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How to Read Adjacency Table

Hi,

When I Issue show adjacency on my 6509 I get following output.

IP       Vlan104                   172.19.104.163(7)
IP       Vlan104                   172.19.104.164(7)
IP       Vlan104                   172.19.104.165(7)
IP       Vlan105                   172.19.105.24(7)
IP       Vlan105                   172.19.105.26(7)
IP       Vlan105                   172.19.105.27(7)
IP       Vlan105                   172.19.105.28(7)

What does (7) signifies?

Regards.

 

 

8 REPLIES
Cisco Employee

Hi,Explanation:172.19.104.163

Hi,

Explanation:

172.19.104.163(7)  :- IP address of the next-hop interface. The value in parenthesis refers to the "refCount" or the number of times that this adjacency is pointed to by FIB entries. The same value appears later in the entry.

RefCount 7
The number of references to the adjacency that are currently stored in the router's memory. There is one for each corresponding entry in the CEF table, plus a few others for a variety of reasons (such as one for the code that performs the show adjacency command).

HTH

Inayath

*Plz rate all usefull posts.

New Member

Hi,Will u plz help me to

Hi,

Will u plz help me to understand how FIB and Adj table work together.

I don't see the layer 2 mapping of next hop in show adjacency

Regards.

 

Cisco Employee

Mitesh,Let me give it a try

Mitesh,

Let me give it a try:

isco Express Forwarding (CEF) switching is a proprietary form of scalable switching intended to tackle the problems associated with demand caching. With CEF switching, the information which is conventionally stored in a route cache is split up over several data structures. The CEF code is able to maintain these data structures in the Gigabit Route Processor (GRP), and also in slave processors such as the line cards in the 12000 routers. The data structures that provide optimized lookup for efficient packet forwarding include:

  • The Forwarding Information Base (FIB) table - CEF uses a FIB to make IP destination prefix-based switching decisions. The FIB is conceptually similar to a routing table or information base. It maintains a mirror image of the forwarding information contained in the IP routing table. When routing or topology changes occur in the network, the IP routing table is updated, and these changes are reflected in the FIB. The FIB maintains next-hop address information based on the information in the IP routing table.

    Because there is a one-to-one correlation between FIB entries and routing table entries, the FIB contains all known routes and eliminates the need for route cache maintenance that is associated with switching paths such as fast switching and optimum switching.

  • Adjacency table - Nodes in the network are said to be adjacent if they can reach each other with a single hop across a link layer. In addition to the FIB, CEF uses adjacency tables to prepend Layer 2 addressing information. The adjacency table maintains Layer 2 next-hop addresses for all FIB entries.

 

1.  CAM table records the incoming packet's MAC address, Port & VLAN.to enable Switching at Layer 2.

2. The Adjacency table records IP address and Layer 2 header for the IP and then references the TCAM table and at this point the switch will have enough information to rewrite the packets headers and send them out the egress port..... The idea is that all of the information is loaded into the Adjacency table & TCAM table as these are run on the ASIC's which makes it hardware/wire speed Layer 3/MLS switching.

Adjacency table.jpg

 

Now coming to reality:

 

C6K1#show adjacency
Protocol Interface                 Address
IP       GigabitEthernet5/2        10.104.149.1(7)
IP       Vlan10                    10.10.10.2(10)


C6K1#sh ip arp   10.10.10.2
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.10.10.2             16   0026.9809.7dc1  ARPA   Vlan10


C6K1#sh mac address-table address  0026.9809.7dc1
Legend: * - primary entry
        age - seconds since last seen
        n/a - not available

  vlan   mac address     type    learn     age              ports
------+----------------+--------+-----+----------+--------------------------
Module 1:
*  201  0026.9809.7dc1   dynamic  Yes          0   Po100
*   10  0026.9809.7dc1   dynamic  Yes          0   Po100
Module 4:
*   50  0026.9809.7dc1   dynamic  Yes          0   Gi4/6 >>> Mac address and port is seen here.
*    1  0026.9809.7dc1   dynamic  Yes          0   Gi4/6

 

HTH

Cisco Employee

Inayath and Mitesh,I

Inayath and Mitesh,

I apologize if this looks like an advertisement (well... perhaps it is ;) ). You may also check the Chapter 6 from the new CCIE Official Certification Guide v5.0, Volume 1, available in this PDF as a sample content from the book:

http://www.ciscopress.com/content/images/9781587143960/samplepages/9781587143960.pdf

I wrote that chapter, and I've gone to great lengths there to explain how the entire CEF with the FIB and ADJDB play together.

Best regards,
Peter

Cisco Employee

Amazing Peter ,I didnt know

Amazing Peter ,

I didnt know about this book I will go through it today and tommorow.

I would appriciate if you can share any book in depth on STP and multicast as I would like to go indepth on the same.

 

 

Cisco Employee

Inayath,Regarding STP,

Inayath,

Regarding STP, Chapter 3 in the same book (CCIE Official Certification Guide v5.0, Volume 1) covers STP extensively. I have incorporated several experiences from issues here on CSC into that chapter.

Multicast routing will be covered in the Volume 2 of the book written by Narbik Kocharians and Terry Vinson that should be available during October. Apart from that, however, I was personally very satisfied with a Cisco-authored training material about IP Multicast that was formerly available on an FTP server at ftp-eng.cisco.com. This FTP server has recently been brought down - I do not know if it still runs internally in Cisco. I have mirrored the contents, however, so if the server does not run anymore, let me know - I will post the slide decks somewhere.

Thank you!

Best regards,
Peter

Cisco Employee

Hi Peter,That FTP is not

Hi Peter,

That FTP is not working...tried all the possibilities.

 

Cisco Employee

Hi Mitesh,The (7) is not

Hi Mitesh,

The (7) is not really interesting to you. Documentation says that this is the count of internal references pointing to this particular entry. It has been my experience that this number is always higher than the number of routing table entries resolving to this particular adjacency table entry. There are obviously another internal references to each adjacency table entry - this is really of interest only to IOS developers and Cisco specialists

In short - do not worry too much about it. What is more interesting is the show adjacency detail output that provides data that are more relevant to your device's operation.

Best regards,
Peter

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