CEF punted packets

The packets that cannot be CEF-switched in a box with CEF switching enabled are punted to the next switching level (fast switching or process switching). The incoming packets can be punted for a number of reasons, for example:

  • If the destination is reachable over an interface that cannot use CEF-switching due to a feature not supported by CEF (for example, X.25 link), the packet has to be fast- or process-switched.

These destinations are easily discovered by inspecting the punt adjacencies).

  • All packets destined for the router itself are process switched (thus punted).
  • If the router needs to reply back to the source with an ICMP packet (redirect, unreachable ...), the reply can be generated only in the process-switching path.
  • All packets with the IP options are punted to process switching.
  • Fragments that have to be processed by the router are also process-switched.
You can inspect the amount of punted packets with the show cef not-cef-switched command.

This article is part of You've asked for it series.

3 comments:

  1. Hi there.
    I have topology:
    R-Sw-SwUser
    R-7609
    Sw-3560
    SwUser-D-Link DES3526
    On Sw i have user's vlans with dhcp pools. Users using pptp to go in internet.
    And i have many-many punted packets (about 1-10 kpps in fast-switched cpu-queue), that destinated to some users.
    I make SPAN-session, and don't see any reason for "punting" them - it is regular torrent traffic (lot of udp packets and tcp-sessions).
    And here is something strange - count of sw-forwarded packets is not equal with table "show cef not-cef-switched".
    Can you help me understand the reason of that situation.

    ReplyDelete
  2. No idea, I'm not a 7600 guru. Defragmentation?

    ReplyDelete
  3. No, hi cpu load is on 3560, not on 7600.
    And there is too small % of fragments.

    ReplyDelete

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Ivan Pepelnjak, CCIE#1354, is the chief technology advisor for NIL Data Communications. He has been designing and implementing large-scale data communications networks as well as teaching and writing books about advanced technologies since 1990. See his full profile, contact him or follow @ioshints on Twitter.