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Unknown unicast traffic consists of unicastpackets with unknown destination MAC addresses. By default, the switchfloods these unicast packets that traverse a VLAN to all interfacesthat are members of that VLAN. Forwarding this type of traffic cancreate unnecessary traffic that leads to poor network performanceor even a complete loss of network service. This flooding of packetsis known as a traffic storm.

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

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.

The destination MAC address in the packet is looked up in the FIB table for the VPLS in-stance. Again, there are two possibilities: either the destination MAC address has already been learned (known MAC address) or the destination MAC address has not been learned on the access side of PE-Router-C (unknown MAC address). Known MAC address (Figure 4). With conversational MAC learning, the interface learns only the source MAC address of an ingressing frame if that interface already has the destination MAC address present in the MAC address table. If the source MAC address interface does not already know the destination MAC address, it does not learn that MAC.

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

• Next, if the destination MAC address is a unicast address, the switch will look for a match between the destination MAC address of the frame and an entry in its MAC address table. If the destination MAC address is in the table, it will forward the frame out the specified port. If the destination MAC address is not in the table, the switch will forward the frame out all ports except the incoming port. This is known as an unknown unicast. If the destination MAC address.
• And i think for a normal ARP request for an address, that is not already present in the arp table, the destination MAC address of the frame will be ff:ff:ff:ff:ff:ff (broasdcast, because it has to reach all host in the local network), but the target MAC address in the payload of the ARP packet is 0:0:0:0:0:0,because it is unknown at the time where the ARP request is sent out (if we already know that MAC address, it.

(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.

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Configuring Unknown Unicast Forwarding (ELS)

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

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.

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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:
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:
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:
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 TrunkInterface

Purpose

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

Action

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

Display the Ethernet switching table:

Meaning

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The sample output from the show configuration ethernet-switching-options command shows that the unknown unicast forwarding interface forVLAN v1 is interface ge-0/0/7. The showethernet-switching table command shows that an unknown unicastpacket is received on interface ge-0/0/3 with the destinationMAC address (DMAC) 00:01:09:00:00:00 and the source MACaddress (SMAC) of 00:11:09:00:01:00. This shows that theSMAC 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)

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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.

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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).

To configure unknown unicast forwarding options:

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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