422 | Quality of Service (QoS)
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Create a Layer 2 Class Map
All class maps are Layer 3 by default; you can create a Layer 2 class map by specifying the option layer2
with the
class-map command. A Layer 2 class map differentiates traffic according to the 802.1p value and/
or characteristics defined in a MAC ACL.
1. To create a match-any class map, use the
class-map match-any command or to create a match-all class
map, use the
class-map match-all command from CONFIGURATION mode, and enter the keyword
layer2.
2. After you create a class-map, FTOS places you in CLASS MAP mode. From this mode, specify your
match criteria using the
match mac command. Match-any class maps allow up to five access-lists.
Match-all class-maps allow only one access list. You can match against only one VLAN ID.
3. After you specify your match criteria, link the class-map to a queue using the
service-queue command
from POLICY MAP mode.
Determine the Order in Which You Use ACLs to Classify Traffic
When you link class-maps to queues using the service-queue command, FTOS matches the class-maps
according to queue priority (queue numbers closer to 0 have lower priorities). For example, in Figure 24-7,
class-map cmap2 is matched against ingress packets before cmap1.
ACLs acl1 and acl2 have overlapping rules because the address range 20.1.1.0/24 is within 20.0.0.0/8.
Therefore, (without the keyword
order) packets within the range 20.1.1.0/24 match positive against cmap1
and are buffered in queue 4, though you intended for these packets to match positive against cmap2 and be
buffered in queue 1.
In cases such as these, where class-maps with overlapping ACL rules are applied to different queues, use
the keyword
order to specify the order in which you want to apply ACL rules (Figure 24-7). The order can
range from 0 to 254. FTOS writes to the content addressable memory (CAM) ACL rules with lower order
numbers (order numbers closer to 0) before rules with higher order numbers so that packets are matched as
you intended. By default, all ACL rules have an order of 254.
Set DSCP Values for Egress Packets Based on Flow
Match-any Layer 3 flows may have several match criteria. All flows that match at least one of the match
criteria are mapped to the same queue because they are in the same class map. Setting a DSCP value from
QOS-POLICY-IN mode (refer to Set a DSCP Value for Egress Packets on page 424) assigns the same
DSCP value to all of the matching flows in the class-map. The flow-based DSCP marking feature allows
you to assign different DSCP to each match criteria CLASS-MAP mode using the
set-ip-dscp option with
the
match command so that matching flows within a class map can have different DSCP values
(Figure 24-8). The values you set from CLASS-MAP mode override the QoS input policy DSCP value,
and packets matching the rule are marked with the specified value.
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