterval. Each time receives a Hello message from a neighbor, it will reset this timer to the RouterDeadInterval. If it does not receive a Hello message before this timer expires, then the neighbor will be set to the OSPF DOWN state.

You can adjust the HelloInterval and RouterDeadInterval with the ip ospf hello-interval and ip ospf dead-interval interface configuration commands. Doing so can reduce OSPF convergence, but you should be careful to take into consideration the quality (and possibly speed) of your links before doing so.