Transparent Bridging

Sept 09, 2004 CS573: Network Protocols and Standards

1

Transparent Bridging

Network Protocols and Standards

Autumn 2004-2005

Sept 09, 2004 CS573: Network Protocols and Standards 2

Reasons for Bridges On a single LAN, there are

limitations: Number of stations Size of segment Bandwidth per segment

Bridges connect LAN segments to make “extended” LANs LANs, LAN Segments, Extended LANs


Example: Bridging Benefits

Consider a LAN segment with average traffic R pkts/sDivide it into two segments and connect with a BridgeAverage traffic on each segment is R/2 pkts/s

Bridge

Stations Stations

R/2 pkts/s R/2 pkts/s


Example: Bridging Benefits On average:

Each segment generates a traffic of R/2 pkts/s

Half of the traffic is for “local” stations Half of the traffic is for “other” segment Traffic on each segment is R/2+(1/2) R/2

Average traffic on each segment is 3R/4 This traffic must not exceed the capacity of

the segment


Example: Bridging Benefits Therefore 3R/4 < C

C is the capacity of the physical link R < 4C/3

Effective R exceeds the capacity i.e. Rmax < 4C/3 rate on any segment must not exceed the capacity

What was the maximum rate allowed when the LAN was not segmented?

(Rmax < C) Does the maximum effective R (i.e., Rmax)

increase when three segments are used? Depends how the segments are connected!


Can we use a router instead? The answer is “It depends” Inter-segment traffic may be handled by routers

if all stations understand layer 3 Older machines did not understand layer 3, but new

ones do Does this mean that with newer stations, we did

not need bridges? Not really! Bridges handle all layer 3 protocols while

early routers usually handled a single layer 3 protocol Don’t multiprotocol routers do address this

issue? And what about convergence to IP? Does that not eliminate the need for multiprotocol routers

An IP router can replace a bridge then, right?


Do we still need a Bridge? What if stations want to move on

the “extended” LAN without reconfiguring their IP addresses? Bridges can help! Bridges have high performance Bridges are simple


Transparent Bridging

…

Bridge

For stations, the two topologies are the same transparent bridging

stations


Transparent Bridge Functions Promiscuous Listening

Every packet passed up to software Store and Forward

Based on a forwarding database Filtering

Also based on forwarding database


Can a Bridge act smart? For the two segment-one bridge topology for

which the maximum rate was 4/3 of the link capacity, was Bridge doing something smart?

Yes, the Bridge forwarded the traffic smartly Manual entry of station addresses? Stations use addresses from a range? Station addresses are assigned such that a

portion indicates the LAN number? Bridges can also “learn” on their own!!!


Forwarding Database (FDB):Creation and Maintenance

The bridge promiscuously listens to every packet/frame received on each port

For each received frame, address in the source field is stored together with the port on which the frame is received. The FDB is created in Station Cache.

Each entry in the FDB is deleted if no traffic is received from that source address for a given period of time (Aging time)


Forwarding Frames For each received frame, the bridge looks

at the destination address: If the address is multicast or broadcast (all 1’s)

then the frame is forwarded to all the interfaces (ports) except for the one on which it is received

For unicast addresses: If the address is not found in FDB, the frame is

forwarded to all the ports except for the one on which it is received

If the address is found in FDB, the frame is forwarded to the port in FDB entry. If the FDB entry has same port on which the frame is received, frame is dropped (filtered)


Example 1: Learning and Forwarding

Transmission order A D

Ports 2, 3 D A

Port 1 Q A

Filtered Z C

Ports 1, 3

BPort 1

Port 2

Port 3

A Q

Z C

D M


Example 2: Two Bridges

B1Port 1 Port 2

B2Port 1 Port 2

A Q D M K T

What are the Station Caches after “complete” learning?


Topologies with Loops Problems

Frames proliferate Learning process unstable Multicast traffic loops forever

B1 B2 B3

LAN 1

LAN 2

A


Topologies with Loops Solutions

Require that the topologies be loop-free through careful deployment of segments and bridges

Design Bridges to detect loops and complain and, perhaps, stop working

Not a good idea because loops provide redundancy Design into the bridges an algorithm that

prunes the topology into a loop-free subset (a spanning tree)

Blocking of some ports may be required Automatically adapt to the changes in topology


Reconfiguration Algorithm Configures an arbitrary topology into a

spanning tree Automatic reconfiguration in case of

topology changes The algorithm should converge for any size

LAN; the stability should be achieved within a short, bounded time

Active topology should be reproducible and manageable

Transparency to end-stations is required Must not use a lot of bandwidth


Spanning Tree Algorithm A distributed Algorithm

Elects a single bridge to be the root bridge Calculates the distance of the shortest path

from each bridge to the root bridge (cost) For each LAN segment , elects a

“designated” bridge from among the bridges residing on that segment

The designated bridge for a LAN segment is the one closest to the root bridge

And…


Spanning Tree Algorithm For each bridge

Selects ports to be included in spanning tree The ports selected are:

The root port --- the port that gives the best path from this bridge to the root

The designated ports --- ports connected to a segment on which this bridge is designated

Ports included in the spanning tree are placed in the forwarding state

All other ports are placed in the blocked state


Forwarding frames along the spanning tree

Forward and Blocked States of Ports

Data traffic (from various stations) is forwarded to and from the ports selected in the spanning tree

Incoming data traffic is always discarded (this is different from filtering frames. Why?) and is never forwarded on the blocked ports


Root Selection: Bridge ID Each port on the Bridge has a unique LAN

address just like any other LAN interface card. Bridge ID is a single bridge-wide identifier that could be: A unique 48-bit address Perhaps the LAN address of one of its ports

Root Bridge is the one with lowest Bridge ID

BPort Address

Transparent Bridging

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