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Adjacent Node
Networking, explained. No BS.

ACLs And Policy

What It Is

An access control list matches packets and takes an action. On routers and switches, ACLs are used for interface filtering, control-plane protection, route filtering, NAT matching, QoS classification, and management access. The modern goal is not just syntax correctness. The goal is policy that is readable, scoped, logged appropriately, and tied to intent.

ACL Types

Type Matches Common Use
Standard IPv4 ACL Source IPv4 address Simple source filtering, route policy, NAT match
Extended IPv4 ACL Protocol, source, destination, ports, flags, options Traffic filtering and classification
Named ACL Human-readable name Preferred for maintainability
Numbered ACL Legacy number ranges Still seen in older configs
IPv6 ACL IPv6 source/destination and next-header fields IPv6 filtering
Object-group policy Groups addresses and services Firewalls and some IOS-XE platforms

Modern note: Use named ACLs with remarks for operational policy. Numbered ACLs are harder to review and maintain.

Matching Basics

Element Example Notes
Action permit, deny First match wins
Protocol ip, tcp, udp, icmp ip means all IPv4 protocols
Source 10.10.20.0 0.0.0.255 Wildcard mask, not subnet mask
Destination host 10.10.30.10 host equals wildcard 0.0.0.0
Port operator eq, neq, lt, gt, range TCP/UDP only
Logging log, log-input Use carefully on busy paths
Time range time-range NAME Operationally risky if undocumented
TCP flags established, syn, ack, rst Platform support varies

Watch out: ACLs normally have an implicit deny at the end. If you do not permit it, it is denied.

Wildcard Masks

Subnet Wildcard Matches
/32 0.0.0.0 One host
/30 0.0.0.3 Four addresses
/29 0.0.0.7 Eight addresses
/28 0.0.0.15 Sixteen addresses
/24 0.0.0.255 256 addresses
/16 0.0.255.255 65,536 addresses
/8 0.255.255.255 16,777,216 addresses

Wildcard math: subtract the subnet mask from 255.255.255.255.

Placement And Direction

Decision Guidance
Inbound Filters before routing decision on that interface
Outbound Filters after routing decision toward egress interface
Close to source Good for stopping unwanted traffic early
Close to destination Useful when policy protects a specific destination
Interface ACL Transit traffic through interface
VTY ACL Management access to device
Control-plane policy Traffic to the device CPU

Design note: Document whether the ACL protects users, servers, management, control plane, NAT eligibility, or route policy. Same syntax, very different intent.

Cisco IOS/IOS-XE Examples

Readable extended ACL:

ip access-list extended USERS-TO-SERVERS
 remark Allow HTTPS to app server
 permit tcp 10.10.20.0 0.0.0.255 host 10.10.30.50 eq 443
 remark Allow DNS to resolver
 permit udp 10.10.20.0 0.0.0.255 host 10.10.30.53 eq 53
 permit tcp 10.10.20.0 0.0.0.255 host 10.10.30.53 eq 53
 remark Deny SMB from users to server segment
 deny tcp 10.10.20.0 0.0.0.255 10.10.30.0 0.0.0.255 eq 445 log
 remark Permit remaining routed traffic by explicit design
 permit ip 10.10.20.0 0.0.0.255 any

Apply inbound:

interface Vlan20
 description User gateway
 ip access-group USERS-TO-SERVERS in

VTY management ACL:

ip access-list standard MGMT-SOURCES
 permit 10.10.10.0 0.0.0.255
 deny any log
!
line vty 0 15
 access-class MGMT-SOURCES in

Notes:

  • Add remarks before meaningful groups of rules.
  • Put specific denies before broad permits.
  • Avoid logging every deny on high-volume paths.
  • Validate direction with packet flow, not memory.

Troubleshooting

Symptom Check Likely Cause
Traffic denied unexpectedly Rule order, implicit deny, direction Wrong sequence or missing permit
ACL counters do not increment Interface, direction, routed path ACL not in packet path
NAT does not match ACL permit/deny meaning for NAT Wrong ACL semantics
CPU/log spike log on high-volume entry Excessive ACL logging
Return traffic blocked Stateful vs stateless behavior ACL only allows one direction
IPv6 still works through block Separate IPv6 policy IPv4 ACL does not filter IPv6
Management lockout VTY ACL, source address, fallback Bad management ACL

Commands

show access-lists
show ip access-lists USERS-TO-SERVERS
show ip interface Vlan20
show running-config interface Vlan20
clear access-list counters USERS-TO-SERVERS

Expected clues:

  • ACL is applied to the expected interface and direction.
  • Counters increment on the expected entries.
  • There is an explicit final policy, not an accidental implicit deny.
  • Return traffic is permitted if the device is not stateful.
  • IPv4 and IPv6 policy are both covered where needed.

Watch Out

  • Do not edit ACLs remotely without a rollback path.
  • Do not use ACLs as a substitute for firewall state when state matters.
  • Do not forget that first match wins.
  • Do not confuse wildcard masks with subnet masks.
  • Do not use permit ip any any unless it is an intentional final policy.
  • Do not log broad busy rules unless you want noise and CPU risk.

References