MPLS Tunnel Inference Using Deep Path Analysis

Deep Path Analysis: Demystifying Transit MPLS Tunnels

Using Deep Path Analysis, you can identify partially obscured transit paths that leverage MPLS. This allows users leveraging our Deep Path Analysis (DPA) technology to view the path taken over the Internet between each agent and destination. As a refresher, we identify MPLS networks using the quick selection section of the Path Visualization interface, stating that X links are part of an MPLS tunnel. There are four types of MPLS tunnels that can be in place - controlled through configuration of individual routers within the MPLS network.

Based on the implementation of various configuration options on transit routers, including RFC4950 and TTL-propagate option, MPLS tunnels can be categorized as Explicit, Implicit, Opaque or Invisible.

Tunnel Type	RFC4950	TTL-propagate
Explicit	Enabled	Enabled
Implicit	Disabled	Enabled
Opaque	Enabled	Disabled
Invisible	Disabled	Disabled

We'll cover the three types of MPLS tunnel inference we support (Explicit, Implicit, and Opaque), using the descriptions and explanations below:

Explicit Tunnel

Explicit MPLS tunnels (tunnels not obscured using router configuration) are displayed in full, with the links between each hop of an MPLS network being shown, complete with label information. When an MPLS hop is detected in the path, the quick selection link identifies links that are part of MPLS networks, and displays the label stack when you hover over an affected link.

Implicit Tunnel

When devices are configured not to send MPLS stack entries, we can still infer that the link is part of an MPLS tunnel, and attempt to infer the hop number of the tunnel. This is represented in the ThousandEyes app as Hop X in an MPLS Tunnel. While no label information may be available (due to provider router configuration), having the source IPs of the transit network may allow a service provider to cross-reference the externally visible map against internal documentation, to find the true path taken.

Opaque Tunnel

In circumstances where a single MPLS label is encountered but the IP TTL is reset at the ingress router, ThousandEyes shows the single hop as an X-hop MPLS tunnel. This information can be helpful when diagnosing network connectivity issues while transiting a service provider's network, since some hops may be obscured from the path visualization output, inferring a sometimes incorrect one-hop transit across the MPLS network.

Invisible Tunnel

In circumstances where a no MPLS label is shown, and the TTL is not reset, ThousandEyes will be unable to detect the presence of an MPLS tunnel, and an erroneous link may be inferred.