CCNA24 Understanding Frame Realy Concepts

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CCNA 200-120

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CCNA 200-120


CCNA 200-120

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Understanding Frame Relay Concepts

Understanding Frame Relay Concepts

Frame Relay was at one time the most popular WAN technology used in computer networks.

Today, Frame Relay has become less popular, being replaced by several other WAN options.

These include the virtual private network (VPN) technology, and Ethernet WANs, . In addition,

many enterprises use Multiprotocol Label Switching (MPLS) VPNs, which follow the same basic

service model as Frame Relay, usually offered by the same Frame Relay providers but with

significant technical advantages.

Frame Relay Overview

Frame Relay networks provide more features and benefits than simple point-to-point

WAN links, but to do that, Frame Relay protocols are more detailed. For example,

Frame Relay networks are multiaccess networks, which means that more than two

devices can attach to the network, similar to LANs. Unlike with LANs, you cannot

send a data link layer broadcast over Frame Relay. Therefore, Frame Relay networks

are called nonbroadcast multiaccess (NBMA) networks. Also, because Frame Relay

is multiaccess, it requires the use of an address that identifies to which remote router

each frame is addressed.

Frame Relay Components

Figure shows the most basic components of a Frame Relay network. A leased line is

installed between the router and a nearby Frame Relay switch; this link is called the access

link. To ensure that the link is working, the device outside the Frame Relay network, called

the data terminal equipment (DTE), exchanges regular messages with the Frame Relay

switch. These keepalive messages, along with other messages, are defined by the Frame

Relay Local Management Interface (LMI) protocol.

The routers are considered DTE, and the Frame Relay switches are data communications

equipment (DCE).

Virtual Circuits

The connection through a Frame Relay network between two DTEs is called a virtual circuit (VC). The circuits are virtual because there is no direct electrical connection from end to end. The connection is logical, and data moves from end to end, without a direct electrical circuit. With VCs, Frame Relay shares the bandwidth among multiple users and any single site can communicate with any other single site without using multiple dedicated physical lines.

There are two ways to establish VCs: SVCs, switched virtual circuits, are established dynamically by sending signalling messages to the network (CALL SETUP, DATA TRANSFER, IDLE, CALL TERMINATION). PVCs, permanent virtual circuits, are preconfigured by the carrier, and after they are set up, only operate in DATA TRANSFER and IDLE modes. Note that some publications refer to PVCs as private VCs.

Figure shows the physical connectivity at each connection to the Frame Relay network, and

shows the logical, or virtual, end-to-end connectivity associated with a virtual circuit (VC).

The logical communications path between each pair of DTEs is a VC. The dashed line

in the figure represents a single VC; The service provider usually preconfigures all the

required details of a VC; predefined VCs are called permanent virtual circuits (PVC).

Routers use the data link connection identifier (DLCI) as the Frame Relay address; it identifies

the VC over which the frame should travel. So, in Figure when R1 needs to forward a packet to

R2, R1 encapsulates the Layer 3 packet into a Frame Relay header and trailer and then sends

the frame.

The Frame Relay header includes the correct DLCI, identifying the PVC connecting R1 to R2,

so that the provider’s Frame Relay switches correctly forward the frame to R2.

Multiple VCs

Frame Relay is statistically multiplexed, meaning that it transmits only one frame at a time, but that many logical connections can co-exist on a single physical line. The Frame Relay Access Device (FRAD) or router connected to the Frame Relay network may have multiple VCs connecting it to various endpoints. Multiple VCs on a single physical line are distinguished because each VC has its own DLCI. Remember that the DLCI has only local significance and may be different at each end of a VC. The figure shows an example of two VCs on a single access line, each with its own DLCI, attaching to a router (R1).

This capability often reduces the equipment and network complexity required to connect multiple devices, making it a very cost-effective replacement for a mesh of access lines. With this configuration, each endpoint needs only a single access line and interface. More savings arise as the capacity of the access line is based on the average bandwidth requirement of the VCs, rather than on the maximum bandwidth requirement.

The Frame Relay Encapsulation Process

Frame Relay takes data packets from a network layer protocol, such as IP or IPX, encapsulates them as the data portion of a Frame Relay frame, and then passes the frame to the physical layer for delivery on the wire. To understand how this works, it is helpful to understand how it relates to the lower levels of the OSI model. The figure shows how Frame Relay encapsulates data for transport and moves it down to the physical layer for delivery.

DLCI - The 10-bit DLCI is the essence of the Frame Relay header. This value represents the virtual connection between the DTE device and the switch. Each virtual connection that is multiplexed onto the physical channel is represented by a unique DLCI. The DLCI values have local significance only, which means that they are unique only to the physical channel on which they reside. Therefore, devices at opposite ends of a connection can use different DLCI values to refer to the same virtual connection. Extended Address (EA) - If the value of the EA field is 1, the current byte is determined to be the last DLCI octet. Although current Frame Relay implementations all use a two-octet DLCI, this capability does allow longer DLCIs in the future. The eighth bit of each byte of the Address field indicates the EA. C/R - The bit that follows the most significant DLCI byte in the Address field. The C/R bit is not currently defined. Congestion Control - Contains 3 bits that control the Frame Relay congestion-notification mechanisms. The FECN, BECN, and DE bits are the last three bits in the Address field.

Frame Relay Addressing

At a basic conceptual level, Frame Relay addresses, called data link connection identifiers (DLCI), have some

similarity with the more familiar MAC and IP addresses. All these addresses exist as binary values, but they all

have some more convenient format: hex for MAC addresses, dotted decimal for IP, and decimal for DLCIs.

Frame Relay defines the DLCI as a 10-bit value, written in decimal, with the low- and high-end values usually

reserved. (The specific range does not matter much because the service provider assigns the values, but they usually range from around 17 to a little less than 1000.)

Frame Relay Local Addressing

The service provider assigns each PVC two local DLCI values: one on one end of the PVC, and

one for the other end. The term local DLCI has several different origins, but you can think of the

word local as emphasizing the fact that from a router’s perspective, the local DLCI is the DLCI

used on the local end of the PVC where the router sits. Figure shows the idea.

In this example, the PVC between routers A and B has two DLCIs assigned by the provider. Router

A’s end uses local DLCI 41 to identify the PVC, and router B’s end uses DLCI 40 to identify the

same PVC. Similarly, the PVC between routers A and C, as usual, has two local DLCIs assigned,

one on each end. In this case, router A’s end uses 42, and router C’s end uses 40.

The service provider could have used any DLCI values within the range of legal values, with one

exception:

The local DLCIs on a single access link must be unique among all PVCs that use one physical

Frame Relay access link because Frame Relay DLCIs are locally significant

Frame Forwarding with One DLCI Field

To get an idea of how the provider forwards a Frame Relay frame, consider the fact that the

provider knows the local DLCI used on both ends of the PVC, plus the access links that connect

to those routers. For instance, in Figure the provider knows that a PVC exists between router A

and router B. They know it uses local DLCI 41 on the router A side. And they know it uses DLCI

40 on the router B side. Keeping that in mind, take a look at Figure , which shows what happens

when router A sends a frame to router B.

Frame Relay Forwarding: Router A to Router B

Frame Relay Forwarding: Router B to Router A

To complete the process, think about a packet sent by router B back toward router A.

Again, the routers only know local DLCI values, so as shown in Figure B sends the frame

with DLCI 40, which identifies the A-to-B PVC; the cloud changes the DLCI to 41; and

router A receives the frame with DLCI 41 in it.

Frame Relay Topologies

Cost-effective Frame Relay networks link dozens and even hundreds of sites. Considering that a corporate network might span any number of service providers and include networks from acquired businesses differing in basic design, documenting topologies can be a very complicated process. However, every network or network segment can be viewed as being one of three topology types: star, full mesh, or partial mesh. Star Topology (Hub and Spoke)

The simplest WAN topology is a star, as shown in the figure. In this topology, Span Engineering has a central site in Chicago that acts as a hub and hosts the primary services. Notice that Span has grown and recently opened an office in San Jose. Using Frame Relay made this expansion relatively easy. Connections to each of the five remote sites act as spokes. In a star topology, the location of the hub is usually chosen by the lowest leased-line cost. When implementing a star topology with Frame Relay, each remote site has an access link to the Frame Relay cloud with a single VC.


Full Mesh Topology This figure represents a full mesh topology using dedicated lines. A full mesh topology suits a situation in which the services to be accessed are geographically dispersed and highly reliable access to them is required. A full mesh topology connects every site to every other site. Using leased-line interconnections, additional serial interfaces and lines add costs. In this example, 10 dedicated lines are required to interconnect each site in a full mesh topology.


Partial Mesh Topology For large networks, a full mesh topology is seldom affordable because the number of links required increases dramatically. The issue is not with the cost of the hardware, but because there is a theoretical limit of less than 1,000 VCs per link. In practice, the limit is less than that. For this reason, larger networks are generally configured in a partial mesh topology. With partial mesh, there are more interconnections than required for a star arrangement, but not as many as for a full mesh. The actual pattern is dependant on the data flow requirements.

LMI and Encapsulation Types

While the PVC gives two customer routers a logical means to send frames to one another,

Frame Relay has many physical and logical components that have to work together to make

those PVCs work. Physically, each router needs a physical access link from the router to some

Frame Relay switch. The provider has to create some kind of physical network between those

switches, as well. In addition, the provider has to do some work so that the frames sent over one

PVC arrive at the correct destination.

Frame Relay uses the Local Management Interface (LMI) protocol to manage each physical

access link and the PVCs that use that link. These LMI messages flow between the DTE (for

example, a router) and the DCE (for example, the Frame Relay switch owned by the service

provider).

The most important LMI message relating to topics on the exam is the LMI status inquiry

message . LMI status messages perform two key functions:

• They perform a keepalive function between the DTE and DCE. If the access link

has a problem, the absence of keepalive messages implies that the link is down.

• They signal whether a PVC is active or inactive. Even though each PVC is

predefined, its status can change. An access link might be up, but one or more VCs

could be down. The router needs to know which VCs are up and which are down. It

learns that information from the switch using LMI status messages.

LMI Types

Interestingly, due to historical reasons, Cisco routers have three options for different

variations of

LMI protocols: Cisco, ITU, and ANSI. Each LMI option differs slightly and therefore is

incompatible with the other two. As long as both the DTE and DCE on each end of an

access link use the same LMI standard, LMI works fine.

Configuring the LMI type is easy. Today’s most popular option is to use the default LMI setting.

This setting uses the LMI autosense feature, in which the router simply figures out which LMI type

the switch is using. So, you can simply let the router autosense the LMI and never bother coding

the LMI type. If you choose to configure the LMI type, the router disables the autosense feature.

Network Layer Addressing with Frame Relay

Frame Relay networks have both similarities and differences as compared to

LAN and point-to-point WAN links. These differences introduce some

additional considerations for passing Layer 3 packets across a Frame Relay

network. In particular, Frame Relay gives us three different options for assigning subnets and IP addresses on Frame Relay interfaces:

• One subnet containing all Frame Relay DTEs

• One subnet per VC

• A hybrid of the first two options

Frame Relay Layer 3 Addressing: One Subnet Containing All Frame Relay DTEs

Figure shows the first alternative, which is to use a single subnet for the Frame Relay network.

This figure shows a fully meshed Frame Relay network because the single-subnet option is

usually used when a full mesh of VCs exists. In a full mesh, each router has a VC to every

other router, meaning that each router can send frames directly to every other router. This more

closely resembles how a LAN works. So, a single subnet can be used for all the routers’ Frame

Relay interfaces, as configured on the routers’ serial interfaces. Table summarizes the

addresses used in Figure

The single-subnet alternative is straightforward,

and it conserves your IP address space. It also

looks like what you are used to with LANs, which

makes it easier to conceptualize. Unfortunately,

most companies build partial-mesh Frame Relay

networks, and the single-subnet option has some deficiencies when the network is a partial mesh.

Frame Relay Layer 3 Addressing: One Subnet Per VC

The second IP addressing alternative, having a single subnet for each VC, works better with a

partially meshed Frame Relay network, as shown in Figure Boston cannot forward frames

directly to Charlotte because no VC is defined between the two. This is a more typical Frame

Relay network because most organizations with many sites tend to group applications on

servers at a few centralized locations and most of the traffic is between each remote site and

those servers.

The single-subnet-per-VC subnetting design uses

the same logic as a set of point-to-point links.

Using multiple subnets instead of one larger

subnet does waste some IP addresses. However,

using a single subnet in the partial-mesh design of

Figure introduces several problems with routing

protocols because not all routers in the subnet can

send messages directly to each other. Partial-mesh

designs work better with a single-subnet-per-VC approach.

Frame Relay Layer 3 Addressing: Hybrid Approach

The third alternative for Layer 3 addressing is a hybrid of the first two alternatives. Consider Figure

which shows a trio of routers with VCs between each of them and two other VCs to remote sites.

Two options exist for Layer 3 addressing in this case. The first is to treat each VC as a separate

Layer 3 group. In this case, five subnets are needed for the Frame Relay network. However,

Routers A, B, and C create a smaller full mesh between each other. This allows Routers A, B,

and C to use one subnet. The other two VCs—one between Routers A and D and one between

Routers A and E—are treated as two separate Layer 3 groups. The result is a total of three subnets.

To accomplish either style of Layer 3 addressing in this

third and final case, subinterfaces are used.

Point-to-point subinterfaces are used when a single VC

is considered to be all that is in the group— for

instance, between routers A and D and between routers

A and E. Multipoint subinterfaces are used when more

than two routers are considered to be in the same

group—for instance, with routers A, B, and C.

Multipoint subinterfaces logically terminate more than

one VC. In fact, the name multipoint implies the

function, because more than one remote site can be

reached via a VC associated with a multipoint subinterface.

IP Addresses with Point-to-Point and Multipoint Subinterfaces

Implementing Frame Relay

Configuring Using Physical Interfaces and One IP Subnet

The first example shows the briefest possible Frame Relay configuration, one that uses just the

first two steps of the configuration checklist in this chapter. The design for the first example

includes the following choices:

• Install an access link into three routers.

• Create a full mesh of PVCs.

• Use a single subnet (Class C network 199.1.1.0 in this example) in the Frame Relay network.

• Configure the routers using their physical interfaces.

• Take the default settings for LMI, Inverse ARP, and encapsulation.

Mayberry Configuration

Mount Pilot Configuration Raleigh Configuration

The configuration is simple in comparison with the protocol concepts. The

encapsulation frame-relay command tells the routers to use Frame Relay data link

protocols instead of the default, which is High-Level Data Link Control (HDLC). Note

that the IP addresses on the three routers’ serial interfaces are all in the same Class C

network. Also, this simple configuration takes advantage of the following IOS default

settings:

• The LMI type is automatically sensed.

• The (default) encapsulation is Cisco.

• PVC DLCIs are learned via LMI status messages.

• Inverse ARP is enabled (by default) and is triggered when a router receives an LMI

status message declaring that the VCs are up is received.

Configuring the Encapsulation and LMI

In many cases, using the defaults as listed with the first example works just fine.

However, for the purpose of showing an alternative configuration, suppose that the

following requirements were added to the requirements :-

• The Raleigh router requires IETF encapsulation on both VCs.

• Mayberry’s LMI type should be ANSI, and LMI autosense should not be used.

Mayberry Configuration with New Requirements

Raleigh Configuration with New Requirements

Frame Relay Address Mapping

For the exams, and for real networking jobs, engineers need to know the DLCIs, and the

process of Frame Relay mapping.

Frame Relay mapping matches a next-hop IP address that sits on the Frame Relay network with

the right DLCI used to send frames to that next-hop device, with the same goal as ARP on a LAN.

Figure shows the same network, this time with local DLCI values shown.

Frame Relay “mapping” creates a correlation between a Layer 3 address and its

corresponding Layer 2 address. The concept is similar to the ARP cache for LAN

interfaces. Similarly, routers that use Frame Relay need a mapping between a router’s

Layer 3 address and the DLCI used to reach that other router

The information that correlates to the next-hop router’s Layer 3 address and the

Layer 2 address used to reach it is called mapping. Mapping is needed on

multiaccess networks

For example, consider a packet that enters Mayberry’s LAN interface destined for network

199.1.11.0/24, the Class C network off Mount Pilot’s LAN interface. As shown in Figure the

router goes through normal routing steps, removing the packet from between the Ethernet

header and trailer, choosing to route the packet out Mayberry’s S0/1/1 interface to Mount Pilot

next, and so on. But what DLCI should Mayberry put into the new Frame Relay header?

The left side of the figure shows the tables Mayberry uses to choose the right DLCI. First,

Mayberry looks at the route it uses to forward the packet, finding the next-hop router IP address.

Then, the Frame Relay Mapping table lists that same next-hop router IP address, along with the

DLCI used to send frames to that address (the equivalent of an ARP table). Mayberry then puts

that DLCI (52, Mayberry’s local DLCI for the PVC connected to Mount Pilot) into the Frame Relay

header.

Mayberry can use two methods to build the mapping shown in Example One uses a

statically configured mapping, and the other uses a dynamic process called Inverse

ARP. The next two small sections explain the details of each of these options.

Inverse ARP

Inverse ARP dynamically creates a mapping between the Layer 3 address (for example, the IP

address) and the Layer 2 address (the local DLCI). The end result of Inverse ARP is the same as

IP ARP on a LAN: The router builds a mapping between a neighbouring Layer 3 address and the

corresponding Layer 2 address. However, the process used by Inverse ARP differs for ARP on a

LAN. After the VC is up, each router announces its network layer address by sending an Inverse

ARP message over that VC. Figure shows how this works.

Inverse ARP is enabled by default

Static Frame Relay Mapping

You can statically configure the same mapping information instead of using Inverse

ARP. In a production network, you probably would just go ahead and use Inverse

ARP. For the exams, you need to know how to configure the static map command

statements

The broadcast keyword on the frame-relay map command is required when the

router needs to send broadcasts or multicasts to the neighbouring router—for

example, to support routing protocol messages such as Hellos.

Verifying Point-to-point Frame Relay

Output from EXEC Commands on Atlanta

Documents

CCNA24 Understanding Frame Realy Concepts