9 Replies Latest reply: May 19, 2019 5:55 PM by Martin RSS

    Where in the heck is the Adjacency info for CEF?

    David

      I am striking out big time in trying to find it, as I have a healthy FIB table on my practice lab:

       

      SW1#sh ip cef

      Prefix               Next Hop             Interface

      0.0.0.0/0            no route

      0.0.0.0/8            drop

      0.0.0.0/32           receive

      10.0.0.0/24          attached             Port-channel1

      10.0.0.0/32          receive              Port-channel1

      10.0.0.10/32         receive              Port-channel1

      10.0.0.255/32        receive              Port-channel1

      10.1.1.0/24          attached             Vlan10

      10.1.1.0/32          receive              Vlan10

      10.1.1.1/32          receive              Vlan10

      10.1.1.255/32        receive              Vlan10

      10.2.2.0/24          attached             Port-channel1

      127.0.0.0/8          drop

      224.0.0.0/4          drop

      224.0.0.0/24         receive

      240.0.0.0/4          drop

      255.255.255.255/32   receive

       

      Then I tried to do some Adjacency lookups, I understand there is no real "Table", but I am unable to even find details on the Host Prefix 10.1.1.1:

       

      SW1#sh adj 10.1.1.1 detail

      Protocol Interface                 Address

      SW1#

       

      And  I cannot get anything to work:

       

      SW1#sh adj

      Protocol Interface                 Address

      SW1#sh ip adj ?

      % Unrecognized command

       

      SW1#sh adj 10.1.1.1 detail

      Protocol Interface                 Address

       

      SW1#sh adj 10.1.1.1 255.255.255.0 ?

      % Unrecognized command

      SW1#

       

      I tried "sh adj summ" and "sh adj det" but that gave me some output that had to do with the CEF process but nothing to do with the CEF populated FIB table!

       

      Absolutely no "sh adj xxxxx" command is giving me Layer 2 info for the FIB table, can someone please tell me what on Earth I am doing wrong?


      Thanks!

        • 1. Re: Where in the heck is the Adjacency info for CEF?
          Ing_Percy

          Hi!

           

          It is a real switch? or INE/GNS3?

           

          In my real switch. I can see the adjacency table of the prefix that is "attached"

          showadjacency1.jpg

          In case of "receive" you can't see outputs about it

          showadjacency2.jpg

           

          Regards!

          • 2. Re: Where in the heck is the Adjacency info for CEF?
            Martin

            Yes, your IP needs to be "attached" not receive.

            Adjacencies are kept for each next-hop router and each host that is connected directly to the local device (switch).  The adjacency table information comes from the ARP table.

            "attached" means next-hop IP is "attached" to me (both IP and MAC address of of the directly attached host). 

            “receive” means packet will be sent to the Layer 3 engine for further processing (my IP, my network, b-cast).

             

             

            CCNP Switch exam book should say something like "FIB keeps the Layer 3 next-hop address for each entry. To streamline packet forwarding even more, the FIB has corresponding Layer 2 information for every next-hop entry. This portion of the FIB is called the adjacency table, consisting of the MAC addresses of nodes that can be reached in a single Layer 2 hop."

             

            Receive - layer 3 engine interfaces; includes packets destined for IP addresses that are assigned to interfaces on the Layer 3 engine, IP network addresses, and IP broadcast addresses

            • 3. Re: Where in the heck is the Adjacency info for CEF?
              David

              You are correct!

               

              Thank you for explaining that!

               

              Martin had slightly more context to his answer so I will give him correct answer, but yours was definitely helpful and would have answered my question perfectly fine.

               

              Thanks for the help

              • 4. Re: Where in the heck is the Adjacency info for CEF?
                David

                Thank you for the full explanation of why it works that way as well as the information the first person to answer had, I will mark you as correct answer for the fuller context.

                 

                Thank you as always Martin

                • 5. Re: Where in the heck is the Adjacency info for CEF?
                  David P.

                  This also might help

                  Demystifying CEF

                  • 6. Re: Where in the heck is the Adjacency info for CEF?
                    David

                    Very nice blog post, that was a good read sir

                     

                    I found one of my issues was I was turning my lab on and immediately issues pings anywhere, so I wasn't really populating CEF to be able to show an Adjacent neighbors information, after I pinged from my Host A through SW1 over a L3 EtherChannel through SW2 to Host B and back I got all sorts of "sh adj det" now on SW1:

                     

                    SW1#sh adj det

                    Protocol Interface                 Address

                    IP       Port-channel1             10.0.0.20(8)

                                                       0 packets, 0 bytes

                                                       epoch 0

                                                       sourced in sev-epoch 0

                                                       Encap length 14

                                                       58971EABC8411CE6C7C1C8410800

                                                       L2 destination address byte offset 0

                                                       L2 destination address byte length 6

                                                       Link-type after encap: ip

                                                       ARP

                    IP       Port-channel1             10.2.2.2(8)

                                                       0 packets, 0 bytes

                                                       epoch 0

                                                       sourced in sev-epoch 0

                                                       Encap length 14

                                                       58971EABC8411CE6C7C1C8410800

                                                       L2 destination address byte offset 0

                                                       L2 destination address byte length 6

                                                       Link-type after encap: ip

                                                       ARP

                    IP       Vlan10                    10.1.1.2(8)

                                                       0 packets, 0 bytes

                    Protocol Interface                 Address

                                                       epoch 0

                                                       sourced in sev-epoch 0

                                                       Encap length 14

                                                       001EF797F14B1CE6C7C1C8450800

                                                       L2 destination address byte offset 0

                                                       L2 destination address byte length 6

                                                       Link-type after encap: ip

                                                       ARP

                    SW1#

                     

                    It never surprises me how working on a home lab and building / breaking / troubleshooting issues myself, I always find little niche behaviors I would have never thought of reading books or blogs, why I always advise lab lab and more lab!

                    • 7. Re: Where in the heck is the Adjacency info for CEF?
                      Martin

                      Yes, in such case, you have populated ARP table by pining devices. As a result of that, adjacency table grows as information comes from the ARP table.  you can use show adjacency x.x.x.x or  show adjacency interface x

                      FIB is build based on routing table whereas the adjacency table comes from the ARP table.  I think your output will time out. 

                      Do not have the latest version of SWITCH book from Cisco press, is CEF still included in the textbook? Is it on Switch exam syllabus?

                      • 8. Re: Where in the heck is the Adjacency info for CEF?
                        David

                        I don't think I can really confirm it is on the exam... but... you know... it probably is.

                         

                        I don't know that the syllabus covers CEF, but MultiLayer Switching operations which CEF is a big part of newer deployments, so I'd know this info for both ROUTE (which I've passed years ago) and SWITCH.

                        • 9. Re: Where in the heck is the Adjacency info for CEF?
                          Martin

                          you are right. CEF is not on new version of exam; it used to be but no longer is.  still, I wonder if it is in new book.