site_logo

Previous Next


  1. CCNA Exam Sim
  2. CCNA Network Sim
  3. CCNA ExamSim with Netsim

4.0 CCNA - IP Services

4.7 Forwarding per-hop behavior (PHB) for QoS such as classification, marking, queuing, congestion, policing, shaping

Forwarding Per-Hop Behavior (PHB) is a mechanism used in Quality of Service (QoS) to control the behavior of packets as they traverse a network. The following are some of the key components of PHB:

1. Classification: This is the process of categorizing network traffic into different groups based on specific criteria such as IP address, protocol, port, or application type.

2. Marking: This is the process of setting a marking or tag on a packet, indicating its priority level or class. Marking is typically done at the edge of the network and is used by the network devices to make QoS decisions.

3. Queuing: This is the process of holding packets in a queue and scheduling their transmission based on their priority. Queuing algorithms, such as Weighted Fair Queuing (WFQ), ensure that high-priority packets are transmitted before low-priority packets.

4. Congestion: This occurs when the network becomes congested and there is not enough bandwidth to handle all the traffic. PHB includes mechanisms, such as Random Early Detection (RED), to help manage congestion by discarding low-priority packets before high-priority packets.

5. Policing: This is the process of monitoring network traffic and enforcing specified traffic rate limits. If a packet exceeds the specified rate limit, it can be discarded or marked with a lower priority.

6. Shaping: This is the process of controlling the rate at which packets are transmitted into the network. This helps to ensure that the network does not become congested and that all packets are transmitted in a timely manner.

The combination of these components in PHB helps to ensure that network traffic is managed in a way that meets the needs of the different applications and users, while also maintaining network performance and stability.

Configuration example

A basic example of configuring Quality of Service (QoS) using Forwarding Per-Hop Behavior (PHB) on a Cisco IOS router:

1. Classify traffic:

class-map match-all HTTP
match access-group 101
!
access-list 101 permit tcp any any eq 80

In this example, we have created a class map named "HTTP" that matches all traffic matching access-group 101. Access-group 101 permits TCP traffic with destination port 80.

2. Mark traffic:

policy-map MARK-HTTP
class HTTP
set dscp af21
!

In this example, we have created a policy map named "MARK-HTTP" that marks all traffic matching the "HTTP" class with Differentiated Services Code Point (DSCP) value of "af21".

3. Apply the policy-map:

interface FastEthernet0/0
service-policy output MARK-HTTP
!

In this example, we have applied the "MARK-HTTP" policy map to FastEthernet0/0 interface for outbound traffic.

4. Configure queuing:

interface FastEthernet0/0
priority-queue out

In this example, we have enabled priority queuing on the FastEthernet0/0 interface for outbound traffic. This allows high-priority packets to be transmitted before low-priority packets.

In actual practice, one needs to understand the network communication requirements fully before implementation of Forwarding PHB.

Previous Next




simulationexams.com ad

certexams.com ad