ITEM: AS7536

Netmon and Netmasks

Question:

Question:

 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 = 147.40.200.125  if2 = 147.40.200.126

 The ring  has multiple routers and they have a mask of 255.255.255.0 for the
 network.

 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
 routers?

 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". 

Information:

 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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