Cisco IOS Order of Operation
Here we found information on the order of operation of the different features on an interface and the packet traverses the IOS software from Cisco.com, which may not suitable for every case table. Anyway, check it whether is suitable or not.
Inside-to-Outside Outside-to-Inside
- If IPSec then check input access list
- decryption – for CET (Cisco Encryption Technology) or IPSec
- check input access list
- check input rate limits
- input accounting
- policy routing
- routing
- redirect to web cache
- NAT inside to outside (local to global translation)
- crypto (check map and mark for encryption)
- check output access list
- inspect (Context-based Access Control (CBAC))
- TCP intercept
- encryption
- Queueing
- If IPSec then check input access list
- decryption – for CET or IPSec
- check input access list
- check input rate limits
- input accounting
- NAT outside to inside (global to local translation)
- policy routing
- routing
- redirect to web cache
- crypto (check map and mark for encryption)
- check output access list
- inspect CBAC
- TCP intercept
- encryption
- Queueing
All right, the above we delivered is the “official version”. But there are others that were provided by some professional network engineers are pretty complete.
See the following for a larger diagram.
More notes: Some variations in feature ordering may occur in specific router platforms, IOS software releases, and switching paths (i.e. CEF versus process-switched).
Ingress Features | Egress Features |
1. Virtual Reassembly * | 1. Output IOS IPS Inspection |
2. IP Traffic Export (RITE) | 2. Output WCCP Redirect |
3. QoS Policy Propagation through BGP (QPPB) | 3. NM-CIDS |
4. Ingress Flexible NetFlow * | 4. NAT Inside-to-Outside or NAT Enable * |
5. Network Based Application Recognition (NBAR) | 5. Network Based Application Recognition (NBAR) |
6. Input QoS Classification | 6. BGP Policy Accounting |
7. Ingress NetFlow * | 7. Lawful Intercept |
8. Lawful Intercept | 8. Check crytpo map ACL and mark for encryption |
9. IOS IPS Inspection (inbound) | 9. Output QoS Classification |
10. Input Stateful Packet Inspection (IOS FW) * | 10. Output ACL check (if not marked for encryption) |
11. Check reverse crypto map ACL | 11. Crypto outbound ACL check (if marked for encryption) |
12. Input ACL (unless existing NetFlow record was found) | 12. Output Flexible Packet Matching (FPM) |
13. Input Flexible Packet Matching (FPM) | 13. DoS Tracker |
14. IPsec Decryption (if encrypted) | 14. Output Stateful Packet Inspection (IOS FW) * |
15. Crypto inbound ACL check (if packet had been encrypted) | 15. TCP Intercept |
16. Unicast RPF check | 16. Output QoS Marking |
17. Input QoS Marking | 17. Output Policing (CAR) |
18. Input Policing (CAR) | 18. Output MAC/Precedence Accounting |
19. Input MAC/Precedence Accounting | 19. IPsec Encryption |
20. NAT Outside-to-Inside * | 20. Output ACL check (if encrypted) |
21. Policy Routing | 21. Egress NetFlow * |
22. Input WCCP Redirect | 22. Egress Flexible NetFlow * |
23. Egress RITE | |
24. Output Queuing (CBWFQ, LLQ, WRED) |
* A note about virtual-reassembly
Virtual-reassembly causes the router to internally reassemble fragmented packets. It is enabled when an interface is configured with NAT, CBAC, or “ip virtual reassembly”. Operations above marked with a * will process the reassembled version of a packet. All other operations process the individual fragments. After virtual reassembly is complete, the router forwards the original fragments, albeit in proper order. This behavior is very different from PIX/ASA/FWSM and ACE which forward the reassembled packet.
Thus, even if virtual-reassembly is turned on, ACLs used for input access-groups and QoS still need to be aware of how ACLs interact with fragments
http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800949b8.shtml
Routing Features
1. Routing table lookup (if packet isn’t marked with a PBR next-hop) 2. tcp adjust-mss
NOTE: Order of Operation for IOS 12.3(8)T and Later
---Reference from http://etherealmind.com/cisco-ios-order-of-operation/
More Notes: A Related Best Cisco Book
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More Related Tips:
What’s the Order of Operations for Cisco IOS?
More Cisco and Networking Tips you can referto http://blog.router-switch.com/