Dell PowerConnect B-RX Configuration Manual page 483

When the broken link is repaired, the link's interfaces come up in the Preforwarding state, which
allows RHPs to travel through the restored interfaces and reach the secondary interface on the
Master node.
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Foundry MRP alarm RHP enhancement
Previously, detection of Foundry MRP ring breaks was completely timer based. An absence of Ring
Health Packets (RHP) for a period of 3 "hello times" indicated to the Foundry MRP master that the
ring is broken. This initiated the transition to a topology change as described in the previous
section. The convergence time associated with such an event could take several hundreds of
milliseconds.
Now, each Foundry MRP node is made a more active participant in detecting link failures. When a
link is detected to be down by its "downstream" neighbor, a special packet (called the Alarm RHP
packet) is sent to the Foundry MRP master, indicating that the link is down.
This Foundry MRP packet is sent from the Foundry MRP member to the Foundry MRP master only
when the secondary link goes down and it is sent on the primary link. The destination MAC address
in the packet is the ring MAC address. This allows the packet to be hardware forwarded all the way
to the Foundry MRP master. When the Master switch in the ring receives this packet, it is notified of
a break in the ring. At that point, the secondary interface is immediately transitioned from
"Blocked" to "Forwarding" .
NOTE
The Alarm RHP packet is only sent by the secondary link owner ring to prevent multiple Foundry MRP
masters going into forward where shared rings are configured.
Operation of foundry MRP alarm RHP enhancement
Operation of the Foundry MRP Alarm RHP enhancement is described in
following:
When the link between Switch B and Switch C fails, the "downstream" neighbor (Switch C) detects
the failure of the link and triggers corrective action. The following is the complete sequence of
events that occurs.
1. The downstream neighbor (Switch C) detects a link down event of the link between Switch B
2. Switch C sends a single RHP packet with a special Alarm bit set. The RHP packet is sent in the
3. Switch A receives the special RHP packet (on the secondary interface) that was sent by Switch
4. Switch A immediately transitions its secondary interface (previously in Blocked state) to the
BigIron RX Series Configuration Guide
53-1002253-01
If an RHP reaches the Master node's secondary interface, the ring is intact. The secondary
interface changes to Blocking. The Master node sets the forwarding bit on in the next RHP.
When the restored interfaces receive this RHP, they immediately change state to Forwarding.
If an RHP does not reach the Master node's secondary interface, the ring is still broken. The
Master node does not send an RHP with the forwarding bit on. In this case, the restored
interfaces remain in the Preforwarding state until the preforwarding timer expires, then change
to the Forwarding state.
and Switch C
same direction of flow as that of the normal RHP packets (i.e. on the link to Switch D)
C. It is now aware that the ring is broken even though the dead_interval may not have expired.
Forwarding state.
How ring breaks are detected and healed
14
Figure 68 and in the
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