cisco ccna 2020

cisco ccna 2020.You may encounter troubleshooting problems related to this misconfiguration. Be sure to be able to troubleshoot failures caused by incorrectly sequenced ACL statements. To restrict access to an IOS device through a VTY, use the command access-classo On the exam, you are likely to be asked to configure an ACL and activate it on a router interface or VTY line.Be sure to be proficient in identifying the statements in the extended ACL that actually perform the filtering. On the exam, you may be required to configure and activate an extended ACL on a router to filter traffic traveling between two sites. You must know how to create ACL comments and how to use the Sequence ACL feature to edit existing ACLs. be comfortable using the show command to view and troubleshoot ACL configurations.Location of ACLs Keep in mind two rules about where you should place standard and extended ACLs.21.5 Authentication SummaryACLs can be used to filter traffic and routing information, restrict access to IOS devices over Telnet and SSH, set traffic priorities, trigger dial-on-demand routing (DDR) phone calls, and more. An ACL is a series of statements grouped together using numbers or names that specify whether traffic should be allowed to pass or prohibited from passing. ACLs can be applied in either the inbound or outbound direction. For inbound ACLs, they will be processed before any other processing of the packets; for outbound ACLs, the packets will be routed to the outbound interface before they are processed.Standard IPACLs allow you to filter based on source IP addresses, while Extended IPACLs allow you to filter based on source and destination IP addresses, IP protocols, and protocol information (such as port numbers). ACLs are processed in top-to-bottom order, and once a match is found, no other statements are processed. Therefore, the order of the statements is important. If no matching statement is found, the implicit reject rule is followed and the packet is discarded. In each direction on each interface, one ACLo can be applied for each protocol There are two special ACL filtering rules: traffic originating at the current router cannot be filtered, and an empty ACL applied on the interface will allow all traffic to pass.Standard ACLs can be used in the numbering ranges 1.99 and 1300.1999, while extended ACLs can be used in the numbering ranges 100.199 and 2000.2699. standard ACLs should be placed as close as possible to the target device, while extended ACLs should be placed as close as possible to the source device.To create a numbered ACL, use the command access-listo To activate an ACL on an interface, use the command ip access-groupo To filter Telnet traffic to and from the router, use the command access-class To activate a standard IPACLo on a VTY line When modifying an ACL, use the sequence ACL function to insert new statements or delete existing statements. To create a named ACL, use the command ip access-list (standard | extended}, which will switch to the appropriate sub-configuration mode.Wildcard masks allow you to match a single address, a range of addresses, or all addresses. A wildcard mask is equivalent to an inverted subnet mask, where a 0 means the corresponding bits must be the same, and a 1 means the corresponding bits are irrelevant. To convert a subnet mask to a wildcard mask, subtract 255 from the value of each byte in the subnet mask to obtain the value of the corresponding byte of the wildcard mask. In a standard ACL, if a wildcard mask is not specified, it defaults to 0.0.0.0.The command show ip interfaces displays all ACLOs active on the router interface The command show access-lists displays the ACLs configured on the router for all protocols, including the number of matches for each statement in the ACL. The command show ip access-lists displays only 25