Incomplete ARP issues on Catalyst 3750 L3 switches

[trvenkatesan]

I recently implemented a swicthed LAN network using 2 Cisco 3750s L3 switches & and a bunch of 2950s all trunked to the 3750s. The 2-3750s are connected via a stackwise cable. There are 4 VLANs (10.x.60.X, 10.x.61.x, 10.0.62.x & 10.0.63.x). Routing between VLANs is working fine. I have two issues. One, when I look at the ARP cache, I see a lot of "Incomplete ARP" on all the VLANs and as a result cannot get to these devices. Two, on VLAN63 I can get to any VLAN and to other sites on an MPLS WAN. But I just cannot get to the Internet from any device on this VLAN. I'd like top know why I am seeing so may Incomplete ARPs and does anyone know if there are issues using stackwise technology. Any help would be appreciated. Thanks.

thiru

[Kevin Dorrell]

Hi Thiru,

Incomplete ARP means that the router part of the 3750 was trying to send to an IP address, but couldn't see a response to its ARP request.

You have two 3750s, and therefore to routing engines. I am interested in how these interact. Are you using HSRP between them on each VLAN? And if so (... and here comes the crunch ...) is it working correctly on every VLAN. You see, if your HSRP is not working correct on every VLAN, you could have both routers trying to serve the shared IP and MAC addresses at the same time, which could cause cofusion in the switch part and/or the hosts.

Is there any pattern in what cannot reach what? For example, the incomplete ARPs, are they always for hosts that are on the other switch?

It would be useful to see your config to be able to focus on the reasons for the problem. At the moment, I can only guess.

Kevin Dorrell

Luxembourg

[trvenkatesan]

Kevin,

Thanks for the reply. To answer your question, I am not running HSRP at all. The 2 swicthes are connected by a stack cable and managed as a single switch(stackwise technology). The Incomplete ARPs are on all vlans. I have attached the config & ARP cache dump.

thiru

[Prashanth Krishnappa]

Do all these IP addresses which have incomplete entries really exist in your network? If they are not, check for any software application/virus which is sending out these arp requests.

If the IP addresses do exist, track a couple down to the physical port on which they are, capture sniffer traces and see if they are responding to ARP requests.

[trvenkatesan]

I have checked a few of the IP addresses. They do exist on the network. My next step was like you suggested to sniff those devices. Thanks for the feedback.

[Kevin Dorrell]

OK, thanks for the reply, and sorry for the distraction. I guess I'd better go and read up about StackWise!

Kevin Dorrell

Luxembourg