Dell Brocade DCX, Configuration Manual

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BigIron RX Series Configuration Guide 351
53-1001810-01
Point-to-point ports
13
Point-to-point ports
To take advantage of the RSTP features, ports on an RSTP topology should be explicitly configured
as point-to-point links. Shared media should not be configured as point-to-point links.
NOTE
Configuring shared media or non-point-to-point links as point-to-point links could lead to Layer 2
loops.
The topology in Figure 42 is an example of shared media that should not be configured as
point-to-point links. In Figure 42 , a port on a bridge communicates or is connected to at least two
ports.
FIGURE 42
Example of shared media
Bridge port states
Ports roles can have one of the following states:
•
Forwarding – RSTP is allowing the port to send and receive all packets.
•
Discarding – RSTP has blocked data traffic on this port to prevent a loop. The device or VLAN
can reach the root bridge using another port, whose state is forwarding. When a port is in this
state, the port does not transmit or receive data frames, but the port does continue to receive
RST BPDUs. This state corresponds to the listening and blocking states of 802.1D.
•
Learning – RSTP is allowing MAC address entries to be added to the filtering database but
does not permit forwarding of data frames. The device can learn the MAC addresses of frames
that the port receives during this state and make corresponding entries in the MAC table.
•
Disabled – The port is not participating in RSTP. This can occur when the port is disconnected
or RSTP is administratively disabled on the port.
A port on a non-root bridge with the role of Root port is always in a forwarding state. If another port
on that bridge assumes the Root port role, then the old Root port moves into a discarding state as
it assumes another port role.
A port on a non-root bridge with a Designated role starts in the discarding state. When that port
becomes elected to the Root port role, RSTP quickly places it into a forwarding state. However, if
the Designated port is an Edge port, then the port starts and stays in a forwarding state and it
cannot be elected as a Root port.
A port with an Alternate or Backup role is always in a discarding state. If the port’s role changes to
Designated, then the port changes into a forwarding state.
If a port on one bridge has a Designated role and that port is connected to a port on another bridge
that has an Alternate or Backup role, the port with a Designated role cannot be given a Root port
role until two instances of the forward delay timer expires on that port.