1 RST-140 3006_05_2001_c1 2001, Cisco Systems, Inc. All rights reserved. LAN Protocols Bill Dufresne SE-IV Cisco Systems CCIE, Content/Video Specialist

RST-1403006_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved. 2© 2001, Cisco Systems, Inc. All rights reserved. 2© 2001, Cisco Systems, Inc. All rights reserved. 2

Complexity

“Just-in-Time”

Networking!

Familiar LAN Design?

Network Protocols Are Important—But Just One Piece of the Puzzle


So…What Makes for a Solid LAN?

• Three simple ingredients:1. Well thought-out network design2. Complimentary suite of protocols3. Strategic application of protocols

Strategic Application of Protocols and Features Requires Detailed Knowledge


Domain What?

• Collision domain It’s my turn, OK it’s now your turn

• Broadcast domain Attention world, I have something to say

• Failure domain Making one’s problem everyone’s problem

• Spanning Tree domain How long to converge?

• Policy domain Regional policing of the network

• Intelligence domain Value-added Services; the router does what?


LAN Domains Described

3 4

1 2

Basic LAN Router/Layer-3 Switch

CollisionDomain

BroadcastDomain

FailureDomain

STPDomain

PolicyDomain

IntelligenceDomain


Large Campus Design

DistributionLayer 3

CoreLayer 2/3

AccessLayer 2

Server FarmBuilding A A Building B

Sample Design A: Large Campus Design


UniDirectional Link Detection (UDLD)

• What is UDLD? Detects one-way connectivity

Independent of auto-negotiation

Similar to FEFI* in 100Fx

Supports 10/100Tx, 1000X

*FEFI: Far End Fault Indication M SS A

UDLD

TX

RX

RX

TX

RXTX

RXTX RX

TX

TXRX


Spanning Tree 802.1d

Loop-free Connectivity

X

A Switch Is Elected As Root

FFF

FB

F

F

A ‘Tree-like’ Loop-free Topology

Is Established

F

ARoot

B

M SS A


Typical Spanning Tree Layout

• Several enhancements specifically targeted to ‘triangular’ topology

• STP enhancementsQuicker convergence

Greater stability

Load balancing

Root

F

F

F F

F

XB

M SS A


Default Spanning Tree Timers

• Hello2 seconds (minute 1)

• Forward delay15 seconds (minute 4)

• Maximum age20 seconds (minute 6)

Time

Blocking20 Sec

Listening

Learning15 Sec

Forwarding

Max–Age

Pre-Forwarding 1

Pre-Forwarding 215 Sec

IEEE 802.1D Defaults

M SS A


• Event #1—New node

Default is 30 seconds for STP to ensure loop-free connection

• Event #2—Uplink failureDefault is 30 seconds for STP to find alternate path

• Event #3—Indirect failureDefault is 50 seconds for STP to find alternate path

Quicker STP Convergence

1

2

3Root

F

F

F F

F

B

X

Barb:

Made ‘X’ red, as with previous graphic

M SS A


PortFast

• Spanning tree enhancement to reduce convergence time

• Used for ports connected to hosts that cannot possibly cause a loop

• Bypasses ‘listening’ and ‘learning’ stages of STP

• Reduces connection time to 2-3 seconds from 30 seconds

Root

F

F

F F

F

B

1

Root

F

F

F F

F

B

1

M SS A


802.1w

• Spanning tree enhancement to reduce fail-over convergence time

• Used when recovery path is known and predictable

• Bypasses ‘listening’ and ‘learning’ stages of STP

• Reduces fail-over time to 2–3 seconds from 30 seconds

• Auto-populates upstream address tables

2

Root

F

F

F F

F

B

1

Root

F

F

F F

F

B

1

M SS A


?X

VLAN Trunking

• Most common topologies consist of multiple VLANs

• Problem becomes one of how to carry multiple VLANs worth of traffic while maintaining isolation

• Multiple protocols to solve the problem

MASS


VLAN Tag Added by Incoming Port

VLAN Tagging Protocols

• ISL• 802.1Q • 802.10 (FDDI)• LANE (ATM)

Which Tagand Which Protocol?

MASS


MAC Length/Type

MAC DATA

PAD

FCS

2

42-

1500

4

2

Used in:• IEEE 802.3ac• IEEE 802.1Q• IEEE 802.1p

VID (VLAN ID)—12 Bits

3 1

802.1Q Tagging Scheme

Tag Control Information

UserPriority CFI

Destination Address

Source Address

EtherType = TPID

6

6

2

MASS


Spanning Tree Issues

• 802.1Q specifies one spanning tree (STP) per bridge cloud, but it does not preclude multiple spanning trees

• One spanning tree doesn’t allow for load-sharing

MASS


Growing Number of Spanning Trees

• PVST (Per VLAN STP)• One active topology per

VLAN (ISL or 802.1Q)• Only 2 unique topologies• CPU consumed to

maintain 4 topologies• Finite limit to number of

trunks and VLANs

BB

?

VLA

N 1

0VL

AN

20

VLA

N 3

0VL

AN

40

BB

MASS


Multiple Instance Spanning Tree (MIST)

• Separates STP topologies from VLANs• Map VLANs to topologies• Must run on all switches• Similar to 802.1s efforts (SSTP*)• Pseudo-compatible with non-Cisco switches• Cannot run with PVST(+)• Virtual BPDU count scales near 80,000!

*SSTP: Shared Spanning Tree Protocol MASS


BB

?

MIST Function

• Two active topologies• All VLANs mapped to one of two

topologies• Lower BPDU counts• Simpler implementation• BPDUs are not encapsulated in

VLAN tag • BPDUs appear as multicasts to

non-MIST environment

B

VLA

N 1

0VL

AN

20

VLA

N 3

0VL

AN

40

MASS


VLA

N A

VLA

N B

Root

B B

MIST (802.1s) Load Balancing

• Per-VLAN-Spanning-Tree • Load balancing at Layer 2• Alternate ‘root’ switches• Fast fail-over with UplinkFast • Use following command:

set spantree <VLAN> root

*MIST: Multiple Instance Spanning Tree

Root

M SS A


EtherChannel

802.3af

• Load sharing and redundancy provided• Valid link aggregations include 2, 4, and 8 links

A B

EtherChannel

100/1000 Ethernet 7

100/1000 Ethernet 3

100/1000 Ethernet 5100/1000 Ethernet 6

100/1000 Ethernet 2100/1000 Ethernet 1

100/1000 Ethernet 4

100/1000 Ethernet 8


802.3af Load Balancing

How does it load share? • Layer 2 devices

Source/destination MAC• Layer 3 devices

Source/destination IP• Server NICs

Source/destination MAC

Layer 3 Switch or

Router

Layer 2 Switch

Server

ASM

S

24RST-1403006_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.

Documents

1 RST-140 3006_05_2001_c1 2001, Cisco Systems, Inc. All rights reserved. LAN Protocols Bill Dufresne SE-IV Cisco Systems CCIE, Content/Video Specialist