Extending a subnet

Mike Lovell mike at dev-zero.net
Sun Oct 4 10:23:59 MDT 2009

Kenneth Burgener wrote:
> On 10/4/2009 1:32 AM, Mike Lovell wrote:
>> you might want to do a 
>> tcp dump on the network and see if much traffic is going to the 
>> broadcast address and then try to figure out if traffic to the broadcast 
>> breaking will cause application problems.
> The traffic tcpdump shows are:
> * directed TCP and UDP traffic
> * arp requests packets
> * 802.1d packets - spanning tree
> * UDP NBT broadcast packets (ports 137,138)
> * UDP rwhod broadcast packet (port 513)
i doubt the NBT and rwhod stuff will break things for you unless you are 
using NetBIOS or rwho and ruptime.
> Arp is lower level then IP, right?  It is sent to all ports of the 
> switch, not to the IP broadcast address, right?
ummmm. according to wikipedia [1], arp broadcasts should go to the 
ethernet broadcast address. i just did some wireshark action and my arps 
when to the ethernet broadcast and where not IP packets. this was done 
on windows though. but i'm pretty sure it should always go to the 
ethernet broadcast cause arp can be used for more network layers than IP.
> 802.1d is also lower level then IP, right?  Will 802.1d spanning tree be 
> broken by this change?  And yes, we have several switches on this one 
> subnet.
STP should not be affected by this. it doesn't (and can't) use IP.
> This network is a "test" network, and not a production network.  So as 
> long as the only problem will be communication between the old 
> configured servers, and the new configured servers, this is acceptable.  
> All of the systems need to be able to access the Internet, and systems 
> within the old config need to be able to speak to other systems within 
> the old config.
> Is there anything else I should check?
nothing else that i can think of. if this is a test environment, you can 
first change the subnet on the gateway and then check connectivity from 
the hosts in the old net. if you expire their arp caches and things will 
work, then you are probably okay.


[1] http://en.wikipedia.org/wiki/Broadcast_address

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