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Beginner

Order of Operations for processing a packet

Hello everyone!

I would like to know what exactly happens to a packet once it reaches an interface, through which exact steps does it go and the precise order. Both inbound and outbound traffic. Something like a flow chart.. I can't find a proper document that shows this.

Appreciate any help in advance.

6 REPLIES 6
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Cisco Employee

Hi Haji,

Your question is very generic. The way packet processed varies depending on if it routed switched, what all services are configured on the router. Also it depends on the platform

Can you specify if it is a CEF router, or harware swiched platfrom and the kind of traffic

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Hey rajs2,

Thank you for your reply.

But isn't there a generic graph/flow chart/description or whatever that would show this?

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This is nice. Thanks. But it is too specific, about NAT. Do you think there is more general order of operations available somewhere?

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The documet is made for NAT. But will generally help on CEF routers to understand the flow

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Rising star

General packet forwarding steps:

1. Layer 2 engine performs the input VLAN ACL lookup

2. If necessary, an input router ACL check is performed

3. The destination IP addr. is compared against Layer 3 FIB.

4. The Layer 2 and Layer 3 header rewritten (according Adjacency table)

5. Layer 2 forwarding engine forwards the frame.

Link about packet switching methods:

http://fengnet.com/book/IS-ISDesign/ch01lev1sec5.html

Best regards,
Abzal