Life of a Low Priority FTP Packet

CoS=0 DSCP=0

CoS=0 DSCP=0

The low-priority FTP packet begins life as a very low-priority CoS = 0, DSCP = 0.

Note The FTP packet is using TCP rather than UDP (which was used by the voice packet).

CoS=0 DSCP=0

CoS=0 DSCP=0

All traffic arriving from the workstation attached to the IP Phone is set to be untrusted.

As a result, the IP Phone will not accept any CoS marking done by the workstation and will remark the CoS to 0. This ensures that the voice traffic generated by the IP Phone will always receive priority treatment over any traffic generated by the workstation.

Note The IP Phone cannot re-mark the incoming DSCP value. The IP Phone can only re-mark the

Layer 2 CoS value.

Default CoS-to-DSCP mapping, CoS 0 = DSCP 0; CoS 0 will use one of the WRR Queues.

Default CoS-to-DSCP mapping, CoS 0 = DSCP 0; CoS 0 will use one of the WRR Queues.

In the Cisco Catalyst 2950 switch, the CoS-to-DSCP mapping table is used to map the CoS value of the packet to the DSCP equivalent. In the case of the FTP frame, the DSCP = 0 matches the CoS = 0, so the frame DSCP value does not change.

The switch congestion management technology— weighted round robin (WRR) with an expedite queue (priority queue [PQ])—dispatches the frame, but not until all high-priority voice frames have been dispatched.

Note WRR with an expedite queue PQ is explained further in the "Congestion Management"

module in this course.

Default CoS-to-DSCP Mapping in Cisco Catalyst 2950 Switch

COS Value

DSCP Value

0

0

1

8

2

16

3

24

4

32

5

40

6

48

7

56

NBAR, class/remark, FTP = AF11

CBWFQ — AF11 gets minimum bandwidth guarantee

NBAR, class/remark, FTP = AF11

CBWFQ — AF11 gets minimum bandwidth guarantee

In the enterprise router, a classification technology, NBAR, recognizes the packet as an FTP packet and re-marks the packet with a DSCP = "001010" = AF11.

With a DSCP = AF11, the packet is then dispatched as a low-priority class packet by CBWFQ. CBWFQ is the component of LLQ that carefully manages the dispatch of data traffic.

The AF11 class is given a minimum guarantee of bandwidth. If the link to the service provider is congested, the packet has a good probability of being dropped to ensure that higher-priority packets are not delayed.

The service provider provides QoS SLA for the controlled-latency, controlled-load 1 (AF31), controlled-load 2 (AF21), and Best-Effort traffic classes. The service provider maps the enterprise customer QoS classifications into the four defined traffic classes of the service provider.

Upon arriving at the service provider network, the packet is identified as an FTP packet and assigned to the controlled-load 2 (AF21) class.

DSCP-based WRED

DSCP-based WRED

The packet traverses the service provider core marked as a controlled-load 2 (AF21) packet.

While in the service provider core network, the FTP packet has a much better probability of being dropped by WRED than the voice packet.

Before the packet enters the service provider network, it is marked DSCP = AF11, which fits the classification scheme used by the enterprise customer. As the packet leaves the service provider network, the packet is re-marked to DSCP = AF11 for the enterprise customer.

The AF11 class is given a minimum guarantee of bandwidth.

As the packet reenters the enterprise network, it is recognized as an AF11 class packet and is passed through the enterprise router without being re-marked.

The FTP packet is sent out the LAN interface toward the switch using FIFO queuing on the LAN interface.

Using its DSCP-to-CoS mapping, the Cisco Catalyst 2950 switch recognizes the DSCP = AF11 packet (Layer 3) as a CoS = 1 priority frame (Layer 2).

The FTP frame is treated by WRR with the CoS 1.

The FTP packet finally arrives at the destination host, and the payload is delivered.

If the packet had been dropped at any point along the way, TCP would recognize that fact and request retransmission of the packet.

Was this article helpful?

0 0

Post a comment