cisco-ios-patterns

# Cisco IOS Patterns Use this skill when reviewing Cisco IOS or IOS-XE snippets, building a change-window checklist, or explaining how to collect evidence from a router or switch without making the incident worse. ## When to Use - Reviewing IOS or IOS-XE configuration before a planned change. - Choosing read-only `show` commands for troubleshooting. - Checking ACL wildcard masks and interface direction. - Explaining global, interface, routing process, and line configuration modes. - Verifying that a change landed in running config and was saved intentionally. ## Operating Rules Treat IOS examples as patterns, not paste-ready production changes. Confirm the platform, interface names, current config, rollback path, and out-of-band access before making changes on a real device. Prefer this workflow: 1. Capture current state with read-only commands. 2. Review the exact candidate config. 3. Confirm management access cannot be locked out. 4. Apply the smallest change in a maintenance window. 5. Re-read state, compare to the baseline, then save only after validation. ## Mode Reference ```text Router> enable Router# show running-config Router# configure terminal Router(config)# interface GigabitEthernet0/1 Router(config-if)# description UPLINK-TO-CORE Router(config-if)# no shutdown Router(config-if)# exit Router(config)# end Router# show running-config interface GigabitEthernet0/1 ``` `running-config` is active memory. `startup-config` is what survives reload. Do not save a change just because a command was accepted; validate behavior first, then use `copy running-config startup-config` if the change is approved. ## Read-Only Collection ```text show version show inventory show processes cpu sorted show memory statistics show logging show running-config | section line vty show running-config | section interface show running-config | section router bgp show ip interface brief show interfaces show interfaces status show vlan brief show mac address-table show spanning-tree show ip route show ip protocols show ip access-lists show route-map show ip prefix-list ``` Collect the specific section you need instead of dumping full config into a ticket when the config may contain secrets, customer names, or private topology. ## Wildcard Masks IOS ACL and many routing statements use wildcard masks, not subnet masks. ```text Subnet mask Wildcard mask 255.255.255.255 0.0.0.0 255.255.255.252 0.0.0.3 255.255.255.0 0.0.0.255 255.255.0.0 0.0.255.255 ``` Review wildcard masks before deployment. A subnet mask accidentally used as a wildcard can match far more traffic than intended. ```text ip access-list extended WEB-IN 10 permit tcp 192.0.2.0 0.0.0.255 any eq 443 999 deny ip any any log ``` Every ACL has an implicit deny at the end. Add an explicit logged deny when the operational goal includes observing misses, and confirm logging volume is safe. ## ACL Placement Review Before applying an ACL to an interface, answer these questi