Network Load Balancing - Cannot Ping Cluster IP from Different Subnet

IT_Dept used Ask the Experts™
Hi guys,

I have an NLB cluster set up with 4 web servers on Server 2008. the cluster is working fine and is using Unicast mode. I have a cluster IP of but this is only pingable from machines on the 10.35.1.x subnet.

I have already added in a static arp entry to the router which has the MAC address which is displayed on the cluster properties in the box that is greyed out.

Any ideas on how we can get our cluster ip address accessible from other internal subnets?
Watch Question

Do more with

Expert Office
EXPERT OFFICE® is a registered trademark of EXPERTS EXCHANGE®
Please make sure Default Gateway IP Address to be inserted in static route if it is different from the above.

Here is an excerpt from a document I created for a similar scenario. IP address will not mean anything to you. Your network guys should be able to make sense of this:

When you try and Ping the cluster address of: “” from an external network, a configuration on the router is necessary.
This is because the MAC address is virtual Multicast MAC address.
IANA (Internet Assigned Numbers Authority)has a mapping process for assigning Multicast MAC addresses with the first 24 bits (+ 1 zero bit) "01-00-5E."
Full range of multicast MAC addresses as follows:
Ethernet Type Address:       01-00-5E-00-00-00-              
Field:       0800
Usage:       Internet      
Multicast Addresses:       Multicast [RFC1112] 01-00-5E-7F-FF-FF
Basically the remaining 23 bits map to the IP address in our case the 49.50.155 part (hex 31 32 9b).  This means we should have a multicast MAC of "01-00-5E-31-32-9b" what we actually have is "03-bf-0a-31-32-9b" which means Microsoft have assigned their own first 24 bits. So even though it is NLB configured as multicast Microsoft have not stuck with the IANA standard. What we need to configure on the "core routers" is a static arp entry as follows:-
arp 03-bf-0a-31-32-9b ARPA
If there is an increase in CPU utilisation after adding this entry a static mac table entry will be added to the "core switches".
mac-address-table static 0100.5301.0200 vlan 50 interface mod/port mod/port disable-snooping

Do more with

Expert Office
Submit tech questions to Ask the Experts™ at any time to receive solutions, advice, and new ideas from leading industry professionals.

Start 7-Day Free Trial