Guidelines For Configuring Acls On The Catalyst 2950 Switches - Cisco Catalyst 2950 Software Manual

Chapter 12 Configuring Network Security with ACLs
A mask can be a combination of either multiple Layer 3 and Layer 4 fields or of multiple Layer 2 fields.
Note
Layer 2 fields cannot be combined with Layer 3 or Layer 4 fields.
There are two types of masks:
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Note
The Catalyst 2950 switch ACL configuration is consistent with other Cisco Catalyst switches. However,
there are significant restrictions as well as differences for ACL configurations on the Catalyst 2950
switches.

Guidelines for Configuring ACLs on the Catalyst 2950 Switches

These configuration guidelines apply to ACL filters:
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User-defined mask—masks that are defined by the user.
System-defined mask—these masks can be configured on any interface:
Switch (config-ext-nacl)# permit tcp any any
Switch (config-ext-nacl)# deny tcp any any
Switch (config-ext-nacl)# permit udp any any
Switch (config-ext-nacl)# deny udp any any
Switch (config-ext-nacl)# permit ip any any
Switch (config-ext-nacl)# deny ip any any
Switch (config-ext-nacl)# deny any any
Switch (config-ext-nacl)# permit any any
In an IP extended ACL (both named and numbered), a Layer 4 system-defined mask cannot
precede a Layer 3 user-defined mask. For example, a Layer 4 system-defined mask such as
permit tcp any any or deny udp any any cannot precede a Layer 3 user-defined mask such as
permit ip 10.1.1.1 any. If you configure this combination, the ACL is not configured. All other
combinations of system-defined and user-defined masks are allowed in security ACLs.
Only one ACL can be attached to an interface. For more information, refer to the ip access-group
interface command in the Catalyst 2950 Desktop Switch Command Reference.
All ACEs in an ACL must have the same user-defined mask. However, ACEs can have different rules
that use the same mask. On a given interface, only one type of user-defined mask is allowed, but you
can apply any number of system-defined masks. For more information on system-defined masks, see
the "Understanding Access Control Parameters" section on page
This example shows the same mask in an ACL:
Switch (config)#ip access-list extended acl2
Switch (config-ext-nacl)# permit tcp 10.1.1.1 0.0.0.0 any eq 80
Switch (config-ext-nacl)# permit tcp 20.1.1.1 0.0.0.0 any eq 23
In this example, the first ACE permits all the TCP packets coming from the host 10.1.1.1 with a
destination TCP port number of 80. The second ACE permits all TCP packets coming from the host
20.1.1.1 with a destination TCP port number of 23. Both the ACEs use the same mask; therefore, a
Catalyst 2950 switch supports this ACL.
Only four user-defined masks can be defined for the entire system. These can be used for either
security or quality of service (QoS) but cannot be shared by QoS and security. You can configure as
many ACLs as you require. However, a system error message appears if ACLs with more than four
different masks are applied to interfaces. For more information on error messages, see
"Error Messages for Security and QoS Configurations."
Catalyst 2950 Desktop Switch Software Configuration Guide
Understanding ACLs
12-4.
Appendix A,
12-5