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Understanding Overlay ping and traceroute Packet Support

 

In a virtualized overlay network, existing ping and traceroute mechanisms do not provide enough information to determine whether or not connectivity is established throughout the network. The existing ping and traceroute commands can only verify the basic connectivity between two endpoints in the underlying physical network, but not in the overlay network. For example, you can issue the existing ping command on a Juniper Networks device that functions as a virtual tunnel endpoint (VTEP) to another Juniper Networks devices that also functions as a VTEP in a Virtual Extensible LAN (VXLAN) overlay. In this situation, the ping output might indicate that the connection between the source and destination VTEPs is up and running despite the fact that one of the endpoints (physical servers upon which applications directly run) is not reachable.

Starting with Junos OS Release 14.1X53-D30 for QFX5100 switches, Release 16.1 for EX9200 switches, and Release 16.2 for MX Series routers, overlay ping and traceroute are introduced as troubleshooting tools for overlay networks.

For ping and traceroute mechanisms to work in overlay networks, the ping and traceroute packets, also referred to collectively as Operations, Administration, and Management (OAM) packets, must be encapsulated with the same VXLAN UDP headers (outer headers) as the data packets forwarded over the overlay segment. This implementation ensures that transit nodes forward the OAM packets in the same way as a data packet for that particular overlay segment.

If any connectivity issues arise for a particular data flow, the overlay OAM packet corresponding to the flow would experience the same connectivity issues as the data packet for that flow.

When using ping overlay and traceroute overlay, keep the following in mind:

  • The only tunnel type supported is VXLAN tunnels.

  • The VTEPs in the overlay network that send and receive the overlay ping packets must be Juniper Networks devices that support overlay ping and traceroute.

Overlay ping and traceroute Functionality

Overlay ping and traceroute packets are sent as User Datagram Protocol (UDP) echo requests and replies and are encapsulated in the VXLAN header. VTEPs, which initiate and terminate overlay tunnels, send and receive overlay OAM packets. Overlay ping and traceroute are supported only in VXLAN overlay networks in which the sending and receiving VTEPs are both Juniper Networks devices.

The overlay ping functionality validates both the data plane and the MAC address and IP address of the VTEPs. This additional validation is different from the more commonly known IP ping functionality where the actual destination replies to the echo request without the overlay segment context.

While tracing a route in a VXLAN overlay network, Juniper Networks devices that are along the route that support overlay traceroute additionally provide a timestamp. Third-party devices and Juniper Networks devices that do not support overlay traceroute do not provide this timestamp.

Overlay OAM Packet Format for UDP Payloads

The format of overlay OAM packets depends on the type of payload that is carried in the tunnel. In the case of VXLAN tunnels, the inner packet is a Layer 2 packet.

Note

Only Layer 2 UDP payloads are supported.

Figure 1 shows complete headers on a VXLAN-encapsulated overlay OAM packet.

Figure 1: VXLAN-Encapsulated Overlay OAM Packet
VXLAN-Encapsulated Overlay OAM Packet
  • Outer Ethernet header—Contains the source MAC (SMAC) and destination MAC (DMAC) addresses of directly connected nodes in the physical network. These addresses change at every hop.

  • Outer IP header—Contains the source and destination IP addresses of the Juniper Networks devices that function as the VTEPs that initiate and terminate the tunnel.

  • Outer UDP header—Contains the source port associated with the flow entropy and destination port. The source port is an internally calculated hash value. The destination port is the standard UDP port (4789) used for VXLAN.

  • VXLAN header—Contains the VXLAN Network Identifier (VNI) or the segment ID of the VXLAN, and new router alert (RA) flag bits.

  • Inner Ethernet header—Contains a control MAC address (00-00-5E-90-xx-xx) for both the SMAC and DMAC. This address is not forwarded out of the VTEP. Alternatively, the SMAC can be set to a non-control MAC address. However, if a non-control MAC address is used, the VTEP must not learn the SMAC from the overlay OAM packets.

  • Inner IP header—Contains the source IP address that can be set to the IP address of the endpoint or source VTEP. The destination IP address can be set to the 127/8 address, which ensures that the overlay OAM packet is not forwarded out of the ports of the Juniper Networks device that is configured as a VTEP.

  • Inner UDP header—Contains a new reserved value used in the destination port field in the inner UDP header. This value identifies the incoming UDP packet as an overlay OAM packet.

  • Inner UDP payload—Contains all of the overlay OAM-specific message format and type, length, and value (TLV) definitions.

    The Inner UDP payload format is as follows:

    The OAM-specific message type is one of the following:

    The TLV definition for VXLAN ping is as follows:

Multiple Routing Instance Support

Starting in Junos OS Release 19.3R1, you can use the ping overlay and traceroute overlay commands to verify connectivity and detect fault in a static VxLAN tunnel with multiple routing instances. The ping and traceroute packets created for the ping overlay and traceroute overlay commands follow the same underlay network path as the data packets. This allow you to verify the connectivity between two VTEPs in the overlay VxLAN tunnel. The devices that are configured as the source and destination VTEP must both be running a Junos OS release that supports multiple routing instance, but the transit devices do not.

Release History Table
Release
Description
Starting in Junos OS Release 19.3R1, you can use the ping overlay and traceroute overlay commands to verify connectivity and detect fault in a static VxLAN tunnel with multiple routing instances.
Starting with Junos OS Release 14.1X53-D30 for QFX5100 switches, Release 16.1 for EX9200 switches, and Release 16.2 for MX Series routers, overlay ping and traceroute are introduced as troubleshooting tools for overlay networks.