Layer 2 Control Protocol On Acx Series Routers - Juniper ACX1000 Configuration Manual

ACX Series Universal Access Router Configuration Guide

Layer 2 Control Protocol on ACX Series Routers

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Understanding Root Protection for Spanning-Tree Instance Interfaces in a Layer 2
Switched Network on page 429
Root Protect for a Spanning-Tree Instance Interface on page 430
Enabling Root Protection for a Spanning-Tree Instance Interface on page 430
LLDP Overview on page 431
Configuring LLDP in ACX Series on page 432
LLDP Operational Mode Commands on page 434
On ACX Series routers in a Layer 2 environment, you can configure various spanning-tree
protocol versions to create a loop-free topology in Layer 2 networks.
A spanning-tree protocol is a Layer 2 control protocol (L2CP) that calculates the best
path through a switched network containing redundant paths. A spanning-tree protocol
uses bridge protocol data unit (BPDU) data frames to exchange information with other
switches. A spanning-tree protocol uses the information provided by the BPDUs to elect
a root bridge, identify root ports for each switch, identify designated ports for each physical
LAN segment, and prune specific redundant links to create a loop-free tree topology.
The resulting tree topology provides a single active Layer 2 data path between any two
end stations.
NOTE: In discussions of spanning-tree protocols, the terms bridge and switch
are often used interchangeably.
The ACX Series Universal Access Routers support Spanning-Tree Protocol (STP), Rapid
Spanning-Tree Protocol (RSTP), Multiple Spanning-Tree Protocol (MSTP), and VLAN
Spanning-Tree Protocol (VSTP).
The original STP is defined in the IEEE 802.1D 1998 specification. A newer version called
RSTP was originally defined in the IEEE 802.1w draft specification and later incorporated
into the IEEE 802.1D-2004 specification. A recent version called MSTP was originally
defined in the IEEE 802.1s draft specification and later incorporated into the
IEEE 802.1Q-2003 specification. The VSTP is compatible with the Per-VLAN Spanning
Tree Plus (PVST+) and Rapid-PVST+ protocols supported on Cisco Systems routers
and switches.
RSTP provides faster reconvergence time than the original STP by identifying certain
links as point to point and by using protocol handshake messages rather than fixed
timeouts. When a point-to-point link fails, the alternate link can transition to the
forwarding state without waiting for any protocol timers to expire.
MSTP provides the capability to logically divide a Layer 2 network into regions. Every
region has a unique identifier and can contain multiple instances of spanning trees. All
regions are bound together using a Common Instance Spanning Tree (CIST), which is
responsible for creating a loop-free topology across regions, whereas the Multiple
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