Information About Implementing Multiple Spanning Tree
Protocol
To implement Ethernet services access lists, you must understand these concepts:
Spanning Tree Protocol Overview
Ethernet is no longer just a link-layer technology used to interconnect network vehicles and hosts. Its low
cost and wide spectrum of bandwidth capabilities coupled with a simple plug and play provisioning philosophy
have transformed Ethernet into a legitimate technique for building networks, particularly in the access and
aggregation regions of service provider networks.
Ethernet networks lacking a TTL field in the Layer 2 (L2) header and, encouraging or requiring multicast
traffic network-wide, are susceptible to broadcast storms if loops are introduced. However, loops are a desirable
property as they provide redundant paths. Spanning tree protocols (STP) are used to provide a loop free
topology within Ethernet networks, allowing redundancy within the network to deal with link failures.
There are many variants of STP; however, they work on the same basic principle. Within a network that may
contain loops, a sufficient number of interfaces are disabled by STP so as to ensure that there is a loop-free
spanning tree, that is, there is exactly one path between any two devices in the network. If there is a fault in
the network that affects one of the active links, the protocol recalculates the spanning tree so as to ensure that
all devices continue to be reachable. STP is transparent to end stations which cannot detect whether they are
connected to a single LAN segment or to a switched LAN containing multiple segments and using STP to
ensure there are no loops.
STP Protocol Operation
All variants of STP operate in a similar fashion: STP frames (known as bridge protocol data units (BPDUs))
are exchanged at regular intervals over Layer 2 LAN segments, between network devices participating in
STP. Such network devices do not forward these frames, but use the information to construct a loop free
spanning tree.
The spanning tree is constructed by first selecting a device which is the root of the spanning tree (known as
the root bridge), and then by determining a loop free path from the root bridge to every other device in the
network. Redundant paths are disabled by setting the appropriate ports into a blocked state, where STP frames
can still be exchanged but data traffic is never forwarded. If a network segment fails and a redundant path
exists, the STP protocol recalculates the spanning tree topology and activates the redundant path, by unblocking
the appropriate ports.
The selection of the root bridge within a STP network is determined by the lowest Bridge ID which is a
combination of configured bridge priority and embedded mac address of each device. The device with the
lowest priority, or with equal lowest priority but the lowest MAC address is selected as the root bridge.
The selection of the active path among a set of redundant paths is determined primarily by the port path cost.
The port path cost represents the cost of transiting between that port and the root bridge - the further the port
is from the root bridge, the higher the cost. The cost is incremented for each link in the path, by an amount
that is (by default) dependent on the media speed. Where two paths from a given LAN segment have an equal
cost, the selection is further determined by the lowest bridge ID of the attached devices, and in the case of
two attachments to the same device, by the configured port priority and port ID of the neighboring attached
ports.
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
412
Implementing Multiple Spanning Tree Protocol
Information About Implementing Multiple Spanning Tree Protocol
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