Frame Relay and MPLS

download report

Transcript 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

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

Simple Frame Relay Network

Actual equipment involved in a frame relay
network



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



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


In order to provision a Frame Relay circuit
four items are needed
These four items will impact the cost of the
curcuit




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

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 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



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


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

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

In a fully meshed design frame relay reduces
the number of physical connections needed
vs. connections like T1s


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?


How many links are needs for a network with
three router for a fully meshed design?
3(3-1)/2 = 3



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



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




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

Three forms of LMI are available on Cisco
equipment
◦ Cisco
◦ Ansi
◦ Q933a

The DCE device determines the type of LMI
used

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 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



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





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



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
Frame Relay follows the designed path of the
PVC to each site




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




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