When An Rstp Root Bridge Fails; Switches Must Relearn Mac Addresses After A Link Failure; Selecting A Spanning Tree Protocol - Juniper EX9200 Features Manual

Spanning-Tree Protocols Feature Guide for EX9200 Switches

When an RSTP Root Bridge Fails

Switches Must Relearn MAC Addresses After a Link Failure

Selecting a Spanning Tree Protocol

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If the BPDU data matches the existing entry in the MAC address table, the port resets
a timer called max age to zero and then forwards a new BPDU with the current active
topology information to the next port in the spanning tree.
If the topology in the BPDU has been changed, the information is updated in the MAC
address table, max age is again set to zero, and a new BPDU is forwarded with the
current active topology information to the next port in the spanning tree.
When an RSTP port does not receive a BPDU for three hello times, it reacts one of two
ways. If the port is the root port, a complete rework of the spanning tree occurs—see
When an RSTP Root Bridge Fails. If the bridge is any non-root bridge, RSTP detects
that the connected device cannot send BPDUs and converts that port to an edge port.
When a link to the root port goes down, a flag called a topology change notification
(TCN) is added to the BPDU. When this BPDU reaches the next port in the VLAN, the
MAC address table is flushed and the BPDU is sent to the next bridge. Eventually, all
ports in the VLAN have flushed their MAC address tables. Then, RSTP configures a new
root port.
After a root port or a designated port fails, the alternate or backup port takes over after
an exchange of BPDUs called the proposal-agreement handshake. RSTP propagates
this handshake over point-to-point links, which are dedicated links between two network
nodes, or switches, that connect one port to another. If a local port becomes a new root
or designated port, it negotiates a rapid transition with the receiving port on the nearest
neighboring switch by using the proposal-agreement handshake to ensure a loop-free
topology.
Because a link failure causes all associated ports to flush their MAC address table, the
network might be slower as it floods to relearn the MAC addresses. There is a way to
speed up this relearning process. During TCN propagation, the Layer 2 forwarding table
of switches is flushed, resulting in a flood of data packets. The Address Resolution Protocol
(ARP) feature causes the switch to proactively send ARP requests for IP addresses in
the ARP cache (present because of Layer 3 VLAN interface). With ARP on STP enabled,
as the reply comes through, the switches builds up the Layer 2 forwarding table, thus
limiting the flooding later. Enabling ARP on STP is most useful to prevent excessive
flooding in large Layer 2 networks using RVIs.
NOTE: The ARP feature is not available on Junos OS for EX Series switches
with support for the Enhanced Layer 2 Software (ELS) configuration style.
The default factory configuration for EX Series switches is RSTP, a faster version of the
original STP. To determine which spanning-tree protocol is best for your situation, see
Table 3 on page 7 below.
Copyright © 2017, Juniper Networks, Inc.