Inter-VLAN Communication

Inter-VLAN Communication Technologies

VLANIF interfaces and Dot1q termination sub-interfaces require that users in VLANs be located on different network segments. When hosts are located on the same network segment, the source host encapsulates the packets with the destination host' MAC address. The device determines that the packets should be forwarded at Layer 2 but Layer 2 switching can only be performed for devices in the same VLAN. Therefore, the device cannot obtain destination hosts' MAC addresses or forward the packets to the destination host. VLAN aggregation can be used to allow hosts on the same network segment in different VLANs to communicate.

• VLAN aggregation

  VLAN aggregation associates a super-VLAN with a super-VLAN. The sub-VLANs share the IP address of the super-VLAN, which acts as the gateway IP address, to implement Layer 3 connectivity with an external network. Proxy ARP can be enabled between sub-VLANs to implement Layer 3 connectivity between sub-VLANs. VLAN aggregation conserves IP addresses.

  VLAN aggregation applies to scenarios where multiple VLANs share a gateway.

  For details about VLAN aggregation, see VLAN Aggregation Configuration.

• VLAN Switch switch-vlan

  VLAN Switch switch-vlan requires a pre-configured static forwarding path along switching nodes on a network. When a switching node receives VLAN-tagged frames matching VLAN Switch entries, it directly forwards the frames to corresponding interfaces according to the static forwarding path, thus implementing Layer 2 communication.

  Switch-VLAN does not require lookup of the MAC address table, so the forwarding efficiency and security are enhanced. If a switching node connects to many user devices, the network administrator needs to configure each user device in advance to establish a static forwarding path. This increases the manual configuration workload and makes network management inconvenient. Switch-VLAN applies to small-scale networks.

  For details about VLAN Switch switch-vlan, see VLAN Switch Configuration.

Inter-VLAN Communication Through the Same Switch

In Figure 4-14, Host_1 (source host) and Host_2 (destination host) connect to the same Layer 3 switch, are located on different network segments, and belong to VLAN 2 and VLAN 3, respectively. After VLANIF 2 and VLANIF 3 are created on the switch and allocated IP addresses, the default gateway addresses of the hosts are set to IP addresses of the VLANIF interfaces.

Figure 4-14  Using VLANIF interfaces to implement inter-VLAN communication through the same switch

When Host_1 sends a packet to Host_2, the packet is transmitted as follows (assuming that no forwarding entry exists on the switch):

1. Host_1 determines that the destination IP address is on a different network segment from its own IP address, and therefore sends an ARP Request packet to request the gateway MAC address. The ARP Request packet carries the destination IP address of 10.1.1.1 (gateway's IP address) and all-F destination MAC address.
2. When the ARP Request packet reaches IF_1 on the Switch, the Switch tags the packet with VLAN 2 (PVID of IF_1). The Switch then adds the mapping between the source MAC address, VLAN ID, and interface (1-1-1, 2, IF_1) in its MAC address table.
3. The Switch detects that the packet is an ARP Request packet and the destination IP address is the IP address of VLANIF 2. The Switch then encapsulates VLANIF 2's MAC address of 3-3-3 into the ARP Reply packet before sending it from IF_1. In addition, the Switch adds the binding of the IP address and MAC address of Host_1 in its ARP table.
4. After receiving the ARP Reply packet from the Switch, Host_1 adds the binding of the IP address and MAC address of VLANIF 2 on the Switch in its ARP table and sends a packet to the Switch. The packet carries the destination MAC address of 3-3-3 and destination IP address of 10.2.2.2 (Host_2's IP address).
5. After the packet reaches IF_1 on the Switch, the Switch tags the packet with VLAN 2.
6. The Switch updates its MAC address table based on the source MAC address, VLAN ID, and inbound interface of the packet, and compares the destination MAC address of the packet with the MAC address of VLANIF 2. If they are the same, the Switch determines that the packet should be forwarded at Layer 3 and searches for a Layer 3 forwarding entry based on the destination IP address. If no entry is found, the Switch sends the packet to the CPU. The CPU then searches for a routing entry to forward the packet.
7. The CPU looks up the routing table based on the destination IP address of the packet and detects that the destination IP address matches a directly connected network segment (network segment of VLANIF 3). The CPU continues to look up its ARP table but finds no matching ARP entry. Therefore, the Switch broadcasts an ARP Request packet with the destination address of 10.2.2.2 to all interfaces in VLAN 3. The ARP Request packet will be send from IF_2.
8. After receiving the ARP Request packet, Host_2 detects that the IP address is its own IP address and sends an ARP Reply packet with its own. Additionally, Host_2 adds the mapping between the MAC address and IP address of VLANIF 3 to its ARP table.
9. After IF_2 on the Switch receives the ARP Reply packet, IF_2 tags the packet with VLAN 3 to the packet and adds the binding of the MAC address and IP address of Host_2 in its ARP table. Before forwarding the packet from Host_1 to Host_2, the Switch removes the tag with VLAN 3 from the packet. The Switch also adds the binding of Host_2's IP address, MAC address, VLAN ID, and outbound interface in its Layer 3 forwarding table.

The packet sent from Host_1 then reaches Host_2. The packet transmission process from Host_2 to Host_1 is similar. Subsequent packets between Host_1 and Host_2 are first sent to the gateway (Switch), and the Switch forwards the packets at Layer 3 based on its Layer 3 forwarding table.

Inter-VLAN Communication Through Multiple Switches

When hosts in different VLANs connect to multiple Layer 3 switches, you need to configure static routes or a dynamic routing protocol in addition to VLANIF interface addresses. This is because IP addresses of VLANIF interfaces can only be used to generate direct routes.

In Figure 4-15, Host_1 (source host) and Host_2 (destination host) are located on different network segments, connect to Layer 3 switches Switch_1 and Switch_2, and belong to VLAN 2 and VLAN 3, respectively. On Switch_1, VLANIF 2 and VLANIF 4 are created and allocated IP addresses of 10.1.1.1 and 10.1.4.1. On Switch_2, VLANIF 3 and VLANIF 4 are created and allocated IP addresses of 10.1.2.1 and 10.1.4.2. Static routes are configured on Switch_1 and Switch_2. On Switch_1, the destination network segment in the static route is 10.1.2.0/24 and the next hop address is 10.1.4.2. On Switch_2, the destination network segment in the static route is 10.1.1.0/24 and the next hop address is 10.1.4.1.

Figure 4-15  Using VLANIF interfaces to implement inter-VLAN communication through multiple switches

When Host_1 sends a packet to Host_2, the packet is transmitted as follows (assuming that no forwarding entry exists on Switch_1 and Switch_2):

1. The first six steps are similar to steps 1 to 6 in inter-VLAN communication when hosts connect to the same switch. After the steps are complete, Switch_1 sends the packet to its CPU and the CPU looks up the routing table.
2. The CPU of Switch_1 searches for the routing table based on the destination IP address of 10.1.2.2 and finds a static route. In the static route, the destination network segment is 10.1.2.0/24 and the next hop address is 10.1.4.2. The CPU continues to look up its ARP table but finds no matching ARP entry. Therefore, Switch_1 broadcasts an ARP Request packet with the destination address of 10.1.4.2 to all interfaces in VLAN 4. IF_2 on Switch_1 transparently transmits the ARP Request packet to IF_2 on Switch_2 without removing the tag from the packet.
3. After the ARP Request packet reaches Switch_2, Switch_2 finds that the destination IP address of the ARP Request packet is the IP address of VLANIF 4. Switch_2 then sends an ARP Reply packet with the MAC address of VLANIF 4 to Switch_1.
4. IF_2 on Switch_2 transparently transmits the ARP Reply packet to Switch_1. After Switch_1 receives the ARP Reply packet, it adds the binding of the MAC address and IP address of VLANIF4 in its ARP table.
5. Before forwarding the packet of Host_1 to Switch_2, Switch_1 changes the destination MAC address of the packet to the MAC address of VLANIF 4 on Switch_2 and the source MAC address to the MAC address of VLANIF 4 on itself. In addition, Switch_1 records the forwarding entry (10.1.2.0/24, next hop IP address, VLAN, and outbound interface) in its Layer 3 forwarding table. Similarly, the packet is transparently transmitted to IF_2 on Switch_2.
6. After Switch_2 receives packets of Host_1 forwarded by Switch_1, the steps similar to steps 6 to 9 in inter-VLAN communication when hosts connect to the same switch are performed. In addition, Switch_2 records the forwarding entry (Host_2's IP address, MAC address, VLAN, and outbound interface) in its Layer 3 forwarding table.