1. Configuring Virtual Circuits
Module 3

2. Module Overview
After successfully completing this module, you should be able to:
Define the purpose of rate monitoring
Describe how an Ascend switch marks a frame as red, amber, or green
Describe the purpose of graceful discard
Configure a PVC
Monitor a PVC for proper operation
Manually define a PVC path
Move a PVC end point
Configure a Zero CIR PVC
In this module we discuss several issues related to the Frame Relay Protocol. These topics will help you understand the basics of Frame Relay and its advantages when used for wide are networking. Upon completion of this module you will be able to identify the following:
The relationship between Frame Relay switching over a Wide Area Network and Local Area Networking.
The characteristics of Frame Relay.
Virtual Circuit Service and the differences between a Permanent Virtual Circuit and a Switched Virtual Circuit .
Addressing for a Permanent Virtual Circuit.
The purpose and functions of Link Management for circuit management.
The formatting of each frame passing through the Frame Relay network.
Network management capabilities of Frame Relay.
Multiprotocol encapsulation in a Frame Relay network.

3. Virtual Circuit Service
Chicago
Orlando
New York
DLCI 50
DLCI 25
DLCI 20
A PVC is a logical connection from one access point to another
The Data Link Connection Identifiers (DLCIs) define the end points of the PVC
Data follows the same path between source and destination, so packets are always in sequence

4. Customer Contracts: Rate Monitoring
Rate monitoring done at ingress port
A customer enters into a contract with a provider for certain levels of service
The contract is implemented by monitoring the ingress port of the customer’s access line
Important elements of the contract include
Access Rate
Committed Information Rate (CIR)
Committed Burst Size (Bc)
Excess Burst Size (Be)

5. Monitoring Customer Ingress Ports
Bits
1536 Kb
Bursts of Traffic from CPE into FR Network
Bc +Be
DE Bit Set Be
Bc=CIR=512 T1
1 sec T2
1 sec T3
1 sec T0
Frame Relay traffic is typically “bursty,” with every transmission at line speed
Ingress port traffic is sampled in measured time intervals
On an Ascend switch, time interval is 1 second
In each interval, customer may exceed CIR or Be

6. Rate Enforcement on an Ascend Switch
(Bits)
Red Frames
DE Bit Set
Bc +Be Be
Amber Frames
Bc=CIR
Green Frames T0
Measurement Interval T1
On an Ascend switch, all frames at ingress port are categorized by color: green, amber, or red
CIR and Be are used as color boundaries
Color designator is stored in Frame Relay header
Color determines order of discard at a congested port

7. Graceful Discard Graceful discard is a PVC parameter
(Bits)
Red Frames
DE Bit Set
Bc +Be Be
Amber Frames
Bc=CIR
Graceful Discard
Green Frames T0 T1
Graceful discard is a PVC parameter
Applies to red frames received on ingress port
If Graceful Discard = ON (default), then red frames are accepted into network
If Graceful Discard = OFF, then red frames are discarded at ingress

8. Ascend’s VC Manager
VC Manager is software entity that supports the ability to
Add/delete PVCs
Activate/deactivate PVCs
Route/reroute PVCs
Generate “keep-alive” packets across idle PVCs
Maintain PVC traffic statistics
CP holds master VC table, each IOP contains table of PVCs that begin/end on slot
An entry in the VC table contains
Description of the VC path (switch/Lport pairs)
Forward/reverse bandwidth values

9. PVC Formation When a PVC is provisioned, the following occurs
Switch with the higher internal IP address initiates the setup
VC manager on IOP requests a path from OSPF
OSPF returns path string (string of switch/Lport pairs)
Calling IOP issues a Call Request PDU (Protocol Data Unit)
Call Request routed through network
Called IOP responds with Confirm PDU
PVC becomes active

10. Adding a PVC
1. From map window, choose Administer => Cascade Parameters => Set All Circuits => Point-to-Point
2. Click on Add… to select logical end points
Select logical end points

11. Setting PVC Attributes
Every PVC has three three categories of
configuration parameters: 1) Administrative
2) Traffic Type
3) User Preference

12. PVC Administrative Attributes
Define DLCI end points
Declare a circuit name

13. PVC Traffic Type Attributes
Set CIR, Be, Bc

14. PVC User Preference Attributes
Set graceful discard to desired setting
(default = On)

15. PVC Templates
Creates template containing this PVC’s administrative, traffic type, and user preference attributes
Expedites PVC configuration

16. PVC Traffic Statistics (1)
1. From map window, choose Administer => Cascade Parameters => Set All Circuits => Point-to-Point
2. Select a circuit
3. Click on Statistics button
Key to PVC end points
Router
Router
Tx (A)
(PVC)
Tx (B)
Rx (A)
Rx (B)

17. PVC Traffic Statistics (2)
Discards typically due to rate monitoring
Indicator of congestion in FR network
PVC utilization may exceed 100%

18. PVC QoS Statistics Packets lost between endpoint Lports
Cumulative trunk delay

19. Manually Defined PVCs 1. This PVC has already been defined
3. Build new path by selecting each trunk segment and adding to path
2. Existing path of PVC
(found by OSPF)

20. Moving Circuit End Points
From map window, choose Administer => Cascade Parameters => Set All Circuits => Move Circuit Endpoint
Existing Circuit End Point
New Circuit End Point

21. Zero CIR PVCs When set to On: - CIR/Bc fixed at 0
- Be set to access rate
- All frames in tagged amber

22. Module Summary
A Permanent Virtual Circuit is a logical connection whose end points are Lports
Rate monitoring parameters such as CIR, Bc, and Be determine the bandwidth requirements of a PVC
All frames on an Ascend switch are classified as green, amber, or red
A PVC has three levels of configuration attributes: Administrative, Traffic Type, and User Preferences
A PVC can be moved by changing the location of its end points