Protocols for Redundant Paths

If Fibre Channel over Ethernet is integrated into the data center or voice and video into LAN, then chances are good that the network will have to undergo a redesign. This is because current network designs build on the provision of substantial overcapacities.

The aggregation/distribution and core layer are often laid out as a tree structure in a multiple redundant manner. In this case, the redundancy is only used in case of a fault. As a result, large bandwidth resources are squan- dered and available redundant paths are used only insufficiently or ineffi- ciently, if at all.

However, short paths and thus lower delay times are the top priority for new real-time applications in networks. So-called "shortest path bridging" will im- plement far-reaching changes in this area.

The most important protocols developed for network redundancy are listed below. Each of these protocols has its own specific characteristics.

Spanning Tree

The best-known and oldest protocol, that prevents “loops” in star-shaped structured Ethernet networks and there- fore allows redundant paths, is the Spanning Tree Protocol (STP). The method was standardized in IEEE 802.1D and came into use in the world of layer 2 switching.

The STP method works as follows:

• If redundant network paths exist, only one path is active. The others are passive – or actually they are “turned off” (for loop prevention).

• The switches determine independently which paths are active and which are passive (negotiation). Manual configuration is not required.

• If an active network path fails, a recalculation is performed for all network paths and the required redundant connection is built.

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Layer 2 switching with Spanning Tree has two essential disadvantages:

• Half of the network paths that are available are not used – there is therefore no load distribution.

• The calculation of network paths in case of a failure can take a full 30 seconds for a network that is large and very complex. The network functions in a very restricted manner during this time.

The Rapid Spanning Tree protocol (RSTP) was designed as a solution for this second problem, and has also been standardized for quite some time. Its basic idea lies in not stopping communication completely when a network path fails, but continuing to work with the old configuration until the re-calculation is completed – and therefore a few connections do not work. It is true this solution does not resolve the problem immediately. However, in case of a more “minor” failure, such as a path failing in the “edge” of the network, the network does not immediately come to a total standstill and downtime in this case amounts to only 1 second or so.

Since after more 25 years the SpanningTree process is pushing its limits, the IETF(InternetEngineering Task Force) wants to replace it with the Trill Protocol (Transparent Interconnection of Lots of links) which internalizes STP.

The more elegant method is Layer 3 Switching (IP address) which, strictly speaking, represents more routing than a classic switching. Routing protocols such as RIP and OSPF come into use in the layer 3 environment, which is why both the network paths that are switched off as well as the relatively extensive re-calculations do not apply when an active path fails.

Layer 3 switching requires extremely careful planning of IP address ranges, since a separate segment must be implemented for each switch.In practice, one frequently finds some combination which uses layer 3 switching in the core area and or layer 2 switching in the aggregation/access area or floor area.

Planning a high-performance and fail-safe network within a building, building complex or campus is already a costly procedure. It is even a more complex process to construct a network that comprises multiple locations within a city (MAN Metropolitan Area Network).

In contrast to a WAN implementation, all locations in a MAN are connected at a very high speed. The core of a MAN is a ring, based on gigabit Ethernet for example. The ring is formed through point-to-point connections between the switches at separate locations. A suitable method must ensure that an alternate path is selected in case a connection between two locations fails. In general, this is realized through layer 3 switching. The primary location, which is generally also the server location, is equipped with redundant core switches which are all connected to the ring.

Link Aggregation

Link Aggregation is another IEEE standard (802.3ad, 802.1ax since 2008) and designates a method for bundling multiple physical LAN interfaces into one logical channel. This technology was originally used exclusively to increase the data throughput between two Ethernet switches. However, current implementations can also connect servers and other systems via link aggregation, e.g. in the data center.

Since incompatibilities and technical differences exist when aggregating Ethernet interfaces, the following terms, based on manufacturer, are used as synonyms.

• Bonding, in the Linux environment

• Etherchannel, for Cisco

• Load balancing, for Hewlett-Packard

• Trunking, for 3Com and Sun Microsystems

• Teaming, for Novell Netware

• Bundling, as the German term

Link Aggregation is only intended for Ethernet and assumes that all ports that are to be aggregated have identical speeds and support full-duplex. The possible number of “aggregation” ports is different and based on manufacturer implementation.

The method only makes sense when interfaces that are as fast as possible are bundled. Four times Fast Ethernet 100 Mbit/s (full-duplex) corresponds to a maximum 800 Mbit/s, on the other hand four times gigabit Ethernet gives 8 Gbit/s.

Another advantage is increased failure safety. If a connection fails, the logical channel as well as a transmission path continue to exist, even if it is at a reduced data throughput.

Shortest Path Bridging

In Shortest Path Bridging (IEEE 802.1aq) the Shortest Path Tree (SPT) is calculated for each individual node, using the Link State Protocol (LSP). The LSP protocol is used to detect and show the shortest connection structures for all bridges/switches in the SPB area. Depending on the SPB concept, MAC addresses for participating stations and information on interface service affiliations are distributed to those stations that are not participating. This SPB method is called SPBM, which stands for SPB-MAC. The topology data helps the calculation computer to determine the most economical connections from each individual node to all other participating nodes. That way each switch or router always has an overall view of all paths available between the nodes.

In case of a change in topology, for example a switch is added or a link deactivated or activated, all switches must re-learn the entire topology. This process may sometimes last several minutes and depends upon the number of switches and links participating in the switchover process.

Nevertheless, Shortest-Path Bridging offers a number of advantages:

• It is possible to construct networks in the Ethernet that are truly meshed.

• Network loads are distributed equally over the entire network topology.

• Redundant connections are no longer deactivated automatically, but used to actively transport the data that is to be transmitted.

• The network design becomes flatter and performance between network nodes increases.

Dynamic negotiation must be mentioned as one disadvantage of Shortest Path Bridging. Transmission paths in this topology change very quickly, which makes traffic management and troubleshooting more difficult.

The first products to support the new “trill” and “SPB” switch mechanisms are expected by the middle of 2011. It still remains to be seen whether present switches can be upgraded to these standards. Network design will change drastically as a result of Shortest Path Bridging. Therefore one should prepare intensively for future requirements and start to lay the groundwork for a modern LAN early on.