Using a Source Address to Demultiplex Traffic

Figure 21 shows how you can use static subscriber interfaces to differentiate traffic for VPN access, based on the traffic’s source address.

Figure 21: Subscriber Interfaces Using a Source Address to Demultiplex Traffic

To configure the static subscriber interfaces shown in Figure 21, perform the following steps:

1. Configure a primary IP interface on a supported layer 2 interface.
   1. Create a layer 2 interface.

      host1(config)#interface fastEthernet 4/1

   2. Create a primary IP interface.

      host1(config-if)#ip address 10.1.1.1 255.255.255.0

   3. Exit Interface Configuration mode.

      host1(config-if)#exit

2. Configure subscriber interface IP1.
   1. Create the shared IP interface.

      host1(config)#virtual-router vra
      Proceed with new virtual-router creation? [confirm] yes
      host1:vra(config)#interface ip ip1

   2. Associate the shared IP interface with the layer 2 interface by using one of the following methods:
      • Static

        host1:vra(config-if)#ip share-interface fastEthernet 4/1

      • Dynamic

        host1:vra(config-if)#ip share-nexthop 10.1.1.2

   3. To fully configure the shared interface, assign an address or make it unnumbered.

      host1:vra(config-if)#ip unnumbered loopback 0

   4. Specify the source addresses for the subscriber interface to use to demultiplex traffic, then exit Interface Configuration mode.

      host1:vra(config-if)#ip source-prefix 10.10.3.0 255.255.255.0

      host1:vra(config-if)#exit

3. Create a static route that sends traffic for destination address 10.10.3.0 to subscriber interface IP1.

   host1:vra(config)#ip route 10.10.3.0 255.255.255.0 ip ip1

4. Repeat Step 2 to configure subscriber interface IP2.

   host1(config)#virtual-router vrb
   Proceed with new virtual-router creation? [confirm] yes
   host1:vrb(config)#interface ip ip2
   host1:vrb(config-if)#ip share-interface fastEthernet 4/1
   host1:vrb(config-if)#ip unnumbered loopback 0
   host1:vrb(config-if)#ip source-prefix 10.10.4.0 255.255.255.0
   host1:vrb(config-if)#exit

5. Create a static route that sends traffic for destination address 10.10.4.0 to subscriber interface IP2.

   host1:vrb(config)#ip route 10.10.4.0 255.255.255.0 ip ip2

6. Specify that DHCP relay does not install host routes—this avoids a conflict that can causes undesirable ARP behavior.

   host1(config)#set dhcp relay inhibit-access-route-creation

   For details about the cause of this conflict and the use of the set dhcp relay inhibit-access-route-creation command to avoid the conflict, see “Configuring DHCP Relay”.

interface ip

• Use to create an IP interface to share a layer 2 interface.
• Use the specified name to refer to the shared IP interface; you cannot use the layer 2 interface to refer to the shared IP interface, because the shared interface can be moved.
• Example

  host1(config)#interface ip si0

• Use the no version to delete the IP interface.
• See interface ip

ip demux-type da-prefix

• Use to specify that the router use a destination address to demultiplex traffic for the subscriber interface.
• Example

  host1(config-if)#ip demux-type da-prefix

• Use the no version to restore the default situation in which the router uses a source address to demultiplex traffic.
• See ip demux-type da-prefix

ip destination-prefix

• Use to specify a destination address for a subscriber interface or for a primary IP interface.
• On the ERX-1440 router or the E320 router, you can configure up to 1024 subnets for static subscriber interfaces per primary IP interface when each subnet has a variable network mask that is less than /32. The number of subnets identifying a single route (/32) is still limited by the global maximum of 16,000 hosts per line module.
• Example

  host1(config-if)#ip destination-prefix 196.168.2.2 255.0.0.0

• Use the no version to remove the association between the interface and the specified IP destination address and mask.
• See ip destination-prefix

ip share-interface

• Use to specify the layer 2 interface for this IP interface to share. The command fails if the layer 2 interface does not yet exist.
• If you issue this command on a shared IP interface, you cannot issue the ip share-nexthop command for the interface.
• After creating the shared IP interface, you can configure it as you do any other IP interface.
• The shared interface is operationally up when the layer 2 interface is operationally up and IP is properly configured.
• You can create operational shared IP interfaces in the absence of a primary IP interface.
• Example

  host1(config-if)#ip share-interface atm 5/3.101

• Use the no version to remove the association between the layer 2 interface and the shared IP interface. You can delete shared and primary IP interfaces independently.
• See ip share-interface

ip share-nexthop

• Use to specify that the shared IP interface dynamically tracks a next hop. If the next hop changes, the shared IP interface moves to the new layer 2 interface associated with the IP interface toward the new next hop.
• If you issue this command on a shared IP interface, you cannot issue the ip share-interface command for the interface.
• If you issue this command on a shared IP interface, the shared interface cannot dynamically track the next hop for the specified destination if the next-hop IP address is resolvable over MPLS.
• If you specify a virtual router, the command fails if the VR does not already exist. If you do not specify a VR, the current VR is assumed.
• After creating the shared IP interface, you can configure it as you do any other IP interface.
• The shared interface is operationally up when the layer 2 interface associated with the specified next hop is operationally up and IP is properly configured.
• Example

  host1(config-if)#ip share-nexthop 192.168.10.16

• Use the no version to halt tracking of the next hop.
• See ip share-nexthop

ip source-prefix

• Use to specify a source address for a subscriber interface.
• On the ERX-1440 router or the E320 router, you can configure up to 1024 subnets for static subscriber interfaces per primary IP interface when each subnet has a variable network mask that is less than /32. The number of subnets identifying a single route (/32) is still limited by the global maximum of 16,000 hosts per line module.
• Example

  host1(config-if)#ip source-prefix 192.168.0.0 255.0.0.0

• Use the no version to remove the association between the interface and the specified IP source address and mask.
• See ip source-prefix

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