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Understanding and Preventing Unknown Unicast Forwarding

Unknown unicast traffic consists of unicast packets with unknown destination MAC addresses. By default, the switch floods these unicast packets that traverse a VLAN to all interfaces that are members of that VLAN. Forwarding this type of traffic can create unnecessary traffic that leads to poor network performance or even a complete loss of network service. This flooding of packets is known as a traffic storm.

To prevent a traffic storm, you can disable the flooding of unknown unicast packets to all VLAN interfaces by configuring specific VLANs or all VLANs to forward all unknown unicast traffic traversing them to a specific interface. You can configure multiple VLANs to forward unknown unicast packets to the same interface or configure different interfaces for different VLANs. This channels the unknown unicast traffic traversing VLANs to specific interfaces instead of flooding all interfaces.

Note:

The unknown-unicast-forwarding feature is not supported on QFX10000 Series platforms.

Verifying That Unknown Unicast Packets Are Forwarded to a Single Interface

Purpose

Verify that a VLAN is forwarding all unknown unicast packets (those with unknown destination MAC addresses) to a single interface instead of flooding unknown unicast packets across all interfaces that are members of that VLAN.

Note:

This procedure uses Junos OS for EX Series switches with support for the Enhanced Layer 2 Software (ELS) configuration style. For ELS details see: Using the Enhanced Layer 2 Software CLI.

Action

(EX4300 Switches) Display the forwarding interface for unknown unicast packets for a VLAN (here, the VLAN name is v1):

(EX9200 Switches) Display the forwarding interface for unknown unicast packets:

Meaning

The sample output from the show commands show that the unknown unicast forwarding interface for VLAN v1is interface ge-0/0/7.

Configuring Unknown Unicast Forwarding (ELS)

Note:

This task uses Junos OS for EX Series switches or QFX Series with support for the Enhanced Layer 2 Software (ELS) configuration style. For ELS details, see Using the Enhanced Layer 2 Software CLI

The unknown-unicast-forwarding feature is not supported on QFX10000 Series platforms.

Unknown unicast traffic consists of packets with unknown destination MAC addresses. By default, the switch floods these packets that traverse a VLAN to all interfaces associated with that VLAN. This flooding of packets is known as a traffic storm and can negatively impact network performance.

To prevent flooding unknown unicast traffic across the switch, configure unknown unicast forwarding to direct all unknown unicast packets within a VLAN to a specific interface. You can configure each VLAN to divert unknown unicast traffic to a different interface or use the same interface for multiple VLANs.

Configuring Unknown Unicast Forwarding on EX4300 Switches

To configure unknown unicast forwarding options on EX4300 switches:

  • Configure unknown unicast forwarding for a specific VLAN and specify the interface to which all unknown unicast traffic will be forwarded:
  • Configure unknown unicast forwarding for all VLANs and specify the interface to which all unknown unicast traffic will be forwarded:

Configuring Unknown Unicast Forwarding on EX9200 Switches

To configure unknown unicast forwarding on EX9200 switches, you must configure a flood filter and apply it to VLANs for which you want to configure unknown unicast forwarding. Flood filters are firewall filters that are applied only to broadcast, unknown unicast, and multicast (BUM) traffic. If a flood filter is configured, only traffic packets that are of the packet type unknown-unicast are forwarded to the interface on which unicast forwarding is configured. A next-hop group redirects the packets according to the action specified in the flood filter.

To configure the next-hop group that receives Layer 2 packets and then configure the interface to which these packets are forwarded:

  1. Configure the next-hop-group action for the Layer 2 interface expected to receive unknown unicast packets:

    For example:

  2. Configure a firewall filter with family address type ethernet-switching:

    For example:

  3. Configure a term in the firewall filter for the interface that receives unknown unicast packets (the interface specified in Step 1) to discard unknown unicast packets:

    For example:

  4. Configure a term in the firewall filter for unknown unicast packets to be flooded to the interface enabled for unknown unicast forwarding by using next-hop-group (in step 1):

    For example:

  5. Configure a default term for the firewall filter to forward packets other than unknown unicast packets:

    For example:

  6. Apply the filter as a flood filter on the VLAN that includes the interface which will receive unknown unicast packets:

    For example:

Verifying That Unknown Unicast Packets Are Forwarded to a Trunk Interface

Purpose

Verify that a VLAN is forwarding all unknown unicast packets (those with unknown destination MAC addresses) to a single trunk interface instead of flooding unknown unicast packets across all interfaces that are members of the same VLAN.

Action

Display the forwarding interface for unknown unicast packets for a VLAN (here, the VLAN name is v1):

Display the Ethernet switching table:

Meaning

The sample output from the show configuration ethernet-switching-options command shows that the unknown unicast forwarding interface for VLAN v1 is interface ge-0/0/7. The show ethernet-switching table command shows that an unknown unicast packet is received on interface ge-0/0/3 with the destination MAC address (DMAC) 00:01:09:00:00:00 and the source MAC address (SMAC) of 00:11:09:00:01:00. This shows that the SMAC of the packet is learned in the normal way (through the interface ge-0/0/3.0), while the DMAC is learned on interface ge-0/0/7.

Configuring Unknown Unicast Forwarding (CLI Procedure)

Unknown unicast traffic consists of packets with unknown destination MAC addresses. By default, the switch floods these packets to all interfaces associated with a VLAN. Forwarding such traffic to interfaces on the switch can create a security issue.

To prevent flooding unknown unicast traffic across the switch, configure unknown unicast forwarding to direct all unknown unicast packets within a VLAN out to a specific trunk interface. From there, the destination MAC address can be learned and added to the Ethernet switching table. You can configure each VLAN to divert unknown unicast traffic to different trunk interfaces or use one trunk interface for multiple VLANs.

Note:

For Junos OS for EX Series switches or QFX Series with support for the Enhanced Layer 2 Software (ELS) configuration style, see Configuring Unknown Unicast Forwarding (ELS).

The unknown-unicast-forwarding feature is not supported on QFX10000 Series platforms.

To configure unknown unicast forwarding options:

Note:

Before you can configure unknown unicast forwarding within a VLAN, you must first configure that VLAN.

  1. Configure unknown unicast forwarding for a specific VLAN (here, the VLAN name is employee):
  2. Specify the trunk interface to which all unknown unicast traffic will be forwarded:

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