Skip to main content

Missing Peer

A Missing Peer appears when Forward detects traffic leaving a device toward a synthetic node on a VLAN or subinterface that the synthetic node is not configured to accept. It represents unmodeled uplinks and highlights gaps in synthetic device configuration.

A Missing Peer ensures such traffic does not silently disappear from path search. Instead, Forward exposes it as a placeholder so you can decide how the unmodeled VLAN or subinterface should be handled.

Key Points​

  • Missing Peers are created automatically. You cannot manually add them. Forward adds a Missing Peer only when the collected snapshot clearly indicates a real interface/VLAN/subinterface sending traffic to a synthetic-device uplink that has no matching synthetic-device connection.

  • They are not errors. A Missing Peer indicates that the potential for traffic is present, but the model has no destination for it.

  • They help identify incomplete synthetic-device configurations. You decide whether to:
    – Add another synthetic-device connection
    – Create a new synthetic device for that service
    – Ignore it if the traffic is intentionally out of scope

Why Missing Peers Occur​

Layer-3 synthetic devices (such as Intranet, Internet, or L3VPN nodes) can accept traffic via:

  • An L2 parent interface (access port or trunk), or
  • An L3 interface (subinterface or SVI)

A single physical uplink may carry multiple VLAN tags or multiple subinterfaces.

A Missing Peer appears when:

  1. A device sends traffic out specific VLANs/subinterfaces
  2. The physical interface is connected to a synthetic device
  3. But the synthetic device is configured to accept only some of those VLANs
  4. Forward sees traffic on an unconfigured VLAN and creates a Missing Peer placeholder to receive that flow

Example Scenario (Lab Topology)​

This example lab illustrates precisely when and why Missing Peers appear.

Physical Device: SJ-BLDG1-CM-CORE-1
Interface e7 has three routed subinterfaces (e7.20, e7.21, e7.22), each used by route sending traffic toward the synthetic node:

  • 172.17.38.0/26 → e7.20
  • 172.17.38.64/26 → e7.21
  • 172.17.38.128/26 → e7.22

synth-router peers with SJ-BLDG1-CM-CORE-1 using three VLAN-tagged subinterfaces:

  • e0/1.20 — VLAN 20
  • e0/1.21 — VLAN 21
  • e0/1.22 — VLAN 22

Behind synth-router, the following downstream subnets are reachable:

  • 172.17.38.0/26
  • 172.17.38.64/26
  • 172.17.38.128/26

Lab diagram

How It Was Modeled in Forward​

In the Sources → Synthetic, a sj-bldg1-synth1 synthetic node was added to model the behavior of synth-router from the lab. Only VLAN 20 and VLAN 21 were added as uplink connections. VLAN 22 was not configured.

Synth node configuration

Resulting Behavior​

  • Traffic to 172.17.38.2/26 → VLAN 20 → reaches the sj-bldg1-synth1 synthetic node
    Path to synth node

  • Traffic to 172.17.38.66/26 → VLAN 21 → reaches the sj-bldg1-synth1 synthetic node (not shown on screenshot)

  • Traffic to 172.17.38.130/26 → VLAN 22 → SJ-BLDG1-CM-CORE-1-missing-peer Path to missing peer

The SJ-BLDG1-CM-CORE-1-missing-peer node captures all egress traffic on VLAN 22, making the unmodeled path explicit.

What To Do When You See a Missing Peer?​

  • If the traffic should go to the Intranet synthetic device:
    Add an appropriate VLAN as an additional uplink.

  • If the traffic represents a different external service:
    Create a separate synthetic device and connect that VLAN/subinterface to it.

  • If the traffic is out of scope:
    You may ignore the Missing Peer. It will remain visible only as an indicator of unmodeled traffic.

important

  • You cannot manually create a Missing Peer. It appears only when Forward detects real egress traffic lacking a valid synthetic destination.

  • Removing a Missing Peer requires adjusting the synthetic-device configuration, not editing the Missing Peer itself.

  • Missing Peers help maintain modeling accuracy, ensuring unmodeled traffic is visible rather than silently dropped from analysis.