LAN Emulation Overview

The following process normally occurs after a LANE client has been enabled:

• Client requests to join an ELAN--The client sets up a connection to the LECS--a bidirectional point-to-point Configure Direct VCC--to find the ATM address of the LES for its ELAN.

LANE clients find the LECS by using the following methods in the listed order:

• • Locally configured ATM address
  • Interim Local Management Interface (ILMI)
  • Fixed address defined by the ATM Forum
  • PVC 0/17
• Configuration server identifies the LES--Using the same VCC, the LECS returns the ATM address and the name of the LES for the client's ELAN.
• Client contacts the server for its LAN--The client sets up a connection to the LES for its ELAN (a bidirectional point-to-point Control Direct VCC) to exchange control traffic.

Once a Control Direct VCC is established between a LANE client and a LES, it remains up.

• Server verifies that the client is allowed to join the ELAN--The server for the ELAN sets up a connection to the LECS to verify that the client is allowed to join the ELAN--a bidirectional point-to-point Configure Direct (server) VCC. The server's configuration request contains the client's MAC address, its ATM address, and the name of the ELAN. The LECS checks its database to determine whether the client can join that LAN; then it uses the same VCC to inform the server whether the client is or is not allowed to join.
• LES allows or disallows the client to join the ELAN--If allowed, the LES adds the LANE client to the unidirectional point-to-multipoint Control Distribute VCC and confirms the join over the bidirectional point-to-point Control Direct VCC. If disallowed, the LES rejects the join over the bidirectional point-to-point Control Direct VCC.
• LANE client sends LE ARP packets for the broadcast address, which is all 1s--Sending LE ARP packets for the broadcast address sets up the VCCs to and from the BUS.

As communication occurs on the ELAN, each client dynamically builds a local LANE ARP (LE ARP) table. A LE ARP table belonging to a client can also have static, preconfigured entries. The LE ARP table maps MAC addresses to ATM addresses.

Note	LE ARP is not the same as IP ARP. IP ARP maps IP addresses (Layer 3) to Ethernet MAC addresses (Layer 2); LE ARP maps ELAN MAC addresses (Layer 2) to ATM addresses (also Layer 2).

When a client first joins an ELAN, its LE ARP table has no dynamic entries and the client has no information about destinations on or behind its ELAN. To learn about a destination when a packet is to be sent, the client begins the following process to find the ATM address corresponding to the known MAC address:

• The client sends a LE ARP request to the LES for this ELAN (point-to-point Control Direct VCC).
• The LES forwards the LE ARP request to all clients on the ELAN (point-to-multipoint Control Distribute VCC).
• Any client that recognizes the MAC address responds with its ATM address (point-to-point Control Direct VCC).
• The LES forwards the response (point-to-multipoint Control Distribute VCC).
• The client adds the MAC address-ATM address pair to its LE ARP cache.
• Then the client can establish a VCC to the desired destination and send packets to that ATM address (bidirectional point-to-point Data Direct VCC).

For unknown destinations, the client sends a packet to the BUS, which forwards the packet to all clients via flooding. The BUS floods the packet because the destination might be behind a bridge that has not yet learned this particular address.

When a LANE client has broadcast or multicast traffic, or unicast traffic with an unknown address to send, the following process occurs:

• The client sends the packet to the BUS (unidirectional point-to-point Multicast Send VCC).
• The BUS forwards (floods) the packet to all clients (unidirectional point-to-multipoint Multicast Forward VCC).

This VCC branches at each ATM switch. The switch forwards such packets to multiple outputs. (The switch does not examine the MAC addresses; it simply forwards all packets it receives.)

In a single ELAN scenario, the LANE components might be assigned as follows:

• Router 1 includes the following LANE components:
  • The LECS (one per LANE switch cloud)
  • The LES and BUS for the ELAN with the default name man (for Manufacturing)
  • The LANE client for the man ELAN.
• Router 2 includes a LANE client for the man ELAN.
• Router 3 includes a LANE client for the man ELAN.
• Router 4 includes a LANE client for the man ELAN.

The figure below illustrates this single ELAN configured across several routers.

Figure 2

Single ELAN Configured on Several Routers

In the multiple LAN scenario, the same switch and routers are used, but multiple ELANs are configured. See the figure below.

Figure 3

Multiple ELANs Configured on Several Routers

In the following scenario, three ELANs are configured on four routers:

• Router 1 includes following LANE components:
  • The LECS (one per LANE switch cloud)
  • The LES and BUS for the ELAN called man (for Manufacturing)
  • The LES and BUS functions for the ELAN called eng (for Engineering)
  • A LANE client for the man ELAN
  • A LANE client for the eng ELAN
• Router 2 includes only the LANE clients for the man and eng ELANs.
• Router 3 includes only the LANE clients for the man and mkt (for Marketing) ELANs.
• Router 4 includes the following LANE components:
  • The LES and BUS for the mkt ELAN
  • A LANE client for the man ELAN
  • A LANE client for the mkt ELANs

In this scenario, once routing is enabled and network level addresses are assigned, Router 1 and Router 2 can route between the man and the eng ELANs, and Router 3 and Router 4 can route between the man and the mkt ELANs.