Troubleshooting topology problems in NAV

After completing this how-to, you should be able to track down the causes of physical topology detection problems in NAV, either solving them or collecting the necessary information to report a problem to the developers.


This how-to isn’t an exhaustive guide, but covers some of the most commonly experienced issues.


NAV provides several layers of topology information:

  • The physical topology of your network (often referred to as layer 2 topology in NAV, since the topology information comes from what is visible from an OSI layer 2 standpoint - see Ethernet, or IEEE 802.1D).

  • The VLAN topology of your network. These are the virtual sub-topologies of your physical network topology, as implemented through IEEE 802.1Q.

  • Your IP (or layer 3) topology. This is simply derived from the IP addresses and subnet prefixes configured on the interfaces of your routers and switches.

Physical topology

Two connected devices aren’t seen as connected by NAV

The most common issue reported to the NAV developers is that two devices that should be connected in the Netmap are not.


Do not blame Netmap. Not without investigating further.

In such a case, you must first ascertain whether the problem is in Netmap itself, or in the NAV database. It is only a problem with Netmap if what it draws is not consistent with NAV data.

  1. Search for one of the two devices in the NAVbar and click on it to open its information page.

  2. Click on the Ports tab to see the port listing for this device.

  3. In the two rightmost columns of this report, the detected neighbor and it’s connected port (if detected) is listed.

  4. You can click the To Device column twice to sort the ports beginning with the ones that NAV has detected a neighboring device on.

You should see something that looks like this:


As you can see, this device is connected to palin-sw, idle-gsw, gilliam-sw, cleese-sw and chapman-sw. If there are devices you expect to be listed as neighbors here that are not, your problem is not with Netmap.

How NAV builds physical topology information

The collection stage

NAV uses several sources of information to detect your physical topology, all collected from SNMP queries against your monitored devices:

  1. The ARP and ND caches of your routers.

  2. The forwarding tables of your switches (CAM tables).

  3. LLDP (Link Layer Discovery Protocol, IEEE 802.1AB)

  4. CDP (Cisco Discovery Protocol)

The first of these is collected by the ipdevpoll ip2mac job in half-hour intervals, whereas the last three are all collected by the ipdevpoll topo job in 15-minute intervals.

These pieces of information are used to build lists of direct (or next-hop) neighbor candidates for each of the ports in your monitored network.

Whereas protocols like CDP and LLDP, if supported and enabled, can usually provide a pretty precise indication of what a port’s next-hop neighbor is, NAV can usually also detect topology without them.


Also, proprietary protocols, such as CDP, can also be imprecise in heterogeneous networking environments, as CDP packets will be invisible to any man-in-the-middle switch that doesn’t support the protocol.

NAV knows the management IP address of all your monitored devices, and uses ARP/ND data as a means of discovering the Ethernet MAC addresses associated with these (Ethernet addresses of individual ports are also collected on SNMP-enabled devices). Coupled with forwarding tables from your switches, NAV can generate a list of next-hop neighbor candidates for each switch port.

When these collection jobs have run, each uplink/downlink port in your network should have a list of possible next-hop neighbors and which protocol provided each of them.

The analysis stage

The navtopology program runs as a cron job once every hour, analyzing the neighbor candidate lists collected by ipdevpoll. It prefers LLDP information over CDP and CAM information, but will otherwise use a very generic algorithm for discerning which of the candidates are true next-hop neighbors.

In a case where you have made changes to which devices are monitored by NAV, you can run the command manually, once ipdevpoll has finished it’s topo job for any new devices. The command navtopology --l2 will run a physical topology analysis, and should be pretty quick.