# VPLS and Transparent Bridging Overview

A single VPLS instance is analogous to a distributed learning bridge (also known as a bridge group) used for transparent bridging, and performs similar functions. In effect, a VPLS instance is a new or existing bridge group that has additional VPLS attributes configured.

A bridge group is a collection of bridge interfaces stacked on Ethernet layer 2 interfaces to form a broadcast domain. Similarly, a VPLS instance is a collection of network interfaces stacked on Ethernet layer 2 interfaces that transmits packets between the router, or VE device, and the CE device located at the edge of the customer’s network. In addition, the VPLS virtual core interface enables a VPLS instance to forward traffic not only between bridge interfaces, like a bridge group, but also between a bridge (network) interface and the service provider core.

Like a bridge group, each VPLS instance maintains its own set of forwarding tables and filters that enables it to learn the network topology by examining the media access control (MAC) source address of every incoming packet. The VPLS instance then creates an entry in its forwarding table that includes the MAC address and associated network interface where the packet was received. For traffic on the VPLS virtual core interface, the VPLS instance captures additional information that includes an outgoing MPLS label used to reach the remote site and an incoming MPLS label used to process traffic received from the remote site.

Table 108 through Table 111 represent the forwarding tables on PE 1 and PE 2 for the sample VPLS topology illustrated in VPLS Sample Topology.

Table 108: VPLS Forwarding Table on PE 1 for VPLS A

Interface | MAC Address | Outgoing Label | Received Label
--- | --- | --- | ---
Fast Ethernet 3/1 | 1a1a.1a1a.1a1a | – | –
VPLS virtual core interface | 3a3a.3a3a.3a3a | 18 | 324

Table 109: VPLS Forwarding Table on PE 1 for VPLS B

Interface | MAC Address | Outgoing Label | Received Label
--- | --- | --- | ---
Bridged Ethernet 2/0.12 | 2b2b.2b2b.2b2b | – | –
VPLS virtual core interface | 4b4b.4b4b.4b4b | 25 | 526

Table 110: VPLS Forwarding Table on PE 2 for VPLS A

Interface | MAC Address | Outgoing Label | Received Label
--- | --- | --- | ---
Fast Ethernet 3/5 | 3a3a.3a3a.3a3a | – | –
VPLS virtual core interface | 1a1a.1a1a.1a1a | 42 | 107

Table 111: VPLS Forwarding Table on PE 2 for VPLS B

Interface | MAC Address | Outgoing Label | Received Label
--- | --- | --- | ---
Bridged Ethernet 2/0.21 | 4b4b.4b4b.4b4b | – | –
VPLS virtual core interface | 2b2b.2b2b.2b2b | 63 | 872