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Deploying the Dell Force10 MXL into a Cisco Nexus Network Environment
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Out-of-Band
An out-of-band interface provides management connectivity to a device without participating in or
relying on a device’s in-band (normal-use) data interfaces. On a switch this means that an out-of-band
interface does not send or receive traffic from the switched links—neither bridged nor routed. Common
out-of-band interface types are Ethernet and serial console—often both are presented with RJ-45
(8P8C) connectors although on IO modules in the Dell PowerEdge M1000e chassis the serial connector is
sometimes a physical USB type-A port requiring a special cable.
Port Channel
See LAG.
PVST
Per-VLAN Spanning-Tree (PVST) is a vendor specific implantation of the spanning tree protocol that
maintains separate instances for each VLAN, passing that instance’s protocol frames within the VLAN it
manages. This method simplifies deployment of multiple VLANs and is popularly used where available.
RSTP
Rapid Spanning-Tree Protocol (RSTP) is a standards-based modified version of the basic spanning tree
protocol that allows for much faster convergence times of spanning tree instances and provides for
special administratively-assigned port states that improve behavior in certain circumstances. RSTP is
originally defined in the IEEE 802.1w standard and is included in 802.1d IEEE Ethernet bridging
standard.
Spanning Tree
Spanning Tree refers to a family of layer-2 management protocols used by Ethernet bridges to establish
a loop-free forwarding topology. At layer-2, Ethernet is a very simple technology that without
intervening protocols or configuration can easily forward traffic in endless loops—see Bridging for an
explanation. The spanning-tree protocols provide a standard way for an interconnected set of Ethernet
bridging devices to only use links that will not cause traffic flows to loop (Ethernet switches—being very
fast bridges—can forward traffic very quickly and looping traffic flows will rapidly saturate all available
bandwidth with unwanted, repeated traffic). In a spanning tree, a single bridge is elected the root
bridge—either by lowest assigned priority or by having the lowest of the presented MAC addresses.
Once a root bridge is elected, every other bridge keeps its one link with the lowest path cost to the
root bridge active. Links with redundant paths are then blocked by switches that don’t have the lowest
path cost for that link. As a result, with spanning tree each non-root bridge effectively has only one
active link between it and the root bridge and the topology of these unblocked links draws a tree to
the root bridge. Spanning tree has a long and involved history on Ethernet and there are many different
implementations with different timings and protocol feature sets—see RSTP, MSTP, and PVST.
Storm
Ethernet Storms—often called broadcast storms—are a descriptive term for excessive (and generally
unwanted) data frames being continuously sent to all available links. Because Ethernet bridges
dynamically learn destinations based on recently received traffic, do not limit the number of times
that data frames can be forwarded across bridges, and do not recognize when data frames have been
forwarded back to the same bridge; without the oversight provided by bridging management protocols,
looped flows of data frames can quickly cause a storm. A storm can quickly saturate all available
bandwidth—catastrophically affecting a network’s performance and reliability. To keep potential
storms from happening, Ethernet bridging generally relies on spanning-tree protocols to establish loop-
free topologies and MAC learning to keep traffic selectively forwarded.