IPv6 Configuration Guide K/KA/KB.15.15

ACL configuration factors
The sequence of entries in an ACL is significant
When the switch uses an ACL to determine whether to permit or deny a packet, it compares the
packet to the criteria specified in the individual ACEs in the ACL, beginning with the first ACE in
the list and proceeding sequentially until a match is found. When a match is found, the switch
applies the indicated action (permit or deny) to the packet. This is significant because, once a
match is found for a packet, subsequent ACEs in the same ACL are not applied to that packet,
regardless of whether they match the packet.
Example 65 ACE that permits all IPv6 traffic not implicitly denied
Suppose that you have applied the ACL shown in Example 65 (page 113) to inbound
IPv6 traffic on VLAN 1 (the default VLAN):
ipv6 access-list "Sample-List-2"
10 deny ipv6 2001:db8::235:10
1
/128
2
::/0
3
20 deny ipv6 2001:db8::245:89/128 ::/0
30 permit tcp 2001:db8::18:100/128 2001:db8::237:1/128
40 deny tcp 2001:db8::18:100/128 ::/0
50 permit ipv6 ::/0 ::/0
4
(Implicit deny ipv6 any any)
5
exit
1
Source Address
2
Prefix length
3
Destination Address and Prefix Length (Specifies Any IPv6 Destination)
4
5
After the last explicit ACE there is always an Implicit Deny. However, in this case it will not
be used because the last permit ipv6 ACL allows all IPv6 packets that earlier ACEs have not
already permitted or denied.
ActionLine #
Shows IP type (IPv6) and ID (Sample-List-2).N/A
A packet from source address 2001:db8:235:10 will be denied (dropped). This ACE filters out all packets
received from 2001:db8:235:10. As a result, IPv6 traffic from that device will not be allowed, and packets
from that device will not be compared against any later entries in the list.
10
A packet from IPv6 source address 2001:db8::245:89 will be denied (dropped). This ACE filters out all packets
received from 2001:db8::245:89. As the result, IPv6 traffic from that device will not be allowed, and packets
from that device will not be compared against any later entries in the list.
20
A TCP packet from SA 2001:db8::18:100 with a DA of 2001:db8::237:1 will be permitted (forwarded).
Since no earlier ACEs in the list have filtered TCP packets from 2001:db8::18:100 with a destination of
30
2001:db8::237:1, the switch will use this ACE to evaluate such packets. Any packets that meet this criteria
will be forwarded. (Any packets that do not meet this TCP source-destination criteria are not affected by this
ACE.)
A TCP packet from source address 2001:db8::18:100 to any destination address will be denied (dropped).
Since, in this example, the intent is to block TCP traffic from 2001:db8::18:100 to any destination except the
40
destination stated in the ACE at line 30, this ACE must follow the ACE at line 30. (If their relative positions
were exchanged, all TCP traffic from 2001:db8::18:100 would be dropped, including the traffic for the
2001:db8::237:1 destination.)
Configuring and assigning an IPv6 ACL 113