Acquiring Initial Links; Detecting Failures; Failing Over - Juniper JUNOSE SOFTWARE FOR E SERIES 11.3.X - LINK LAYER CONFIGURATION GUIDE 2010-10-13 Configuration Manual

JunosE 11.3.x Link Layer Configuration Guide
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Figure 29 on page 215 displays a network with RSTP enabled on Gigabit Ethernet switches
1 and 2. The local port receives bridge PDUs (BPDU), Ethernet broadcasts, and flooded
unicast packets. If Link 1 is initially active and Link 2 is the backup, initial traffic destined
for the LAG can be Ethernet broadcasts, PPPoE PDUs, or flooded Ethernet unicasts. The
responses are only sent on the active link; in this case, Link 1.
The Ethernet network topology that is managed by RSTP learns that the MAC for the
LAG group is through Link 1. Broadcasts and flooded packets are still sent on Link 2. If
Link 1 is no longer viable, but has not suffered a physical failure, then that address ages
out of the bridge databases and any packets directed to the LAG are flooded. The LAG
detects traffic on Link 2 after the minimum delay time and then fails over.

Acquiring Initial Links

In an RSTP network, the system uses the following process for acquiring new links:
Based on the configuration, the system selects a link as active and the other as
1.
redundant.
The spanning tree converges on a topology.
2.
When convergence occurs and the status of the spanning tree ports change to
3.
forwarding, network traffic appears on the links.
The local port detects the traffic and confirms the active member as active and the
4.
other as the redundant port. Because the initial traffic is broadcast or flooded, both
ports receive the packets. However, because of the timing difference, the selected
active port remains active.

Detecting Failures

In an RSTP network, the system uses the following process for detecting when the link
has switched over due to topology changes:
BPDUs are ignored on the redundant port and system time is not retrieved. Because
1.
MAC learning forces non-flooded unicast packets to the active link, traffic to the
redundant link does not receive non-flooded packets. The most recent system time
is always retrieved when a network packet is received.
When the network cannot reach the active link because of topology changes, traffic
2.
appears on the redundant link. The redundant port detects the traffic and captures
the latest timestamp. When the difference between the timestamp of the first
non-bridged PDU and the time the last packet that was received on the active port is
sufficiently large to account for the minimum spanning tree convergence time and
latency for flooded and broadcast packets, then the port fails over.

Failing Over

In an RSTP network, the system uses the following process to fail over:
When the link has failed over, the system monitors the previously active port.
1.
When a network packet is received on the redundant port, the system retrieves the
2.
timestamp. If the difference in timestamps between that one and the most recent on
the current active port is more than the configured failover delay time, then the link
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