CCNA 2 v6 RSE Chapter 7 Exam Online

An outbound ACL should be utilized when the same ACL filtering rules will be applied to packets coming from more than one inbound interface before exiting a single outbound interface. The outbound ACL will be applied on the single outbound interface.

The only filter that can be applied with a standard ACL is the source IP address. An extended ACL can use multiple criteria to filter traffic, such as source IP address, destination IP address, type of traffic, and type of message.

With an inbound ACL, incoming packets are processed before they are routed. With an outbound ACL, packets are first routed to the outbound interface, then they are processed. Thus processing inbound is more efficient from the router perspective. The structure, filtering methods, and limitations (on an interface, only one inbound and one outbound ACL can be configured) are the same for both types of ACLs.

When a packet comes into a router that has an ACL configured on the interface, the router compares the condition of each ACE to determine if the defined criteria has been met. If met, the router takes the action defined in the ACE (allows the packet through or discards it). If the defined criteria has not been met, the router proceeds to the next ACE. An implicit deny any statement is at the end of every standard ACL.

The ACL statement access-list 10 permit 192.168.16.0 0.0.3.255 will match all four network prefixes. All four prefixes have the same 22 high order bits. These 22 high order bits are matched by the network prefix and wildcard mask of 192.168.16.0 0.0.3.255.

To permit or deny one specific IP address, either the wildcard mask 0.0.0.0 (used after the IP address) or the wildcard mask keyword host (used before the IP address) can be used.

In calculating how many ACLs can be configured, use the rule of “three Ps”: one ACL per protocol, per direction, per interface. In this case, 2 interfaces x 2 protocols x 2 directions yields 8 possible ACLs.

Extended ACLs should be placed as close as possible to the source IP address, so that traffic that needs to be filtered does not cross the network and use network resources. Because standard ACLs do not specify a destination address, they should be placed as close to the destination as possible. Placing a standard ACL close to the source may have the effect of filtering all traffic, and limiting services to other hosts. Filtering unwanted traffic before it enters low-bandwidth links preserves bandwidth and supports network functionality. Decisions on placing ACLs inbound or outbound are dependent on the requirements to be met.

Standard access lists only filter on the source IP address. In the design, the packets would be coming from the 192.168.10.96/27 network (the R2 G0/0 network). The correct ACL is access-list 1 permit 192.168.10.96 0.0.0.31.

Because standard access lists only filter on the source IP address, they are commonly placed closest to the destination network. In this example, the source packets will be coming from the R2 G0/0 network. The destination is the R1 G0/1 network. The proper ACL placement is outbound on the R1 G0/1 interface.

The show access-lists command shows how many packets have met the criteria for each ACE in terms of a specific number of “matches.”

The show access-lists command is only relevant to traffic passing through the router on which the ACL is configured.

Named ACL ACEs can be removed using the no command followed by the sequence number.

Administrative access over SSH to the router is through the vty lines. Therefore, the ACL must be applied to those lines in the inbound direction. This is accomplished by entering line configuration mode and issuing the access-class command.

Access to vty lines can be filtered with an ACL and applied using the access-class in command.

The access-class command is used only on VTY ports. VTY ports support Telnet and/or SSH traffic. The match permit ACE is how many attempts were allowed using the VTY ports. The match deny ACE shows that a device from a network other than 192.168.10.0 was not allowed to access the router through the VTY ports.

Because the new deny ACE is a host address that falls within the existing 172.16.0.0 network that is permitted, the router rejects the command and displays an error message. For the new deny ACE to take effect, it must be manually configured by the administrator with a sequence number that is less than 10.

Because of the implicit deny at the end of all ACLs, the access-list 1 permit any command must be included to ensure that only traffic from the 172.16.4.0/24 subnet is blocked and that all other traffic is allowed.​

After a reboot, access list entries will be renumbered to allow host statements to be listed first and thus more efficiently processed by the Cisco IOS.​

Because there is a deny any ACE at the end of every standard ACL, the effect of having all deny statements is that all traffic will be denied regardless of the direction in which the ACL is applied.

ACEs are commonly reordered from the way they were entered by the network administrator. The ACEs that have host criteria such as in the statement permit host 192.168.10.5, are reordered as the first statements because they are the most specific (have the most number of bits that must match).

Always test an ACL to ensure that it performs as it was designed. Applying an ACL that is applied using the ip access-group in command instead of using the ip access-group out command is not going to work as designed.

The clear access-list counters command is used to reset all numbers relating to ACE match conditions that have been made within a particular ACE. The command is useful when troubleshooting an ACL that has recently been deployed.

Converting the wildcard mask 0.0.3.255 to binary and subtracting it from 255.255.255.255 yields a subnet mask of 255.255.252.0.
Using the host parameter in a wildcard mask requires that all bits match the given address.
192.168.15.65 is the first valid host address in a subnetwork beginning with the subnetwork address 192.168.15.64. The subnet mask contains 4 host bits, yielding subnets with 16 addresses.
192.168.15.144 is a valid subnetwork address in a similar subnetwork. Change the wildcard mask 0.0.0.15 to binary and subtract it from 255.255.255.255, and the resulting subnet mask is 255.255.255.240.
192.168.3.64 is a subnetwork address in a subnet with 8 addresses. Convert 0.0.0.7 to binary and subtract it from 255.255.255.255, and the resulting subnet mask is 255.255.255.248. That mask contains 3 host bits, and yields 8 addresses.