SNMP works between networks even though routing isn't working.
Posted on 2011-09-30
I just ran into an interesting problem. I have a network monitoring host on one network 10.5.0.0/16. I have wireless access points on another network 10.6.1.0/24. Access between the networks is controlled by a firewall.
I have enabled pings and SNMP on the firewall from 10.5.0.0/16 ---> 10.6.1.0/24. All other traffic is blocked. Pings fail, but the network monitoring host on 10.5.0.0/16 is successful in retrieving data from the access points over SNMP. If I do a trace route, I clearly see a routing problem. The first hop is the local gateway. The 2nd hop is out to the internet then it times out. So I understand that I have a routing problem and I know how to fix it.
My question is:
How can SNMP be working if the routing isn't working? I am certain that SNMP is working from 10.5.0.0/16 to 10.6.1.0/24. I set up monitoring on a host 10.5.1.5. It is retrieving real time data from 3 access points 10.6.1.6, 7, 8.
I thought that SNMP relies on UDP which is routable. How is this SNMP traffic traversing the network boundary when the routes are failing?
Just curious ...