Understand the order of operations for Cisco IOS

NAT order of operations

inside-to-outside

Order of operations for the inside-to-outside list:

1. If IPSec, then check input access list
2. Decryption—for Cisco Encryption Technology (CET) or IPSec
3. Check input access list
4. Check input rate limits
5. Input accounting
6. Policy routing
7. Routing
8. Redirect to Web cache
9. NAT inside to outside (local to global translation)
10. Crypto (check map and mark for encryption)
11. Check output access list
12. Inspect context-based access control (CBAC)
13. TCP intercept
14. Encryption

outside-to-inside

Order of operations for the outside-to-inside list:

1. If IPSec, then check input access list
2. Decryption—for CET or IPSec
3. Check input access list
4. Check input rate limits
5. Input accounting
6. NAT outside to inside (global to local translation)
7. Policy routing
8. Routing
9. Redirect to Web cache
10. Crypto (check map and mark for encryption)
11. Check output access list
12. Inspect CBAC
13. TCP intercept
14. Encryption

QoS Order of Operations

The Quality of Service (QoS) order of operations is another important list to know. Of course, this is only really important if you're using QoS. But if you are, you need to be familiar with it.

Inbound traffic order of operations

1. QoS Policy Propagation through Border Gateway Protocol (BGP)—or QPPB
2. Input common classification
3. Input ACLs
4. Input marking—class-based marking or Committed Access Rate (CAR)
5. Input policing—through a class-based policer or CAR
6. IPSec
7. Cisco Express Forwarding (CEF) or Fast Switching

Outbound traffic order of operations

1. CEF or Fast Switching
2. Output common classification
3. Output ACLs
4. Output marking
5. Output policing—through a class-based policer or CAR
6. Queueing—Class-Based Weighted Fair Queueing (CBWFQ) and Low Latency Queueing (LLQ))—and Weighted Random Early Detection (WRED)

links

• Understand the order of operations for Cisco IOS