cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
364
Views
10
Helpful
10
Replies
Highlighted
Participant

No valid CEF entry

Hello all

I found the following lines on the Cisco website, Which not clear to me

can anyone explain this?  

"if the CEF lookup process failed to find a valid entry in the forwarding information base, the packet is punted to the next available switching path or dropped."

https://www.cisco.com/c/en/us/support/docs/ios-nx-os-software/ios-software-releases-120-mainline/47205-cef-whichpath.html#backinfo

My question is when a packet arrives at the interface what is the result if there is no entry for the destination in the FIB table?

-->Punted to the next slowest switching method? (for ex: punted to RP for process switching)

-->or dropped?

 

Thanks 

Siva

Everyone's tags (2)
10 REPLIES 10
Highlighted
VIP Collaborator

Re: No valid CEF entry

@sivam siva Hello,

 

 

-->Punted to the next slowest switching method? (for ex: punted to RP for process switching)

 

R: I think that is dropped.

cef.PNG

 

Look here a overview about: https://www.petri.com/cisco-express-forwarding-cef-overview

Look here more information: https://books.google.com.br/books?id=-SM1Jeu07UYC&pg=PA55&lpg=PA55&dq=fib,+process+switching+and+dropped+cisco&source=bl&ots=wbVfvNoLFm&sig=ACfU3U2dyWvN9TwMdUj5KfCT1vSF4bCAaQ&hl=pt-BR&sa=X&ved=2ahUKEwiTmJK-p4rmAhVLDrkGHcwcCfsQ6AEwC3oECAoQAQ#v=onepage...

Jaderson Pessoa
*** Rate All Helpful Responses ***
Highlighted
Participant

Re: No valid CEF entry

Hi @Jaderson Pessoa 

Thanks for the reply 

 

I agree with the sense of dropping the packet. because the FIB would always have complete information about RIB, so there is no use of sending the packet to the Route processor only for route lookup.

But if you see on the Cisco website which I mentioned above, they said packet can be punted to the route processor.

so why is the discrepancy with cisco for us?

As I'm preparing for the exam what should I believe?

 

Regards 

Siva

Highlighted
VIP Collaborator

Re: No valid CEF entry

@sivam siva Hello,

 

You need consider other charactheristics about Cisco information, for exemple:

 

"With an incomplete adjacency, CEF considers that the router as a whole (including all the other switching paths) does not know how to get to the adjacent node. We punt packets to process switching in order to kick off some resolution protocol like Address Resolution Protocol (ARP),"

 

"Only packets that match a glean adjacency are punted to the Gigabit Routing Processor (GRP). In addition, on the GSR, the LC CPU does not punt packets to the GRP for features, and the LC sends an ICMP unreachable (as long as the no ip unreachables command is not configured). On the GSR, the only traffic punted to the GRP are packets destined to an interface on the router or packets sourced from the router."

 

Looks below;

 

  • Host route adjacency: A host route adjacency entry is used to specify that a specific host is within one layer two hop.
  • Null adjacency: This is used for packets which are destined for the Null0 interface.
  • Glean adjacency: A glean adjacency is used when a device is connected to multiple hosts off the same interface. In this case, the entry contains a prefix for the subnet not just for a specific host entry.
  • Punt adjacency: This is used for those packets that utilize features not currently supported by CEF and which must be forwarding to the next switching level (often to be process switched).
  • Discard adjacency: A discard adjacency entry is used for those packets that are to be automatically dropped.
  • Drop adjacency: A drop adjacency entry is used for those packets which dropped but only after the prefix is checked.

 

A punt occurs under these conditions:

  • The next lower level did not produce a valid path or, in the case of CEF, a valid adjacency. In other words, if the CEF lookup process failed to find a valid entry in the forwarding information base, the packet is punted to the next available switching path or dropped.

  • A particular feature or Layer 2 encapsulation is not supported at the lowest level. If CEF supports a particular feature, ownership of a packet is passed through a set of software routines in the CEF "feature path."

  • A feature requires special handling.

 

In other words, normall is drop the package. For your studies, you should consider the conditions.

Jaderson Pessoa
*** Rate All Helpful Responses ***
Highlighted
VIP Collaborator

Re: No valid CEF entry

if you an mark as solved will help other people found it easier.
Jaderson Pessoa
*** Rate All Helpful Responses ***
Highlighted
Participant

Re: No valid CEF entry

Hi @Jaderson Pessoa 

Thank you for the reply

 

I raised a question for the words from the cisco website,

got the same lines as an answer in your reply (sorry)

A punt occurs under these conditions:

The next lower level did not produce a valid path or, in the case of CEF, a valid adjacency. In other words, if the CEF lookup process failed to find a valid entry in the forwarding information base, the packet is punted to the next available switching path or dropped.

I understood the concept of CEF, and I agree that dropping the packet after FIB lookup (if the destination is not there in FIB) makes sense.

 

If someone can ensure this with a valid cisco link or any other official sites would be very helpful for me.

Again thank you very much for your response and time 

 

Highlighted
VIP Collaborator

Re: No valid CEF entry

@sivam siva Hello,

 

 

Here official cisco notes: https://www.cisco.com/c/en/us/support/docs/ios-nx-os-software/ios-software-releases-120-mainline/47205-cef-whichpath.html

 

PUNTS.PNG

Jaderson Pessoa
*** Rate All Helpful Responses ***
Highlighted
VIP Collaborator

Re: No valid CEF entry

Highlighted
Participant

Re: No valid CEF entry

 
Highlighted
Participant

Re: No valid CEF entry

Hi

Sorry again @Jaderson Pessoa 



I have raised a question from this Link only, please see my original question in which I already attached this link.



I just want to ensure the device can only drop the packet when it can't find destination in FIB. (please let me know if you find any like this)



Thanks 

Siva
Highlighted
VIP Collaborator

Re: No valid CEF entry