Figure 1: Flooding A Packet With An Unknown Destination To All Pe Routers In - Juniper EX9200 Features Manual

VPLS Feature Guide for EX9200 Switches
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Figure 1: Flooding a Packet with an Unknown Destination to All PE Routers
in the VPLS Instance
VPLS can be directly connected to an Ethernet switch. Layer 2 information gathered by
an Ethernet switch (for example, media access control [MAC] addresses and interface
ports) is included in the VPLS routing instance table. However, instead of all VPLS
interfaces being physical switch ports, the router allows remote traffic for a VPLS instance
to be delivered across an MPLS LSP and arrive on a virtual port. The virtual port emulates
a local, physical port. Traffic can be learned, forwarded, or flooded to the virtual port in
almost the same way as traffic is sent to a local port.
The VPLS routing table learns MAC address and interface information for both physical
and virtual ports. The main difference between a physical port and a virtual port is that
the router captures additional information from the virtual port, an outgoing MPLS label
used to reach the remote site and an incoming MPLS label for VPLS traffic received from
the remote site. The virtual port is generated dynamically on a Tunnel Services Physical
Interface Card (PIC) when you configure VPLS on the router.
You can also configure VPLS without a Tunnel Services PIC. To do so, you use a
label-switched interface (LSI) to provide VPLS functionality. An LSI MPLS label is used
as the inner label for VPLS. This label maps to a VPLS routing instance. On the PE router,
the LSI label is stripped and then mapped to a logical LSI interface. The Layer 2 Ethernet
frame is then forwarded using the LSI interface to the correct VPLS routing instance.
One restriction on flooding behavior in VPLS is that traffic received from remote PE routers
is never forwarded to other PE routers. This restriction helps prevent loops in the core
network. However, if a CE Ethernet switch has two or more connections to the same PE
router, you must enable the Spanning Tree Protocol (STP) on the CE switch to prevent
loops. STP is supported on MX Series routers and EX Series switches only.
The Junos OS allows standard Bridge Protocol Data Unit (BPDU) frames to pass through
emulated Layer 2 connections, such as those configured with Layer 2 VPNs, Layer 2
circuits, and VPLS routing instances. However, CE Ethernet switches that generate
proprietary BPDU frames might not be able to run STP across Juniper Networks routing
platforms configured for these emulated Layer 2 connections.
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