This section contains the following topics:
J-series routers have eight queues built into the hardware. If a classifier does not assign a packet to any other queue (for example, for other than well-known DSCPs that have not been added to the classifier), the packet is assigned by default to the class associated with queue 0.
Table 134 shows the four default forwarding classes and queues that Juniper Networks classifiers assign to packets based on the DSCP values in arriving packet headers.
Table 134: Default Forwarding Class Queue Assignments
|
Forwarding Class |
Forwarding Queue |
|
best-effort |
queue 0 |
|
expedited-forwarding |
queue 1 |
|
assured-forwarding |
queue 2 |
|
network-control |
queue 3 |
Because the Services Router supports up to eight queues, you can configure two queues for each forwarding class, one with high loss priority and one with low loss priority.
Each forwarding class has an associated scheduler priority. Only two forwarding classes, best-effort and network-control (queue 0 and queue 3), are used in the JUNOS default scheduler configuration.
By default, the best-effort forwarding class (queue 0) receives 95 percent of the output link bandwidth and buffer space, and the network-control forwarding class (queue 3) receives 5 percent of the output link bandwidth and buffer space. The default drop profile causes the buffer to fill and then discard all packets until it again has space.
The expedited-forwarding and assured-forwarding classes have no schedulers, because by default no resources are assigned to queue 1 and queue 2. However, you can manually configure resources for expedited-forwarding and assured-forwarding.
The default scheduler settings are implicit in the configuration, although they do not appear in the output of the show class-of-service command.
- [edit class-of-service]
- schedulers {
-
- network-control {
- transmit-rate percent 5;
- buffer-size percent 5;
- priority low;
- drop-profile-map loss-priority any protocol any;
- drop-profile terminal;
- }
-
- best-effort {
- transmit-rate percent 95;
- buffer-size percent 95;
- priority low;
- drop-profile-map loss-priority any protocol any;
- drop-profile terminal;
- }
- }
- drop-profiles {
-
- terminal {
-
-
- fill-level 100 drop-probability 100;
- }
- }
Table 135 shows the forwarding class and packet loss priority (PLP) that are assigned by default to each well-known DSCP. Although several DSCPs map to the expedited-forwarding (ef) and assured-forwarding (af) classes, by default no resources are assigned to these forwarding classes. All af classes other than af1x are mapped to best-effort, because RFC 2597, Assured Forwarding PHB Group, prohibits a node from aggregating classes. Assignment to best-effort implies that the node does not support that class.
You can modify the default settings through configuration. For instructions, see Configuring Class of Service with DiffServ.
Table 135: Default Behavior Aggregate (BA) Classification
Typically, a router rewrites the DSCPs in outgoing packets once, when packets enter the DiffServ portion of the network, either because the packets do not arrive from the customer with the proper DSCP bit set or because the service provider wants to verify the that customer has set the DSCP properly. CoS implementations that accept the DSCP and classify and schedule traffic solely on DSCP value perform behavior aggregate (BA) DiffServ functions and do not usually rewrite the DSCP. DSCP rewrites typically occur in multifield (MF) DiffServ scenarios.
For instructions for configuring rewrite rules, see Configuring and Applying Rewrite Rules (Required).
Table 136 shows the router forwarding classes associated with each well-known DSCP code point and the resources assigned to their output queues for a sample DiffServ CoS implementation. This example assigns expedited forwarding to queue 1 and a subset of the assured forwarding classes (af1x) to queue 2, and distributes resources among all four forwarding classes.
Other DiffServ-based implementations are possible. For configuration information, see Configuring Class of Service with DiffServ.
Table 136: Sample BA Classification Forwarding Classes and Queues