Acquiring Initial Links; Detecting Failures; Figure 29: Dual-Homed Heterogeneous Configuration In An Rstp - Juniper JUNOSE SOFTWARE 11.0.X - LINK LAYER CONFIGURATION GUIDE 4-1-2010 Configuration Manual

Figure 29: Dual-Homed Heterogeneous Configuration in an RSTP Network
Figure 29 on page 219 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
4.
the 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.
1.
Because 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.
Chapter 6: Configuring 802.3ad Link Aggregation and Link Redundancy
Ethernet Link Redundancy Behavior
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