Re: cdp neighbors question

Armegeden · ‎11-26-2005

1CC1#sh cdp neighbors

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

S - Switch, H - Host, I - IGMP, r - Repeater

Device ID Local Intrfce Holdtme Capability Platform Port ID

1CC1 Fas 3/3 134 R S I WS-C4006 Fas 3/9

1CC1 Fas 3/9 134 R S I WS-C4006 Fas 3/3

CAMERA Gig 6/4 177 S I WS-C2950G-Gig 0/1

The first two entries are puzzling me. Switch CC1 sees itself as it's own neighbors on 3/3 and 3/9? Huh?

Is this a hub/switch on the network or something?

ankurbhasin · ‎11-26-2005

Hi Armegeden,

Can you please confirm what is exactly connected to port 3/9 and 3/3 and also what is the config on this ports?

Looks to be a HUB.

Regards,

Ankur

Armegeden · ‎11-26-2005

1CC1#sh run | begin 3/3

interface FastEthernet3/3

switchport access vlan 12

description I N 28

no snmp trap link-status

spanning-tree portfast

!

interface FastEthernet3/9

switchport access vlan 21

description I N 40

no snmp trap link-status

!

1CC1#sh cdp neighbors detail

-------------------------

Device ID: 1CC1

Entry address(es):

IP address: 10.10.68.2

Platform: cisco WS-C4006, Capabilities: Router Switch IGMP

Interface: FastEthernet3/3, Port ID (outgoing port): FastEthernet3/9

Holdtime : 135 sec

Version :

Cisco Internetwork Operating System Software

IOS (tm) Catalyst 4000 L3 Switch Software (cat4000-IS-M), Version 12.1(8a)EW1, EARLY DEPLOYMENT RELE

ASE SOFTWARE (fc1)

TAC Support: http://www.cisco.com/tac

Compiled Wed 20-Feb-02 18:02 by eaarmas

advertisement version: 2

VTP Management Domain: 'DMN'

Native VLAN: 21

Duplex: full

-------------------------

Device ID: 1CC1

Entry address(es):

IP address: 10.10.67.3

Platform: cisco WS-C4006, Capabilities: Router Switch IGMP

Interface: FastEthernet3/9, Port ID (outgoing port): FastEthernet3/3

Holdtime : 135 sec

Version :

Cisco Internetwork Operating System Software

IOS (tm) Catalyst 4000 L3 Switch Software (cat4000-IS-M), Version 12.1(8a)EW1, EARLY DEPLOYMENT RELE

ASE SOFTWARE (fc1)

TAC Support: http://www.cisco.com/tac

Compiled Wed 20-Feb-02 18:02 by eaarmas

advertisement version: 2

VTP Management Domain: 'DMN'

Native VLAN: 12

Duplex: full

10.10.68.2 is the Layer3 address of a Vlan, and 10.10.67.3 is the Layer3 addres of the backup VTP servers Vlan (a different Vlan, though). 10.10.68.1 is the standby ip.

So this is what a CDP neighbor advertisement looks like through a HUB?

ankurbhasin · ‎11-26-2005

Hi Armegeden,

Yes I think that these 2 ports are copnnected to a hub? Can you confirm that please?

Because the destination address for CDP frame is a multicast address and when it coems out from your port 3/3 it will reach the hub and hub will broadcast the frame and it will see port 3/9 as its naighbor and vice versa it will happen when the CDp frame will move out from port 3/9.

Now both the ports are in different vlan so this could be the behaviour is it possible can you configure both the ports into same vlan just to verify what will happen I think one port will go into blocking state.

Can you just make a change for a minute and verify?

Regards,

Ankur

Armegeden · ‎11-26-2005

Okay, I put port 3/3 into a different Vlan (4). Watched it for about 5 minutes, and nothing changed as far as status goes.

Fa3/3 I N 28 connected 4 a-full a-100 10/100BaseTX

Changing it back to Vlan12 now.

I'll try and go track down what it's plugged into in a bit. The switch is in a completely different building than where I am and I need to finish what I'm working on now 8)

Thanx much for the input so far though. Much appreciated.

Though I always thought having a hub on a network is really bad?

glen.grant · ‎11-26-2005

Someone has introduced a hardwire loop into your switch . I bet if you look at spanning tree for that vlan it would show one of the ports blocked . You need to investigate and get that out of there .

Armegeden · ‎11-28-2005

Hello,

I went over to the switch and tried tracing the cable. Not possible without my Probe&Toner which is on loan at the moment. So I come back to the office and turn off portfast (no spanning portfast), and shut, no shut the ports.

If there is indeed a loop on those ports, which it looks like there is, shouldn't spanning-tree put those in blocking state? I guess I know the theory of spanning-tree, but have never put it to the test. Shouldn't this be a good test?

!

interface FastEthernet3/3

switchport access vlan 12

description 28

no snmp trap link-status

!

interface FastEthernet3/9

switchport access vlan 21

description 40

no snmp trap link-status

!

1CC1#sh spanning-tree blockedports

Name Blocked Interfaces List

---------------------- -----------------------------

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

1CC1#

glen.grant · ‎11-28-2005

Seeing that the ports are in 2 different vlans you are actually bridgeing the 2 vlans and is something can cause real problems,I'm not sure if spanning tree will take care of this if they are in 2 different vlans , maybe someone else can chime in , not a good situation if they connected to one another .