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MICROSOFT NLB on satck ERS4800 with ip routing activate

  • 23 May 2019
  • 5 replies

  • Contributor
  • 18 replies
Hi team,

in ERS4800 stack doing ip routing between servers and user we need deploy two server with MICROSOFT NLB in multicast mode.

I found this document that describes this scenario on section 2.2:


But I´m confused about the config describes the section 2.2.2, as a "mandatory configuration steps" :

"No mandatory configuration steps need to be performed. By default Avaya Ethernet switching platforms will flood Network Load Balancing cluster traffic with no additional configuration being required."

but in "optional configuration steps":

"If Network Load Balancing clusters are deployed using multicast or IGMP-multicast modes, administrators must create a static ARP entry on the core mapping the multicast MAC address to the server VLAN and server switch uplink port."

So, Will it works with no configuration or optional config is needed? Has someone experience with this config?

Thanks in advance.


5 replies

Userlevel 4
It's complicated. It depends on the NLB operating mode (Unicast / Multicast / Multicast+IGMP) and who is acting as the L3 router on the NLB segment.
The ERS does not have explicit support for Microsoft NLB, you would be much better off if using a VSP platform.
If the NLB mode is Unicast, when the router ARPs for the NLB Cluster IP, the Microsoft servers reply with a bogus MAC (which does not exist in the FDB MAC table); ERS are happy to install this ARP, however the ARP entry will point to the port where it was received. When using this ARP to send traffic the ERS will only send traffic out of that port. So if the ERS is acting as router, it cannot be the switch where all the NLB Servers are conected, because it will not flood traffic to multiple ports (the topology shown in 2.2.2 is ok in this respect, as the ERS router is different from the ERS switches where servers are connected).
If the NLB mode is Multicast, when the router ARPs for the NLB Cluster IP, the Microsoft servers reply with a multicast MAC address; ERS won't install this; you need to create a static ARP entry. You will need to provide a single port when you do so; same constraint as above.
In NLB mode is Multicast+IGMP is similar to the vanilla Multicast mode but in addition the Microsft servers will generate IGMP report messages; the idea is that the switch can leverage this info to dynamically flooding traffic where is sees IGMP membership; this will never work on ERS and you should avoid this mode as most modern L2 switches perform snooping on the IP Multicast address (which Microsoft NLB does not use) and not the multicast MAC (which Microsoft NLB uses in Multicast+IGMP mode). If you want to limit the flooding to just the NLB Servers, then simply put them in the same VLAN (instead of trying to use the Multicast+IGMP mode) and use either Unicast or simple Multicast modes.
Thanks Ludovico, the NLB mode is multicast but the ARP entry is learnt by the ERS4800 stack:

STACK1#show arp 172.A.B.C
IP Address Age (min) MAC Address VLAN-Unit/Port/Trunk Flags
172.A.B.C 0 03🇧🇫ac:1b:0a:0f VLAN#10-2/7 D
Total ARP entries : 1
Flags Legend:
S=Static, D=Dynamic, L=Local, B=Broadcast, V=VRRP, v=VRRPOwner

But anyway, Could the recomentadion be to put both servers with Multicast mode in an isolated VLAN and ERS with no config as I understand?


Userlevel 4
Oh ok, I thought that would require static ARP config.
But you see that port 2/7 ?
The ERS is not going to flood packets in the VLAN when it uses that ARP record.
It will only send one copy of the multicast packet out of port 2/7.

To your question, yes you can do that. But who is going to IP route packets to/from that VLAN ? It cannot be the same ERS.
Hi Ludovico,

Yes, it see the port 2/7 because is its own port, sorry but the L2 and L3 is made for the same stack or ERS4800 in which are plugged both servers.

Sorry but I´m very confused about the config or support of this topology.


Userlevel 4
If you have both NLB servers plugged into the same ERS, then this ERS has to act as L2 for that NLB VLAN.
If you give this ERS an IP address to act as gateway on this VLAN, then when routing packets into the NLB VLAN towards the NLB Cluster IP, it will not replicate a copy to both servers, and NLB won't work.
You would need to have a separate ERS acting as the IP router for this VLAN and connected to the 1st ERS via 1 physical (or logical, MLT) connection. This way, the 2nd ERS routes the packet out of its single port to the 1st ERS and the 1st ERS then floods the packet to all NLB Servers in the VLAN.
It's convoluted, because ERS was never expressly designed to support Microsoft NLB.
If you take a VOSS VSP, it has Microsoft NLB support (Unicast and Multicast modes - except VSP4450/4850 models) and you can NLB-enable the VLAN, have servers directly connected and an VSP IP interface on the same VLAN.