Netmon and Netmasks
I have an interesting situation where a Cabletron hub is reporting via SNMP
it's two IP interfaces both have a network mask of 255.255.0.0 but the
discovered interface has 255.255.255.0 in the ovwdb database.
The Cabletron specific are: if1 = 22.214.171.124 if2 = 126.96.36.199
The ring has multiple routers and they have a mask of 255.255.255.0 for the
I traced a demand poll and discovery of the hub and confirmed that the mask
responses from the hub did have the value of 255.255.0.0.
Why would the database modify the mask that was returned by the node? Is it
trying to resolve the fact that the network mask was 255.255.255.0 on the
The fact that the database does not reflect the true data is unsettling. I
hope this is a unique problem to our installation and not a design "feature".
It's hard to say for sure, but this might be working as designed.
It's kind of complicated, but basically, netmon determines what
subnets are in existence by logical ANDing your ROUTER's IP addresses
with the IP addresses' subnet masks (which are taken from the MIB).
For all non-router devices, the subnet mask is deduced
from WHICH SUBNET NETMON THINKS THE NODE IS IN, based on various
tables in routers.
Believe it of not, this is much more reliable tha relying on the
MIB subnet mask value for most nodes.
So, if you have a node in your network, and its subnet mask
is incorrect in the MIB, netmon should put the right subnet mask value
in the object database, even though it differs from the
value in the MIB. netmon is totally brainy, as the kids say.
The bottom line is this: the subnet mask in the object database
should be consistent with the subnet in which netmon places the
node. If you think that something is wrong, delete the node
from all maps and rediscover it.
Support Line: Netmon and Netmasks ITEM: AS7536
Dated: December 1995 Category: N/A
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Comments or suggestions?