Download Spanning Tree Protocol (STP)

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Spanning Tree Protocol (STP)
The 802.1D Spanning Tree Protocol (STP)
standard was designed at a time when the
recovery of connectivity after an outage within a
minute or so was considered adequate
performance. With the advent of Layer 3
switching in LAN environments, bridging now
competes with routed solutions where protocols,
such as Open Shortest Path First (OSPF) and
Enhanced Interior Gateway Routing Protocol
(EIGRP), are able to provide an alternate path in
less time.
Spanning Tree Protocol (STP)
Cisco enhanced the original 802.1D specification
with features such as Uplink Fast, Backbone
Fast, and Port Fast to speed up the convergence
time of a bridged network. The drawback is that
these mechanisms are proprietary and need
additional configuration.
Spanning Tree Protocol (STP)
STP runs on bridges and switches that are 802.1Dcompliant. There are different flavors of STP, but 802.1D
is the most popular and widely implemented. You
implement STP on bridges and switches in order to
prevent loops in the network. Use STP in situations where
you want redundant links, but not loops. Redundant links
are as important as backups in the case of a failover in a
network. A failure of your primary activates the backup
links so that users can continue to use the network.
Without STP on the bridges and switches, such a failure
can result in a loop. If two connected switches run
different flavors of STP, they require different timings to
converge. When different flavors are used in the switches,
it creates timing issues between Blocking and Forwarding
states. Therefore, it is recommended to use the same
flavors of STP.
Spanning Tree Protocol (STP)
Rapid Spanning Tree Protocol (RSTP)
Rapid Spanning Tree Protocol (RSTP; IEEE
802.1w) can be seen as an evolution of the 802.1D
standard more than a revolution. The 802.1D
terminology remains primarily the same. Most
parameters have been left unchanged so users
familiar with 802.1D can rapidly configure the new
protocol comfortably. In most cases, RSTP performs
better than proprietary extensions of Cisco without
any additional configuration. 802.1w can also revert
back to 802.1D in order to interoperate with legacy
bridges on a per-port basis. This drops the benefits it
introduces.
Rapid Spanning Tree Protocol (RSTP)
RSTP (IEEE 802.1w) natively includes most of the Cisco
proprietary enhancements to the 802.1D spanning tree, such as
BackboneFast, UplinkFast, and PortFast. RSTP can achieve much
faster convergence in a properly configured network, sometimes
in the order of a few hundred milliseconds. Classic 802.1D
timers, such as forward delay and max_age, are only used as a
backup and should not be necessary if point-to-point links and
edge ports are properly identified and set by the administrator.
Also, the timers should not be necessary if there is no interaction
with legacy bridges.
BPDU Cisco Feature
There are rules to describe one way of determining what
spanning tree will be computed by the algorithm, but the rules as
written require knowledge of the entire network. The bridges
have to determine the root bridge and compute the port roles
(root, designated, or blocked) with only the information that they
have. To ensure that each bridge has enough information, the
bridges use special data frames called Bridge Protocol Data Units
(BPDUs) to exchange information about bridge IDs and root path
costs.
BPDU Cisco Feature
A bridge sends a BPDU frame using the unique MAC address of
the port itself as a source address, and a destination address of the
STP multicast address 01:80:C2:00:00:00.
There are three types of BPDUs:
Configuration BPDU (CBPDU), used for Spanning Tree
computation
Topology Change Notification (TCN) BPDU, used to
announce changes in the network topology
Topology Change Notification Acknowledgment (TCA)
BPDUs are exchanged regularly (every 2 seconds by default) and
enable switches to keep track of network changes and to start and
stop forwarding at ports as required.
UplinkFast Cisco Feature
UplinkFast
Another form of immediate transition to the forwarding state
included in RSTP is similar to the Cisco UplinkFast proprietary
spanning tree extension. Basically, when a bridge loses its root
port, it is able to put its best alternate port directly into the
forwarding mode (the appearance of a new root port is also
handled by RSTP). The selection of an alternate port as the new
root port generates a topology change. The 802.1w topology
change mechanism clears the appropriate entries in the Content
Addressable Memory (CAM) tables of the upstream bridge. This
removes the need for the dummy multicast generation process of
UplinkFast.
UplinkFast does not need to be configured further because the
mechanism is included natively and enabled in RSTP
automatically.
Edge Port Cisco Feature
Edge Ports
The edge port concept is already well known to Cisco spanning
tree users, as it basically corresponds to the PortFast feature. All
ports directly connected to end stations cannot create bridging
loops in the network. Therefore, the edge port directly transitions
to the forwarding state, and skips the listening and learning
stages. Neither edge ports or PortFast enabled ports generate
topology changes when the link toggles. An edge port that
receives a BPDU immediately loses edge port status and becomes
a normal spanning tree port. At this point, there is a userconfigured value and an operational value for the edge port state.
The Cisco implementation maintains that the PortFast keyword
be used for edge port configuration. This makes the transition to
RSTP simpler.
Root Port
The role is now a variable assigned to a
given port.
IEEE 802.1 Standards
The port that receives the best BPDU on a bridge is the
root port. This is the port that is the closest to the root
bridge in terms of path cost. The STA elects a single
root bridge in the whole bridged network (per-VLAN).
The root bridge sends BPDUs that are more useful than
the ones any other bridge sends. The root bridge is the
only bridge in the network that does not have a root
port. All other bridges receive BPDUs on at least one
port.
IEEE 802.1 Standards
The STP (802.1D) defines five different port states:
Disabled
Listening
Learning
Blocking
Forwarding
The RSTP (802.1w) port states are:
Discarding
Learning
Forwarding
Network Address Translation (NAT)
NAT allows an Internet Protocol (IP) network to
maintain public IP addresses separately from
private IP addresses. NAT is a popular technology
for Internet connection sharing. It is also sometimes
used in server load balancing applications on
corporate networks.
In it's most common configuration, NAT maps all of
the private IP addresses on a home network to the
single IP address supplied by an Internet Service
Provider (ISP). This allows computers on the home
LAN to share a single Internet connection.
Additionally, it enhances home network security by
limiting the access of external computers into the
home IP network
NAT Overload
PAT configuration is many to one.
NAT
Static NAT is the simplest form of NAT, one to
one.
Transparent Bridge
The spanning-tree calculation occurs when the
bridge is powered up and whenever a topology
change is detected. The calculation requires
communication between the spanning-tree bridges,
which is accomplished through configuration
messages (sometimes called bridge protocol data
units, or BPDUs). Configuration messages contain
information identifying the bridge that is presumed
to be the root (root identifier) and the distance from
the sending bridge to the root bridge (root path
cost). Configuration messages also contain the
bridge and port identifier of the sending bridge, as
well as the age of information contained in the
configuration message.
Transparent Bridge
Bridges exchange configuration messages at
regular intervals (typically one to four seconds).
If a bridge fails (causing a topology change),
neighboring bridges will detect the lack of
configuration messages and initiate a spanningtree recalculation.
Transparent Bridge
Border Gateway Protocol (BGP)
BGP Revealed
BGP basically determines how an Autonomous
System (AS), or independent network, passes
packets of data to and from another AS. Rather
than depend on a calculated metric to determine
the best path, BGP uses attribute information that
is included in route advertisements to determine
the chosen path.
Border Gateway Protocol (BGP)
ASBGP
Autonomous System (AS)
Within the Internet, an Autonomous System (AS)
is a collection of connected Internet Protocol (IP)
routing prefixes under the control of one or more
network operators that presents a common,
clearly defined routing policy to the Internet.
Interior routing protocols:
packetlife.net/posters/ISO_Interior_Routing_Protocol.pdf
Root Bridge
Root Bridge
Root Bridge
Root Bridge
Root Bridge
Root Bridge