Terminal Server NLB

1. the address is working locally on my LAN. Workstations on the network can access the without a problem. Any networks past my router are not able to get to the IP address. The router is not routing the requests to that IP address. and experts have determined that I should add a static ARP entry into the router in order to get the NLB address routed.

Can you explain why this needs to be done. Our router people are questioning it.
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Why do you need a static route? Unless you have setup routing protocols like RIP or OSPF in your network (where routers can automatically discover things), routers need to know how to access other networks and where to send the packets.

Setting a static route tells your router which interface it should use to access another network. It is common practice. Why is it being questioned?

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I had this same exact issue with microsoft NLB here's a little more information


"Possible Cause

If you are using multicast support, you might find that your router has difficulty resolving the primary IP address into a multicast media access control (MAC) address using the ARP protocol.

I'm not sure if that gives you an explaination as to what your issue is but thats why we had to do it. I had to go back and forth over this with the ISP for a while.

To configure a static ARP mapping, issue the arp ip-address hardware-address type [alias] from the global configuration mode of the router.

Nayyar HH (CCIE RS)Network ArchitectCommented:
The issue is because in MS NLB multicast mode your cluster (or vip) address is a unicast address however when the Router/Switch ARPs for the MAC address it recieves a Multicast MAC address (usually starts with 01:00:5e:xx:xx). The router/switch doesnt accept this as it expects a Unicast IP to resolve to a Unicast MAC address, hence router/switch cannot forward packets to the cluster as it doesnt have its MAC. It will show up as incomplete in the arp tacnle of the router/switch.

To fix the issue add a static arp entry as follows;

arp <unicast_IP> <multicast_MAC>

Note that the packets will be process switched. See link below .....


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