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Network recovery
Layer 2 mechanisms to increase reliability
Spanning tree
IEEE 802.1D spanning tree is an Ethernet loop avoidance protocol. It allows network managers
to connect redundant network links within their networks. Prior to the advent of spanning tree,
loops within a switched Ethernet network would forward traffic around the loop forever, which
saturated the network and prevented new traffic from getting through. Spanning tree selects one
switch as a root and creates a loop-free topology connecting to the root. If loops are discovered,
one switch blocks that port until its alternate path to the root is disrupted. Then the blocked port
is brought back into service. There are several drawbacks to spanning tree:
By default, all switches have the same priority, which means that root bridge selection is
random. This can be suboptimal in a network.
Spanning tree is slow to converge. It typically takes at least 50 seconds from link failure for
a backup link to become active. As Layer 2 complexity increases, so does convergence
time.
Although there are mechanisms for speeding up spanning tree, most are proprietary.
Traditional spanning tree is not VLAN aware. Thus, it will block links even if VLAN
provisioning would have prevented a loop.
To solve these issues, the IEEE has recently introduced 802.1s and 802.1w enhancements.
802.1w introduces rapid spanning tree protocol (RSTP). RSTP uses active handshaking to
speed up convergence times. 802.1s introduces multiple spanning trees (MST), which is a way
of grouping different VLANs into different spanning tree instances. These features might not be
present in data network switches yet, but look for them soon.
Link Aggregation Groups
Link Aggregation Groups (LAGs) are a mechanism for combining multiple real inter-switch links
(typically four, Avaya products are configurable from two to eight) into one point-to-point virtual
interswitch link. The advantage of this mechanism over spanning tree is that an organization
can have the redundant links in if a failure occurs in one of the LAG links, the two switches will
quickly discover it, and remove the failed link from the LAG., which reduces the convergence
time to nearly instantaneous. Not all implementations interoperate, so care must be taken when
the LAG connects switches from multiple vendors. Also, LAG links are a point-to-point
technology. They cannot be used to connect a backup switch in case the primary fails. When
available, this is a very good mechanism for improving the resiliency of LANs.
352 Avaya Application Solutions IP Telephony Deployment Guide
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