Re: Explain why change from unicast to multicast prevents port flooding

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From: Glenn L (the.only(delete)_at_gmail)
Date: 01/13/05 

Date: Wed, 12 Jan 2005 17:38:08 -0800

inline............ 

-- 
Glenn L
CCNA, MCSE 2000/2003 + Security
"Phil" <phil.marsden.google@softwire.co.uk> wrote in message 
news:76fe3757.0501121657.63c1d0f4@posting.google.com...
> I'm trying to get my head around if switching from unicast to
> multicast NLB will solve my switch port flooding problem
>
> Scenario:
> - Assume cluster IP address is c.c.c.c
> - Dual NIC so can use multi or unicast if want inter-host
> communication
> - Just consider the cluster NIC for now
>
> I have a general knowledge of Ethernet and IP but not an in depth
> understanding so I am going to put my thoughts down in laymans terms
> It might even help some people understand better
>
> So, please correct me....
>
> Unicast
> -------
> 1  MaskSourceMac set to 1
> 2  All hosts have MAC address overridden to u-u-u-u-u-u on their
> Cluster NIC
[SNIP]
not sure what you mean here....only one node at a time owns the cluster IP 
address and I think the MAC address as well.
> 3  Inbound packet for c.c.c.c arrives at router
> 4  Router sends ARP request
> 5  One of the cluster host sends ARP reply saying "MAC address for
> c.c.c.c is u-u-u-u-u-u"
> 6  Question:Which host sends this reply?
[snip]
whichever node currently owns the cluster IP address resource.
> 7  Router then knows to send c.c.c.c traffic to MAC u-u-u-u-u-u
> 8  Because MaskSourceMac is set to 1, any ethernet frames from a
> cluster host to the switch have their MAC addresses masked
[SNIP]
I guess I don't follow you on this one.  I don't know what a MaskSourceMac 
is.
> 9  This means that the switch doesn't know which port has MAC
> u-u-u-u-u-u so sends traffic to all ports
> 10 Hence flooding
> 11 Question:Is the ARP reply totally independent of the assigning a
> MAC address to a port?
> 12          i.e. surely if an ARP reply comes back from a port saying
> "send c.c.c.c traffic to MAC u-u-u-u-u-u" then that port has MAC
> u-u-u-u-u-u?
>
>
> Multicast
> ---------
> 1 All hosts have MAC address m-m-m-m-m-m as well as their unique
> factory ones set on the cluster NIC
> 2 Inbound packet for c.c.c.c arrives at router
> 3 Router sends ARP request
> 4 The ARP reply says "MAC address for c.c.c.c is m-m-m-m-m-m
> 5 Question:Which host sends this reply?
[SNIP]
I suppose they all would.  If they all share the same IP multicast address, 
then they also share the same multicast MAC address. 
http://www.microsoft.com/resources/documentation/Windows/2000/server/reskit/en-us/Default.asp?url=/resources/documentation/Windows/2000/server/reskit/en-us/intwork/inaf_mul_wrfn.asp
> 5 Such a mapping in an ARP reply is rejected by some routers and so
> administrator must add a static ARP entry in the router mapping the
> Cluster IP Address to its MAC Address
> 6 Router now broadcasts and traffic for c.c.c.c to the multicast MAC
> m-m-m-m-m-m
> 7 Question:Will this not also result in flooding of other non-cluster
> host ports?
>
Multicasting is new to W2K3 clustering and only happens on the private 
heartbeat NIC and only for clusters of 3 or more nodes.  I am unclear on 
what happens if the private network communication fails, and the public 
network is cofigured for mixed mode, hence the public network takes over 
control of heartbeats.  The documentation does not spell this out.  Does the 
public network start using multicasting? I doubt it.
Easy test though.
Your private heartbeat network should not be going through a router in most 
cases.  It should be connected through a dumb hub or a layer 2 switch on an 
isolated VLAN.
> So, have I got it about right?
>
> Many thanks in advance for any replies.
> Phil.

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