CSIS 4823Data Communications

Networking – Frame Relay and MPLS

Mr. Mark Welton

WAN transportation method that formats data into frames and sent over a network controlled by a service provider

Frame Relay is often represented in network diagrams with a cloud depiction, representing an unknown environment

Frame Relay uses VC (virtual circuits) through the cloud to allow delivery to endpoints

The endpoints appear as directly connected circuits

Frame Relay

Two types of VCs◦ Permanent (PVC)

Circuit is always up Path is “hard coded” through the provider’s system

◦ Switched (SVC) Are on-demand circuits Path is created through the provider’s system when

used

Frame Relay

Simple Frame Relay Network

Frame Relay

Actual equipment involved in a frame relay network

Frame Relay

Each VC is given a Layer-2 address called a Data Link Control Identifier (DLCI)

DLCIs are only visible to the customer and the service provider

Other customers of the service provider do not see the DLCIs or other customer data even though the Frame Relay network is shared

Frame Relay

The primary benefits of Frame Relay are cost and flexibility

Dedicated point-to-point circuits (like T1s) are priced based on distance between the locations

Frame Relay is priced based on the components of the Frame Relay circuit

Frame Relay

In order to provision a Frame Relay circuit four items are needed

These four items will impact the cost of the curcuit

Frame Relay

Location for the circuit Port speed – is the physical size of the

circuit, such as a T1 or T3 CIR – Committed Information Rate

◦This is the guaranteed bandwidth allocated by the service provider in bps

Burst Rate – is the amount of additional bandwidth available on the port

Frame Relay

Burst Rate – is the amount of additional bandwidth available on the port◦ Typically this is ordered at 2 times the CIR or

the full port speed◦ If the CIR is exceeded but not the burst rate the

frames are marked as Discard Eligible (DE)◦ This means that if the frame relay switch

becomes congested they will be dropped◦ If the frames exceed the burst rate they are

automatically dropped

Frame Relay

Frame Relay

Frame Relay allows for multiple links to multiple locations to terminate on the same physical circuit and router interface

common designs for frame relay◦ Hub and spoke◦ Partial mesh◦ Fully mesh

Frame Relay

All circuits are terminated to a central location (usual the data center)

Branch sites can not communicate with each other without going through the central router

Design is commonly used when branches to not need to communication directly

Hub and Spoke

Similar to spoke and hub but some branch site will also have circuits to each other

If two or more branch sites need to communicate to each other often a frame relay circuit is added directly between them

Partial mesh

If an addition router was added to the right design and only had a circuit to router A I would consider it a partial mesh not spoke and hub

Spoke and Hub and Partial Mesh Design

In a fully meshed design frame relay reduces the number of physical connections needed vs. connections like T1s

Fully Meshed Design

To determine the number of links needed for a fully meshed network the formula N(N-1)/2 is used where N is the number of nodes (routers) in the network

How many links are needs for a network with three router for a fully meshed design?

Fully Meshed Design

How many links are needs for a network with three router for a fully meshed design?

3(3-1)/2 = 3

Fully Meshed Design

How many links are needs for a network with six router for a fully meshed design? 6(6-1)/2 = 15

Using T1s you would need 15 T1 In Frame Relay you would need 6 Frame

Relay circuits with 15 DLCIs

Fully Meshed Design

Since multiple locations can terminate on the same physical circuit, oversubscription is possible

Oversubscription occurs when the amount of bandwidth provisioned in all the CIRs exceeds the port speed of the circuit

CIR cannot be guaranteed if it exceeds the amount of bandwidth physically available

Oversubscription

LMI – (Local Management Interface) provides communication between the

Data Terminal Equipment (DTE) – or customer equipment like routers, and the

Data Communication Equipment (DCE) – or service provider equipment

LMI provides an exchange of status messages regarding the VCs

Frame Relay

Three forms of LMI are available on Cisco equipment◦ Cisco◦ Ansi◦ Q933a

The DCE device determines the type of LMI used

Frame Relay

Three statuses of PVCs◦ Active – normal good status for PVC◦ Inactive – indicates the service provider has

configured a PVC, but the customer equipment is not configured for that DLCI

◦ Deleted – indicates the customer equipment is configured for a DLCI, but the PVC does not exist from the service provider

Frame Relay

Frame Relay networks will detect congestion and mark frames with Forward Explicit Congestion Notification (FECNs) and Backward Explicit Congestion Notifications (BECNs)

They are sent to the DTE equipment DTE equipment can adjust the flow of traffic

to reduce congestion on the network The DCE equipment does not perform flow

control for customers

Frame Relay

The FECN bits are marked when congestion occurs

When FECN bits are seen in the frames, the BECN bits get marked

Now traffic leaving and coming from a Frame Relay switch is notified of the congestion

Frame Relay

MPLS – Multi-Protocol Label Switching networks are another common WAN network

Like Frame Relay network diagrams represent them as a cloud

packets in an MPLS network are prefixed with an MPLS header (called a label stack)

The header contains one or more labels, a traffic-class field (used for quality of service [QoS]), a bottom-of-stack flag, and

an 8-bit time-to-live (TTL) field

MPLS

The label stack is the only thing examined by the MPLS switches

no traditional routing table lookups are required, which in theory makes this a much faster solution than more traditional IP-based solutions

The MPLS header allows MPLS networks to be shared among multiple customers, without the customers seeing each other’s traffic

MPLS

Multisite Frame Relay Design

Frame Relay follows the designed path of the PVC to each site

MPLS Design

From the telecom view MPLS uses switching in the cloud to move the data to the next hop

For the customer view an IP routing protocol provides a next hop for the destination IP address

traditional Interior Gateway Protocols (IGPs) like RIP, OSPF, and EIGRP do not work well over MPLS networks

BGP is the most common protocol in use to communicate to customer endpoints

How Does it Work?

So MPLS is then a Layer-3 protocol, not really

So it must be Layer-2 protocol, not completely

It is like layer-2.5 If you could make Layer-2 that had routing

intelligence without the table overhead and processing delay and fixed problems small fixed cell size ATM adds you would be closer to MPLS

How Does it Work?