5: datalink layer 5a-1 bridges and spanning tree protocol reference: mainly peterson-davie
DESCRIPTION
5: DataLink Layer 5a-3 Forwarding How do determine to which LAN segment to forward frame? Looks like a routing problem...TRANSCRIPT
5: DataLink Layer 5a-1
Bridges and spanning tree protocol
• Reference: Mainly Peterson-Davie
5: DataLink Layer 5a-2
Bridges Link layer device
stores and forwards Ethernet frames examines frame header and selectively
forwards frame based on MAC dest address when frame is to be forwarded on segment,
uses CSMA/CD to access segment transparent
hosts are unaware of presence of bridges plug-and-play, self-learning
bridges do not need to be configured
5: DataLink Layer 5a-3
Forwarding
How do determine to which LAN segment to forward frame?• Looks like a routing problem...
5: DataLink Layer 5a-4
Self learning A bridge has a bridge table entry in bridge table:
(Node LAN Address, Bridge Interface, Time Stamp)
stale entries in table dropped (TTL can be 60 min)
bridges learn which hosts can be reached through which interfaces when frame received, bridge “learns” location
of sender: incoming LAN segment records sender/location pair in bridge table
5: DataLink Layer 5a-5
Filtering/ForwardingWhen bridge receives a frame:
index bridge table using MAC dest addressif entry found for destination
then{ if dest on segment from which frame arrived
then drop the frame else forward the frame on interface
indicated } else flood
forward on all but the interface on which the frame arrived
5: DataLink Layer 5a-6
Bridge exampleSuppose C sends frame to D and D replies back
with frame to C.
Bridge receives frame from from Cnotes in bridge table that C is on interface 1because D is not in table, bridge sends frame into
interfaces 2 and 3 frame received by D
5: DataLink Layer 5a-7
Bridge Learning: example
D generates frame for C, sends bridge receives frame
notes in bridge table that D is on interface 2 bridge knows C is on interface 1, so selectively
forwards frame to interface 1
5: DataLink Layer 5a-8
Needed: RoutingComplex large LANs need alternative routes
Load balancing Fault tolerance
Bridge must decide whether to forward frame Bridge must decide which LAN to forward frame on
Routing selected for each source-destination pair of LANs Done in configuration Usually least hop route Only changed when topology changes
5: DataLink Layer 5a-9
Spanning Tree Bridge automatically develops routing
table Automatically update in response to
changes Frame forwarding Address learning Loop resolution
Reference: Stallings
5: DataLink Layer 5a-10
Frame forwardingMaintain forwarding database for each port
List station addresses reached through each portFor a frame arriving on port X:
Search forwarding database to see if MAC address is listed for any port except X
If address not found, forward to all ports except X If address listed for port Y, check port Y for blocking
or forwarding state• Blocking prevents port from receiving or transmitting
If not blocked, transmit frame through port Y
5: DataLink Layer 5a-11
Address LearningWhen frame arrives at port X, it has come form
the LAN attached to port XUse the source address to update forwarding
database for port X to include that addressTimer on each entry in database (reset
whenever frame received)Each time frame arrives, source address
checked against forwarding database
5: DataLink Layer 5a-12
Loop of Bridges
5: DataLink Layer 5a-13
Loop Resolving The simple learning mechanism described
fails in presence of loops in the LAN Loops may be present by mistake, or
deliberately provided for redundency This problem is resolved by running a
distributed spanning tree algorithm
5: DataLink Layer 5a-14
Spanning Tree Algorithm Creates a logical, or “active” topology
that behaves like a spanning tree Makes alternate bridges redundant Is run periodically, so will discover failures
and use alternate bridges if necessary
Reference: Fred Halsall: “Data Communications, Computer Networks and Open Systems”, 4th Edition.
5: DataLink Layer
Spanning tree Think of the LAN as a graph that possibly has
loops (LAN segments as nodes, bridges as edges)
The spanning tree is a subgraph of this graph that covers all vertices (LAN segments), but contains no cycles.
(a) (b)
5: DataLink Layer
Spanning tree algorithm Spanning tree algorithm is a protocol used by
a set of bridges to agree upon a spanning tree for a particular extended LAN.
Essentially, this means that each bridge decides the ports over which it is and is not willing to forward packets.
Some ports (or even entire bridges) may not participate in a spanning tree
How does the bridge select the ports to include (/exclude)?
5: DataLink Layer 5a-17
Spanning Tree Algorithm Working: Bridges regularly exchange frames known as
Bridge Protocol Data Units (BPDUs). This exchange does the following:
1. Each bridge has a unique Identifier2. Bridge with highest priority and smallest ID is selected
as root bridge.3. Each bridge determines for each port, the least cost
path from root bridge to this port. This is the Root Path Cost (RPC) for this port.a) Select the port which has the least RPC and
designate it as the Root Port (RP). This is the port which will be used for communicating with the root.
5: DataLink Layer
•Algorithm...
1. Once root port is determined, one bridge port is selected for each LAN segment as the designated bridge port (DP) over which which frames will be sent for that LAN segment.
a) This is a port (which is NOT a root port) which has the least path cost to the root
b) The ports of the root bridge are always DPs for the LAN segments connected to the root bridge
2. The state of the bridge ports can be set either to forwarding or blocking.
a) All ports that are either RPs or DPs are forwarding, the rest are blocking.
5: DataLink Layer
Example:A
C
E
D
B
K
F
H
J
G
I
B3
B7
B4
B2
B5
B1
B6
A
C
E
D
B
K
F
H
J
G
I
B5
B2
B3
B7
B4
B1
B6
5: DataLink Layer
Example: working B1 is the root bridge B3 and B5 are both connected to LAN A, but
B5 is the designated port since it's closer to root
B5 and B7 are both connected to LAN B, but B5 is the designated port due to smaller ID (equal distance).
5: DataLink Layer 5a-21
Topology InitializationBPDUs are sent to a broadcast MAC address of all bridges
on the LANAll bridges initially assume they are the root bridgeEach BPDU contains (self ID, root ID, transmitting port ID,
RPC of this port)A bridge updates its own info if it receives an update which
identifies a root with smaller id or identifies a root with equal id but with shorter distance the root id and distance are equal, but the sending
bridge has a smaller idThe bridge adds 1 to the received RPC in the above update
and saves this info.
5: DataLink Layer
•Initialization...
When a bridge determines that it's not the root bridge, it stops generating messages on its own and only forwards configuration messages from others
Also, when a bridge determines that it's the DP for that LAN, it stops sending messages on that port
When the system stabilizes, only the root is generating messages, and the messages are forwarded over only the spanning tree
5: DataLink Layer 5a-23
Topology ChangeRoot bridge regularly transmits BPDUs, forwarded by all
bridges on all portsBridges will keep timers associated with each of its
forwarding portsWhen timers expire, procedure similar to topology
initialization is done
5: DataLink Layer 5a-24
Some bridge features Isolates collision domains resulting in higher
total max throughput limitless number of nodes and geographical
coverage Can connect different Ethernet types Transparent (“plug-and-play”): no
configuration necessary
5: DataLink Layer 5a-25
Bridges vs. Routers both store-and-forward devices
routers: network layer devices (examine network layer headers)
bridges are link layer devices routers maintain routing tables, implement
routing algorithms bridges maintain bridge tables, implement
filtering, learning and spanning tree algorithms
5: DataLink Layer 5a-26
Routers vs. BridgesBridges + and - + Bridge operation is simpler requiring less
packet processing+ Bridge tables are self learning - All traffic confined to spanning tree, even when
alternative bandwidth is available- Bridges do not offer protection from broadcast
storms
5: DataLink Layer 5a-27
Routers vs. BridgesRouters + and -+ arbitrary topologies can be supported, cycling is
limited by TTL counters (and good routing protocols)
+ provide protection against broadcast storms- require IP address configuration (not plug and
play)- require higher packet processing
bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)
5: DataLink Layer 5a-28
Ethernet Switches Essentially a multi-
interface bridge layer 2 (frame)
forwarding, filtering using LAN addresses
Switching: A-to-A’ and B-to-B’ simultaneously, no collisions
large number of interfaces
often: individual hosts, star-connected into switch Ethernet, but no
collisions!
5: DataLink Layer 5a-29
Ethernet Switches cut-through switching: frame forwarded
from input to output port without awaiting for assembly of entire frameslight reduction in latency
combinations of shared/dedicated, 10/100/1000 Mbps interfaces