Specifications

130 IBM Flex System p260 and p460 Planning and Implementation Guide
Topology 2 in Figure 5-1 on page 129 has each switch module in the chassis with
two direct connections to two enterprise switches. This topology is more
advanced, and it has a higher level of redundancy, but certain specific protocols
such as Spanning Tree or Virtual Link Aggregation Groups must be
implemented. Otherwise, network loops and broadcast storms can cause the
meltdown of the network.
Spanning Tree Protocol
Spanning Tree Protocol is a 802.1D standard protocol used in Layer 2 redundant
network topologies. When multiple paths exist between two points on a network,
Spanning Tree Protocol or one of its enhanced variants can prevent broadcast
loops and ensure that the switch uses only the most efficient network path.
Spanning Tree Protocol is also used to enable automatic network reconfiguration
in case of failure. For example, enterprise switches 1 and 2, together with switch
1 in chassis, create a loop in a Layer 2 network (see Topology 2 in Figure 5-1 on
page 129). We must use Spanning Tree Protocol in that case as a loop
prevention mechanism (because a Layer 2 network cannot operate in a loop).
Assume that the link between enterprise switch 2 and chassis switch 1 is
disabled by Spanning Tree Protocol to break a loop, so traffic is going through
the link between enterprise switch 1 and chassis switch 1. If there is a link failure,
Spanning Tree Protocol reconfigures the network and activates the previously
disabled link. The process of reconfiguration can take tenths of a second, and
the service is unavailable during this time.
Whenever possible, plan to use trunking with VLAN tagging for interswitch
connections, which can help you achieve higher performance by increasing
interswitch bandwidth, and higher availability by providing redundancy for links in
the aggregation bundle.
STP modifications, such as Port Fast Forwarding or Uplink Fast, might help
improve STP convergence time and the performance of the network
infrastructure. Additionally, several instances of STP might run on the same
switch simultaneously, on a per-VLAN basis (that is, each VLAN has its own
copy of STP to load-balance traffic across uplinks more efficiently).
For example, assume that a switch has two uplinks in a redundant loop topology,
and several VLANs are implemented. If single STP is used, then one of these
uplinks is disabled and the other carries traffic from all VLANs. However, if two
STP instances are running, then one link is disabled for one set of VLANs while
carrying traffic from another set of VLANs, and vice versa. Both links are active,
thus enabling more efficient use of available bandwidth.