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New Member



I am trying to map out a remote network (no physical access) using cdp as we have a suspected stp issue.

Something is throwing me though, we have multiple switches connected to the same local interface as shown in cdp - how can this be? I thought cdp was a single l2 hop?

See below, I am at a loss as to how to translate and draw the L2 topology - please help!

swixl-2#sh cdp nei
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
                 Gig 0/1            157          S I      WS-C2950G-Gig 0/1
                 Gig 0/1            153          S I      WS-C2950G-Gig 0/2
                 Gig 0/1            147          S I      WS-C2950G-Gig 0/2
                 Gig 0/2            162         R S I     WS-C6509  Gig 3/5
swixl-6          Gig 0/1            150          S I      WS-C2950G-Gig 0/2

Cisco Employee

Re: CDP/STP Issue

A hub maybe?


Lei Tian


Re: CDP/STP Issue

I agree you have a hub in between your switch and all those 2950's .  The 2950's ar

e hung off the hub which is hung off your switch.

New Member

Re: CDP/STP Issue

No hubs.

I have found out they are using the older GBICs and splitting the copper links A/B between different switches.

Is this a normal practice? I thought A and B was tx rx?

New Member

Re: CDP/STP Issue

In the end I had to jump on a plane to solve this one!

It turns out that the gigstack modules get very messy when it comes to STP, especially so if the gigastacks have not been cabled properly as the specs for them are very specific - easy for somebody to adjust this and get it wrong.

The most common type of configuration in the cascaded redundent stack which employs a half duplex shared link. This is the reason why you see multiple neighbors (more than 2 sometimes up to nine) on the sh cdp nei command. This only adds to the troubleshooting confusion.

Even when spaning tree blocks one of the ports on the gigastack, this does not show in the sh spanning tree blocked command, again adding to confusion. The only way to tell is to look at the LED on the gigastack (bear in mind the physical is up if there are two ports used on the gstack module) or to issue the show interface command which details gigastack link status and redundant link.

This makes me wonder if it uses standard STP or some proprietary cisco gstack modified version for half duplex gigastack. Either way, Cisco dropped this for a reason, its a mine field in my eyes!

Even after tralling through the many forums many experts do not understand the gigastack stp operations and simply state in depth detail about how stp works on standard links which is no help (after easy points).

Obviously this is not covered on the ccie lab as very little information out there. I realise this has been dropped for sfp these days but in the real world, engineers (like me) have to still deal with legacy tech and fly miles to fix issues like this!

Here's my ouput below to help anybody in the future who has gigastack stp issues

swixl-4#sh int gi0/1
GigabitEthernet0/1 is up, line protocol is up (connected)
  Hardware is Gigabit Ethernet, address is 000c.3070.7031 (bia 000c.3070.7031)
  MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Half-duplex, 1000Mb/s, media type is CX
  input flow-control is off, output flow-control is off
  ARP type: ARPA, ARP Timeout 04:00:00
  GigaStack module(0.2) in GBIC slot. link1 is up, link2 is redundant link
  Last input 00:00:00, output 00:00:01, output hang never
  Last clearing of "show interface" counters never
  Input queue: 1/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 2142000 bits/sec, 494 packets/sec
  5 minute output rate 74000 bits/sec, 26 packets/sec
     195820137 packets input, 3059874038 bytes, 0 no buffer
     Received 18755551 broadcasts (0 multicast)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 12245420 multicast, 0 pause inputswixl-4#sh cdp nei
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
                 Gig 0/1            123          S I      WS-C2950G-Gig 0/2
                 Gig 0/1            177          S I      WS-C2950G-Gig 0/2
                 Gig 0/1            144          S I      WS-C2950G-Gig 0/1
                 Gig 0/1            121          S I      WS-C2950G-Gig 0/2
swixl-6          Gig 0/1            120          S I      WS-C2950G-Gig 0/2

swixl-4#sh spanning-tree blockedports

Name                 Blocked Interfaces List
-------------------- ------------------------------------

Number of blocked ports (segments) in the system : 0

     0 input packets with dribble condition detected
     85301608 packets output, 2416013666 bytes, 0 underruns

     0 output errors, 117243 collisions, 2 interface resets
     0 babbles, 0 late collision, 228810 deferred
     0 lost carrier, 0 no carrier, 0 PAUSE output
     0 output buffer failures, 0 output buffers swapped out


Re: CDP/STP Issue

  Ugh , gigastack.  Would never use that . Had that on one account and its impossible trying to track down devices in a gigastack .  Can you say redesign ???  

New Member

Re: CDP/STP Issue

Funnily enough I am under contract to re-design the entire global network. Problem is, when something goes down I get sent to fix it and train the local engineers!