Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

6500 and native 12.1(19)E1a ARP oddity

We have encountered a very strange problem with

Cat 6506:




we asked for Native IOS on the box and got 12.1(19)E1

which we upgraded to 12.1(19)E1a. Bench testing looked OK

but when we cut it into service routing fell to pieces.

The minimal criminal is two Layer 3 VLAN interfaces:

one having a port on the WS-6148-RJ45 and a single host attached,

the other including a GE port on the Supervisor (1000BaseT) connected

to a Layer 2 network (3550-12T)

Pinging from the host on the WS-6148-RJ45 to a host on the subnet

connected to the GE port fails if the destination host is not already

in the router ARP cache. debug arp did not show the router issuing the

ARP request. If I sit on the router and issue the ping the ARP cache is correctly

issued and subsequent connectivity is OK, until the ARP is flushed.

Bizzarely if the GE link is broken out on to a 10Mbps hub

(we did this to look for the ARP request on a 10Mps monitor) the router

will ARP for the destination. Breaking the GE link down to 100Mbps the

router does not ARP. After the breakdown to 10Mbps, breaking back to

GE the router does continues to ARP as would be expected.

Doing a shut/ no shut on the Layer 3 VLAN restores the broken behaviour.

Jumping branches to 12.1(13)E11 fixes the problem and we have gone

with that in production so I cannot do any further repros of the problem

but I thought I would ask on the forum if anyone else had seen anything

like this. The only difference I spotted between 12.1(19)E1a and 12.1(13)E11

that might be significant is that the former showed dCEF in the CEF flags

under 'sh ip interface'

Lee Flight

Network Support, Computer Centre

University of Leicester


Re: 6500 and native 12.1(19)E1a ARP oddity

I haven't seen anything like this before but I suspect that this could be a bug. Use the bugtoolkit to check if it is one.