11-27-2019 03:27 AM
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."
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
11-27-2019 03:42 AM - edited 11-27-2019 03:44 AM
@sivam siva Hello,
-->Punted to the next slowest switching method? (for ex: punted to RP for process switching)
R: I think that is dropped.
Look here a overview about: https://www.petri.com/cisco-express-forwarding-cef-overview
11-27-2019 08:28 AM
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
11-27-2019 09:15 AM
@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;
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.
12-02-2019 03:35 AM
12-02-2019 05:24 AM
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
12-02-2019 05:50 AM
@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
12-02-2019 05:56 AM
12-02-2019 06:46 AM - edited 12-02-2019 06:49 AM
12-02-2019 07:27 AM
12-02-2019 08:48 AM
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