CCIE Pursuit Blog

December 1, 2007

LFU 10: No VLAN…No CDP!!!

I was working on a practice lab today and was using CDP to verify that all of the connections were correct.  I cruised through sw1 and sw2 but when I hit sw3 I started to see strange issues with CDP.
 
sw3 fa0/3 is connected to r3 fa0/1 but I cannot see a CDP neighbor:
 
sw3#sh run int fa0/3
Building configuration…
 
Current configuration : 95 bytes
!
interface FastEthernet0/3
 switchport access vlan 33
 switchport mode dynamic desirable
end
 
Port      Name               Status       Vlan       Duplex  Speed Type
Fa0/3                        connected    33         a-full  a-100 10/100BaseTX
 
sw3#sh int fa0/3
FastEthernet0/3 is up, line protocol is up (connected)
 
sw3#sh cdp neigh fa0/3
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
sw3#

The interface is up/up and CDP is running (it would have given me an error if it was not) but CDP does not see r3’s fa0/1 interface. 

Let’s look at r3:

r3#sh run int fa0/1
Building configuration…
 
Current configuration : 95 bytes
!
interface FastEthernet0/1
 ip address 132.1.33.3 255.255.255.0
 duplex auto
 speed auto
end
 
r3#sh ip int br | i net0/1
FastEthernet0/1
            132.1.33.3      YES manual up                    up
 
r3#sh cdp neigh fa0/1
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
r3#

r3’s fa0/1 interface is up and up and CDP is running.  I can actually physically trace that cable to sw3’s fa0/3 interface.  So why can’t sw3 see r3 (and vice versa) via CDP? 

Here’s the problem:
 
sw3#sh int fa0/3 switch
Name: Fa0/3
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: native
Negotiation of Trunking: On
Access Mode VLAN: 33 (Inactive)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: none

I configured the port as an accesss port assigned to VLAN 33, but VLAN 33 does not exit on the switch (it will once VTP does it’s magic).  Once VLAN 33 is configured on the switch CDP will work:

sw3#sh vlan id 33

VLAN Name                             Status    Ports
—- ——————————– ——— ——————————-
33   VLAN0033                         active    Fa0/3, Po2

VLAN Type  SAID       MTU   Parent RingNo BridgeNo Stp  BrdgMode Trans1 Trans2
—- —– ———- —– —— —— ——– —- ——– —— ——
33   enet  100033     1500  –      –      –        –    –        0      0

Remote SPAN VLAN
—————-
Disabled

Primary Secondary Type              Ports
——- ——— —————– ——————————————

sw3#sh int fa0/3 switch
Name: Fa0/3
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: native
Negotiation of Trunking: On
Access Mode VLAN: 33 (VLAN0033)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: none

sw3#sh cdp neigh fa0/3
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
r3                  Fas 0/3               179           R S I     2651XM    Fas0/1

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