LevelOne GEL-1061, Руководство пользователя

Страница: 262 / 500

Chapter 12
| Security Measures
Access Control Lists
– 262 –
possible depends on too many factors to be precisely determined. It depends
on the amount of hardware resources reserved at runtime for this purpose.
Auto ACE Compression is a software feature used to compress all the ACEs of an
ACL to utilize hardware resources more efficiency. Without compression, one
ACE would occupy a fixed number of entries in TCAM. So if one ACL includes 25
ACEs, the ACL would need (25 * n) entries in TCAM, where “n” is the fixed
number of TCAM entries needed for one ACE. When compression is employed,
before writing the ACE into TCAM, the software compresses the ACEs to reduce
the number of required TCAM entries. For example, one ACL may include 128
ACEs which classify a continuous IP address range like 192.168.1.0~255. If
compression is disabled, the ACL would occupy (128*n) entries of TCAM, using
up nearly all of the hardware resources. When using compression, the 128 ACEs
are compressed into one ACE classifying the IP address as 192.168.1.0/24,
which requires only “n” entries in TCAM. The above example is an ideal case for
compression. The worst case would be if no any ACE can be compressed, in
which case the used number of TCAM entries would be the same as without
compression. It would also require more time to process the ACEs.
◆
If no matches are found down to the end of the list, the traffic is denied. For this
reason, frequently hit entries should be placed at the top of the list. There is an
implied deny for traffic that is not explicitly permitted. Also, note that a single-
entry ACL with only one deny entry has the effect of denying all traffic. You
should therefore use at least one permit statement in an ACL or all traffic will be
blocked.
Because the switch stops testing after the first match, the order of the
conditions is critical. If no conditions match, the packet will be denied.
The order in which active ACLs are checked is as follows:
1.
User-defined rules in IP and MAC ACLs for ingress or egress ports are checked in
parallel.
2.
Rules within an ACL are checked in the configured order, from top to bottom.
3.
If the result of checking an IP ACL is to permit a packet, but the result of a MAC
ACL on the same packet is to deny it, the packet will be denied (because the
decision to deny a packet has a higher priority for security reasons). A packet
will also be denied if the IP ACL denies it and the MAC ACL accepts it.
Showing
TCAM Utilization
Use the Security > ACL (Configure ACL - Show TCAM) page to show utilization
parameters for TCAM (Ternary Content Addressable Memory), including the
number policy control entries in use, the number of free entries, and the overall
percentage of TCAM in use.
Command Usage
Policy control entries (PCEs) are used by various system functions which rely on
rule-based searches, including Access Control Lists (ACLs), IP Source Guard filter