HP ProLiant BL p-Class GbE2 Interconnect Switch Compatibility with Cisco-based Networks - White Paper

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The GbE2 Interconnect Switch further provides interoperability with Cisco’s Per-VLAN Spanning Tree
Plus (PVST+) 801.Q tagging proprietary protocol via the use of spanning tree groups (STG). In the
GbE2 implementation, an administrator creates an STG and then assigns a VLAN to it. This differs
from the Cisco implementation where an administrator creates a VLAN, and then a spanning tree
instance (i.e. STG) is automatically created and assigned to the VALN.
The PVST+ interoperability feature on the GbE2 Interconnect Switch includes the following:
• Tagged ports may belong to more than one STG, but untagged ports can belong to only one
STG.
• When a tagged port belongs to more than one STG, egress BPDUs are tagged to identify their
STG membership.
• An untagged port cannot span multiple STGs.
• Sixteen STGs operating simultaneously are supported per GbE2 Interconnect Switch.
• The default STG 1 can hold multiple VLANs; all other STGs (groups 2–16) can hold one
VLAN.
The GbE2 Interconnect Switch provides two methods to interoperate with PVST+:
1. All GbE2 Interconnect Switch VLANs configured on the ports connected to the Catalyst
switches may be added to the default STG (STG 1).
2. An unique GbE2 Interconnect Switch STG may be created for each of the configured VLANs
connecting to the Catalyst switches.
For rapid spanning tree convergence, many Catalyst switches support Cisco’s proprietary features
PortFast, UplinkFast, and BackboneFast, as well as the industry-standard IEEE 802.1w. The 802.1w
extension is an enhancement to the original 802.1D standard. As noted by Cisco, 802.1w provides
similar convergence time improvements to the Cisco methods, but 802.1w provides the added benefit
of interoperability between vendors. Support for the 802.1w standard is planned for a future GbE2
Interconnect Switch software release. In the meantime, the GbE2 Interconnect Switch does allow the
disabling of spanning tree on a per switch or port basis. This capability is ideal for networks designed
without loops or individual switch ports connected to server blades or other devices where a loop
does not exist.
Multi-link trunking
Multi-link trunking (MLT), also know as link aggregation, port trunking, and Cisco EtherChannel,
combines multiple physical switch ports into a single logical port called a trunk. The bandwidth of the
trunk is the aggregate of the bandwidth of the individual links.
The industry standard for multi-link trunking is IEEE 802.3ad
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. Cisco has developed a similar multi-link
trunking method known as EtherChannel. The GbE2 Interconnect Switch supports IEEE 802.3ad
(802.3-2002) without LACP
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that is compatible with EtherChannel. The GbE2 Interconnect
interoperates with both Fast EtherChannel, providing link aggregation for Fast Ethernet (100MB)
ports, and Gigabit EtherChannel, which aggregates Gigabit Ethernet (1000MB) links. The GbE2
Interconnect Switch supports twelve trunks per switch. Each trunk may contain two to six ports
providing a 12-Gbps aggregate throughput full duplex.
An algorithm automatically applies load balancing to the ports in the trunk. A port failure within the
group causes the network traffic to be directed to the remaining ports. Load balancing is maintained
whenever a link in a trunk is lost or returned to service. This provides flexible and scalable bandwidth
with resiliency and load sharing across the links between the GbE2 Interconnect Switch and Cisco
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The IEEE 802.3 standards have been merged into a single standard defined as IEEE 802.3-2002. IEEE 802.3-2002, section 43 (Link
Aggregation) defines the standards specified in IEEE 802.3ad.
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Link aggregation control protocol (LACP) is an enhancement over EtherChannel and other static multi-link trunking methods. LACP dynamically
learns about the link status and takes decisions on which links to use for and load balancing and failback in case of link failure. As a result,
IEE 802.3ad with LACP is often called dynamic trunking.