[c-nsp] Duplicate IP add on CAt6509

[Phil Mayers]

Dave Lim wrote:
> Anyone have faced this log message of duplicate IP address before?
> 
> Here is the network topology
> 
> C6509CS01 --------------- C6509CS02
> ---------------
> 
> This problem occured for 5 min and brought the customer vlan 1 network. Note
> that the
> 
> C6509CS01
> Feb 8 08:08:32: %STANDBY-3-DUPADDR: Duplicate address 10.1.1.253 on Vlan1,
> sourced by 0000.0c07.ac09
> 
> VLAN 1 MAC address on C6509CS01
> Vlan1 is up, line protocol is up
> Hardware is Cat6k RP Virtual Ethernet, address is 0009.b63e.cffc (bia
> 0009.b63
> 
> C6509CS02
> *Feb 9 01:29:49: %STANDBY-3-DUPADDR: Duplicate address 10.1.1.252 on Vlan1,
> sourced by 0009.b63e.bffc
> 
> VLAN 1 MAC address on C6509CS02
> Vlan1 is up, line protocol is up
> Hardware is Cat6k RP Virtual Ethernet, address is 0009.b63e.bffc (bia
> 0009.b63e.bffc)
> Internet address is 10.1.1.252/24
> 
> I am suspecting this is a spanning tree loop and causes the switch to see a

Yes, it is.

> duplicate IP address. But logically, there is no loop between the 2 6509
> unless the port-channel connecting the 2 switches fail and in turns there is
> a loop. But in this cases, STP should kick in.

The loop need not be on your equipment. It could be downstream on
customer equipment which does not itself have STP enabled. Imagine this
topology:

6500-1 ======= 6500-2 ----- cust-swi } LOOP

Broadcast and link-layer multicast packets (e.g. the HSRP hellos itself)
will go down to cust-swi, out of one of the loops ports, back in the
other, and back up to the 6500s.