Chapter 6. Troubleshooting provider networks

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A deployment of virtual routers and switches, also known as software-defined networking (SDN), may seem to introduce complexity. However, the diagnostic process of troubleshooting network connectivity in OpenStack Networking is similar to the diagnostic process for physical networks. If you use VLANs, you can consider the virtual infrastructure as a trunked extension of the physical network, rather than a wholly separate environment.

6.1. Basic ping testing

The ping command is a useful tool for analyzing network connectivity problems. The results serve as a basic indicator of network connectivity, but might not entirely exclude all connectivity issues, such as a firewall blocking the actual application traffic. The ping command sends traffic to specific destinations, and then reports back whether the attempts were successful.


The ping command is an ICMP operation. To use ping, you must allow ICMP traffic to traverse any intermediary firewalls.

Ping tests are most useful when run from the machine experiencing network issues, so it may be necessary to connect to the command line via the VNC management console if the machine seems to be completely offline.

For example, the following ping test command validates multiple layers of network infrastructure in order to succeed; name resolution, IP routing, and network switching must all function correctly:

$ ping

PING ( 56(84) bytes of data.
64 bytes from ( icmp_seq=1 ttl=54 time=13.4 ms
64 bytes from ( icmp_seq=2 ttl=54 time=13.5 ms
64 bytes from ( icmp_seq=3 ttl=54 time=13.4 ms

You can terminate the ping command with Ctrl-c, after which a summary of the results is presented. Zero percent packet loss indicates that the connection was stable and did not time out.

--- ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2003ms
rtt min/avg/max/mdev = 13.461/13.498/13.541/0.100 ms

The results of a ping test can be very revealing, depending on which destination you test. For example, in the following diagram VM1 is experiencing some form of connectivity issue. The possible destinations are numbered in blue, and the conclusions drawn from a successful or failed result are presented:

path troubleshooting

1. The internet - a common first step is to send a ping test to an internet location, such as

  • Success: This test indicates that all the various network points in between the machine and the Internet are functioning correctly. This includes the virtual and physical network infrastructure.
  • Failure: There are various ways in which a ping test to a distant internet location can fail. If other machines on your network are able to successfully ping the internet, that proves the internet connection is working, and the issue is likely within the configuration of the local machine.

2. Physical router - This is the router interface that the network administrator designates to direct traffic onward to external destinations.

  • Success: Ping tests to the physical router can determine whether the local network and underlying switches are functioning. These packets do not traverse the router, so they do not prove whether there is a routing issue present on the default gateway.
  • Failure: This indicates that the problem lies between VM1 and the default gateway. The router/switches might be down, or you may be using an incorrect default gateway. Compare the configuration with that on another server that you know is functioning correctly. Try pinging another server on the local network.

3. Neutron router - This is the virtual SDN (Software-defined Networking) router that Red Hat OpenStack Platform uses to direct the traffic of virtual machines.

  • Success: Firewall is allowing ICMP traffic, the Networking node is online.
  • Failure: Confirm whether ICMP traffic is permitted in the security group of the instance. Check that the Networking node is online, confirm that all the required services are running, and review the L3 agent log (/var/log/neutron/l3-agent.log).

4. Physical switch - The physical switch manages traffic between nodes on the same physical network.

  • Success: Traffic sent by a VM to the physical switch must pass through the virtual network infrastructure, indicating that this segment is functioning correctly.
  • Failure: Check that the physical switch port is configured to trunk the required VLANs.

5. VM2 - Attempt to ping a VM on the same subnet, on the same Compute node.

  • Success: The NIC driver and basic IP configuration on VM1 are functional.
  • Failure: Validate the network configuration on VM1. Or, firewall on VM2 might simply be blocking ping traffic. In addition, verify the virtual switching configuration and review the Open vSwitch (or Linux Bridge) log files.

6.2. Troubleshooting VLAN networks

OpenStack Networking can trunk VLAN networks through to the SDN switches. Support for VLAN-tagged provider networks means that virtual instances can integrate with server subnets in the physical network.

To troubleshoot connectivity to a VLAN Provider network, complete these steps:

  1. Ping the gateway with ping <gateway-IP-address>.

    Consider this example, in which a network is created with these commands:

    # openstack network create --provider-network-type vlan --provider-physical-network phy-eno1 --provider-segment 120 provider
    # openstack subnet create --no-dhcp --allocation-pool start=,end= --gateway --network  provider public_subnet

    In this example, the gateway IP address is

    $ ping
  2. If the ping fails, do the following:

    1. Confirm that you have network flow for the associated VLAN.

      It is possible that the VLAN ID has not been set. In this example, OpenStack Networking is configured to trunk VLAN 120 to the provider network. (See --provider:segmentation_id=120 in the example in step 1.)

    2. Confirm the VLAN flow on the bridge interface using the command, ovs-ofctl dump-flows <bridge-name>.

      In this example the bridge is named br-ex:

      # ovs-ofctl dump-flows br-ex
       NXST_FLOW reply (xid=0x4):
        cookie=0x0, duration=987.521s, table=0, n_packets=67897, n_bytes=14065247, idle_age=0, priority=1 actions=NORMAL
        cookie=0x0, duration=986.979s, table=0, n_packets=8, n_bytes=648, idle_age=977, priority=2,in_port=12 actions=drop

6.2.1. Reviewing the VLAN configuration and log files

  • OpenStack Networking (neutron) agents - Use the openstack network agent list command to verify that all agents are up and registered with the correct names:

    (overcloud)[stack@undercloud~]$ openstack network agent list
    | id                                   | agent_type         | host                  | alive | admin_state_up |
    | a08397a8-6600-437d-9013-b2c5b3730c0c | Metadata agent     |   | :-)   | True           |
    | a5153cd2-5881-4fc8-b0ad-be0c97734e6a | L3 agent           |   | :-)   | True           |
    | b54f0be7-c555-43da-ad19-5593a075ddf0 | DHCP agent         |   | :-)   | True           |
    | d2be3cb0-4010-4458-b459-c5eb0d4d354b | Open vSwitch agent |   | :-)   | True           |
  • Review /var/log/containers/neutron/openvswitch-agent.log - this log should provide confirmation that the creation process used the ovs-ofctl command to configure VLAN trunking.
  • Validate external_network_bridge in the /etc/neutron/l3_agent.ini file. If there is a hardcoded value in the external_network_bridge parameter, you cannot use a provider network with the L3-agent, and you cannot create the necessary flows. The external_network_bridge value must be in the format `external_network_bridge = "" `.
  • Check the network_vlan_ranges value in the /etc/neutron/plugin.ini file. For provider networks, do not specify the numeric VLAN ID. Specify IDs only when using VLAN isolated project networks.
  • Validate the OVS agent configuration file bridge mappings, confirm that the bridge mapped to phy-eno1 exists and is properly connected to eno1.

6.3. Troubleshooting from within project networks

In OpenStack Networking, all project traffic is contained within network namespaces so that projects can configure networks without interfering with each other. For example, network namespaces allow different projects to have the same subnet range of without interference between them.

To begin troubleshooting a project network, first determine which network namespace contains the network:

  1. List all the project networks using the openstack network list command:

    # (overcloud)[stack@osp13-undercloud ~]$ openstack network list
    | id                                   | name        | subnets                                               |
    | 9cb32fe0-d7fb-432c-b116-f483c6497b08 | web-servers | 453d6769-fcde-4796-a205-66ee01680bba |
    | a0cc8cdd-575f-4788-a3e3-5df8c6d0dd81 | private     | c1e58160-707f-44a7-bf94-8694f29e74d3      |
    | baadd774-87e9-4e97-a055-326bb422b29b | private     | 340c58e1-7fe7-4cf2-96a7-96a0a4ff3231 |
    | 24ba3a36-5645-4f46-be47-f6af2a7d8af2 | public      | 35f3d2cb-6e4b-4527-a932-952a395c4bb3  |

    In this example,examine the web-servers network. Make a note of the id value in the web-server row (9cb32fe0-d7fb-432c-b116-f483c6497b08). This value is appended to the network namespace, which helps you identify the namespace in the next step.

  2. List all the network namespaces using the ip netns list command:

    # ip netns list

    The output contains a namespace that matches the web-servers network id. In this example the namespace is qdhcp-9cb32fe0-d7fb-432c-b116-f483c6497b08.

  3. Examine the configuration of the web-servers network by running commands within the namespace, prefixing the troubleshooting commands with ip netns exec <namespace>:

    # ip netns exec qrouter-62ed467e-abae-4ab4-87f4-13a9937fbd6b route -n
    Kernel IP routing table
    Destination     Gateway         Genmask         Flags Metric Ref    Use Iface         UG    0      0        0 qg-8d128f89-87 U     0      0        0 qg-8d128f89-87   U     0      0        0 qr-8efd6357-96

6.3.1. Performing advanced ICMP testing within the namespace

  1. Capture ICMP traffic using the tcpdump command:

    # ip netns exec qrouter-62ed467e-abae-4ab4-87f4-13a9937fbd6b tcpdump -qnntpi any icmp
  2. In a separate command line window, perform a ping test to an external network:

    # ip netns exec qrouter-62ed467e-abae-4ab4-87f4-13a9937fbd6b ping
  3. In the terminal running the tcpdump session, observe detailed results of the ping test.

    tcpdump: listening on any, link-type LINUX_SLL (Linux cooked), capture size 65535 bytes
    IP (tos 0xc0, ttl 64, id 55447, offset 0, flags [none], proto ICMP (1), length 88) > ICMP host unreachable, length 68
    	IP (tos 0x0, ttl 64, id 22976, offset 0, flags [DF], proto UDP (17), length 60) > [bad udp cksum 0xfa7b -> 0xe235!] UDP, length 32

When you perform a tcpdump analysis of traffic, you might observe the responding packets heading to the router interface rather than the instance. This is expected behavior, as the qrouter performs DNAT on the return packets.

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