Lucent Technologies 6000 - Manual (Page 522)

14-2 MAX 6000/3000 Network Configuration Guide
Configuring Packet Bridging
Introduction to Lucent bridging
Disadvantages of bridging
Bridges examine all packets on the LAN (in what is termed promiscuous mode), so they incur
greater processor and memory overhead than routers. On heavily loaded networks, this
increased overhead can result in slower performance.
Routers also have other advantages over bridging. Because they examine packets at the
network layer (instead of the link layer), you can filter on logical addresses, providing
enhanced security and control. In addition, routers support multiple transmission paths to a
given destination, enhancing the reliability and performance of packet delivery.
Note: If you have a MAX unit running Multiband Simulation, disable bridging.
How the MAX initiates a bridged WAN connection
When you configure the MAX unit for bridging, it accepts all packets on the Ethernet network
and forwards only those that have one of the following:
• A physical address that is not on the local Ethernet segment (the segment to which the unit
connects).
• A broadcast address.
The important thing to remember about bridging connections is that they operate on physical
and broadcast addresses, not on logical (network) addresses.
Physical addresses and the bridge table
A physical address is a unique, hardware-level address associated with a specific network
controller. A device’s physical address is also called its Media Access Control (MAC) address.
In an Ethernet network, the physical address is a six-byte hexadecimal number assigned by the
Ethernet hardware manufacturer. For example:
0000D801CFF2
If the MAX unit receives a packet whose destination MAC address is not on the local network,
it first checks its internal bridge table. (For a description of the table, see “Transparent
bridging” on page 14-4). If it finds the packet’s destination MAC address in its bridge table,
the unit dials the connection and bridges the packet.
If the address is not specified in its bridge table, the unit checks for active sessions that have
bridging enabled. If there are one or more active bridging links, the unit forwards the packet
across all active sessions that have bridging enabled.
Broadcast addresses
Multiple nodes in a network recognize a broadcast address. For example, the Ethernet
broadcast address at the physical level is:
FFFFFFFFFFFF
All devices on the same network receive all packets with that destination address. The MAX
discards broadcast packets when you configure the MAX as a router only. When you configure
the MAX as a bridge, it forwards packets with the broadcast destination address across all
active sessions that have bridging enabled.