Reference Guide

L3 ACL rules
Use ACL commands for L3 packet filtering. TCP packets from host 10.1.1.1 with the TCP destination port equal to 24 are
permitted, and all others are denied.
TCP packets that are first fragments or non-fragmented from host 10.1.1.1 with the TCP destination port equal to 24 are
permitted, and all TCP non-first fragments from host 10.1.1.1 are permitted. All other IP packets that are non-first fragments are
denied.
Permit ACL with L3 information only
If a packet’s L3 information matches the information in the ACL, the packet's fragment offset (FO) is checked:
● If a packet's FO > 0, the packet is permitted
● If a packet's FO = 0, the next ACL entry processes
Deny ACL with L3 information only
If a packet's L3 information does not match the L3 information in the ACL, the packet's FO is checked:
● If a packet's FO > 0, the packet is denied
●
If a packet's FO = 0, the next ACL line processes
Permit all packets from host
OS10(config)# ip access-list ABC
OS10(conf-ipv4-acl)# permit tcp host 10.1.1.1 any eq 24
OS10(conf-ipv4-acl)# deny ip any any fragment
Permit only first fragments and non-fragmented packets from host
OS10(config)# ip access-list ABC
OS10(conf-ipv4-acl)# permit tcp host 10.1.1.1 any eq 24
OS10(conf-ipv4-acl)# permit tcp host 10.1.1.1 any fragment
OS10(conf-ipv4-acl)# deny ip any any fragment
To log all packets denied and to override the implicit deny rule and the implicit permit rule for TCP/ UDP fragments, use a similar
configuration. When an ACL filters packets, it looks at the FO to determine whether it is a fragment:
● FO = 0 means it is either the first fragment or the packet is a non-fragment
● FO > 0 means it is the fragments of the original packet
Assign sequence number to filter
IP ACLs filter on source and destination IP addresses, IP host addresses, TCP addresses, TCP host addresses, UDP addresses,
and UDP host addresses. Traffic passes through the filter by filter sequence. Configure the IP ACL by first entering IP
ACCESS-LIST mode and then assigning a sequence number to the filter.
User-provided sequence number
● Enter IP ACCESS LIST mode by creating an IP ACL in CONFIGURATION mode.
ip access-list access-list-name
Access Control Lists
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