Hello David
Hmm, I haven’t seen precedence values 6 and 7 being reserved. Values for the set ip precedence command range from 0 to 7, and values 6 and 7 correspond to “internetwork control” and “network control”. However, bits 6 and 7 of the Type of Service (ToS) bits are reserved for future use. Could that be what you mean?
OSPF packets are marked by default to ensure they receive priority handling by the network. Note that these markings are applied to control plane traffic (OSPF messages) and not data plane or user traffic. Even if QoS isn’t explicitly configured on network devices, many switches, and some newer routers already have some default settings for CoPP which helps to ensure that control plane traffic is given the appropriate priorities. As for the use of “CS6”, it’s a common marking for network control traffic such as OSPF messages.
Class Selector markings were created to ensure backward compatibility with older devices that only understand IPP. While it’s true that an IPP device would only check the first 3 left-most bits of the ToS byte, the Class Selector markings provide a convenient way to explicitly map the DSCP values to IPP values rather than relying simply on the reading of the first tree bits. This ensures that even if a packet is forwarded through a network that only supports IPP, the QoS information will not be completely lost.
To answer this question, let me try to give a definition of “integrated” vs “differentiated” services so that we can understand the difference and understand the specific definitions…
The “Integrated Services” term refers to integrating resource reservation into the network’s operation. This means that each flow explicitly reserves resources (bandwidth, buffer, etc.) along its path using a signaling protocol like RSVP.
The term “Differentiated Services” refers to differentiating traffic by class rather than treating each flow individually. Traffic is grouped and handled based on predefined policies, offering “differentiated” levels of service.
If you need any further clarification or if you would like to further discuss any of these responses, please let me know!
I hope this has been helpful!
Laz