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• Protocols and Versions
• Timers
• Switched VS Routed
• Trunk Encapsulations
• VTP Configs
• Router to Switch Config
• Switch VLAN Config
• Port-Ether-Channel
• Spanning tree Priority
• VTP Sniffer trace
• Performance issues with asymmetric routing and backup segments
• Performance issues with STP changes
• Link to IOS Switch commands

A virtual LAN (VLAN) is a group of hosts or network devices that form a single bridging domain.

VLANs are formed to group related users (logically), regardless of the physical connections of their hosts to the network.
 

• Know as  frame-tagging or label switching.
• Sets a BROADCAST DOMAIN
• Works at the Layer 2 level.
• All devices in the same vlan must reside on the same IP network.
• IEEE's 802.1Q Virtual LANs Standard.
• Each VLAN establishes its own spanning tree 802.1d table.
• VLANs are differentiated by assigning each VLAN a "color" (or VLAN ID)
• Native VLAN frames are carried over the trunk "UNTAGGED"

Reasons
     Broadcast control - Just as switches physically isolate collision domains for attached hosts and only forward traffic out a particular port, VLANs provide logical collision domains that confine broadcast and multicast traffic to the bridging domain.

     Security - If you do not include a router in a VLAN, no users outside of that VLAN can communicate with the users in the VLAN and vice versa. This extreme level of security can be highly desirable for certain projects and applications. Packets must move up a layer and route.

     Performance  -You can assign users that require high performance networking to their own VLANs. Segmenting bandwidth and improving thruput.

     Network management - Software on the switch allows you to assign users to VLANs and later reassign them to another VLAN. Recabling to change connectivity is no longer necessary in the switched LAN environment because network management tools allow you to reconfigure the LAN logically in seconds.
 

When a LAN switch first starts up and as the devices that are connected to it request services from other devices, the switch builds a table that associates the MAC address of each local device with the port number through which that device is reachable.
Caching the MAC address and delivering subsequent packets only to that port A switch can also handle multiple simultaneous conversations.
Whenever a device connected to the LAN switch sends a packet to an address that is not in the LAN switch's table (for example, to a device that is beyond the LAN switch), or whenever the device sends a broadcast or multicast packet, the LAN switch sends the packet out all ports (except for the port from which the packet originated) - a technique known as unicast flooding.
 

Protocols

• The Spanning Tree Protocol (STP) provides network link redundancy so that a Layer 2 switched network can recover from failures without intervention in a timely manner. The STP is defined in the IEEE 802.1D standard.
• The IEEE 802.1Q standard specifies how VLANs are to be trunked between switches. It also specifies only a single instance of STP that encompasses all VLANs. Known as the Common Spanning Tree (CST).

Spanning-Tree 802.1D - Sends Bridge Protocol Data Units:

Two Types of BPDU Packets:

• Configuration
• Topology change notification (TCN) - these get acknowledged

Common Spanning Tree
The IEEE 802.1Q standard specifies how VLANs are to be trunked between switches.

• CST 802.1Q specifies only a SINGLE instance of STP that encompasses all VLANs.
• All CST BPDUs are transmitted over trunk links using the native VLAN with untagged frames.

IEEE 802.10  MAN - defines a method for secure bridging of data across a shared metropolitan area network
(MAN)  Switched or Routed backbone.
 

Spanning-Tree Interoperability and Backward Compatibility

Lists the interoperability and compatibility among the supported spanning-tree modes in a network.

In a mixed MSTP and PVST+ network, the common spanning-tree (CST) root must be inside the MST backbone, and a PVST+ switch cannot connect to multiple MST regions.

When a network contains switches running rapid PVST+ and switches running PVST+, we recommend that the rapid-PVST+ switches and PVST+ switches be configured for different spanning-tree instances. In the rapid-PVST+ spanning-tree instances, the root switch must be a rapid-PVST+ switch. In the PVST+ instances, the root switch must be a PVST+ switch. The PVST+ switches should be at the edge of the network.

STP - 802.1D

• One spanning tree instance

CST -Common Spanning Tree

• 802.1Q

• One spanning tree instance

• Multiple Links (Only One forwards)

• Standards based

PVST Per-VLAN Spanning Tree

• One instance per vlan

• Cisco Proprietary - Requires ISL

• Problems with CST

PVST+

• Solves Interoperability Problems (uses ISL or 802.1q)

• IEEE version

• Default now in code

RPVST+

• Rapid PVST - Enhanced PVST

• RSTP - Faster Convergence 802.1W

• Removes link between ports role and state

• RSTP Synchronization - Proposal and agreement

MST - Multiple Spanning Tree Protocol 802.1S

• Reduces STP regions, takes multiple VLANS creates on instance of STP

• Sets up regions of control (Name, Revision and Instance map)

• IST - Internal # = 0

Timers:

• Hello timer Controls how often the switch broadcasts hello messages to other switches.

• Forward-delay timer Controls how long each of the listening and learning states last before the interface begins
  forwarding.

• Maximum-age timer Controls the amount of time the switch stores protocol information received on an interface. This gets reset when a TCN takes place. On a trunk all VLANs change (Default from 300 seconds to Forward delay 15)

Timing Summary

• Max Age - A port stays in blocking for 20s.
• Forward Delay - A port stays in listening for 15s, and then learning for another 15s. 2 x 15 = 30 seconds
• Time for a backup link to move from blocking to forwarding after a failure is 20+15+15s = 50s.

Switched Versus Routed
 

Switched	.	Routed	.
Advantage	Disadvantage	Advantage	Disadvantage
Propagates color  information across  entire network.	Backbone is running bridging.	No bridging in backbone.	Color information is not  propagated across backbone   and must be configured  manually.
Allows greater  scalability by  extending bridge  domains.	Broadcast traffic  increases drastically on   the backbone.	Easy to integrate into  existing           internetwork.	If subnets are split, a bridged  path has to be set up between switches.
.	.	Can run native protocols in the  backbone.	.

Trunking Encapsulations:

	DOT1Q	ISL
Proprietary	NO	YES (default Cisco to Cisco)
TAG	YES except Native VLAN	YES
Encapsulation	NO (Adds 4 bit tag + 18 CRC) 1522	YES (Entire Frame) 30 bytes (26 Header + 4 Trailer)
CRC Recalculated	YES	YES - But its in the FCS (trailer)
Supported VLANS	4096	1024

Inter-Switch Link
ISL is a CISCO proprietary protocol for interconnecting multiple switches and maintaining VLAN information as traffic goes between switches. This technology is similar to IEEE  802.10 in that it is a method of multiplexing bridge groups over a high speed backbone.

With ISL, an Ethernet frame is encapsulated with a header that maintains VLAN IDs between switches. A 30-byte header is prepended to the Ethernet frame, and it contains a two-byte VLAN ID. This is applied only on trunks LEAVING the switch, known as tagging.

802.1Q Industry standard for Trunking

• Does not add external encapsulation instead Adds 4 byte tag inside the frame
• Supports Multi vendor
• Frame size becomes 1522 bytes:   1500 MTU + 4 TAG + 18 CRC
• Works with 802.3 standard of 1518 bytes untagged

Dynamic Trunk Protocol

Will send DTP packets out every 30 seconds

Access	Sets switch to permanent NON-Trunking mode
Trunk	Set and negotiate to become trunk
Nonegotiate	Will be a trunk but will not send DTP
Dynamic Auto	Will respond to DTP, but not send
Dynamic Desirable	Will send and respond to DTP

Antispoof Method - Hard code  non trunks as access ports: switchport mode access

LANE
LAN Emulation (LANE) is a service that provides interoperability between ATM-based workstations and devices connected to existing legacy LAN technology.
LANE uses MAC encapsulation (OSI Layer 2) because this approach supports the largest number of existing OSI Layer 3 protocols. The end result is that all devices attached to an emulated LAN appear to be on one bridged segment. In this way, AppleTalk, IPX, and other protocols should have similar performance characteristics as in a traditional bridged environment. In ATM LANE environments, the ATM switch handles traffic that belongs to the same emulated LAN (ELAN), and routers handle routing

VTP VLAN Trunking Protocol

• Cisco's VTP provides a means to distribute and synchronize identifying  information about VLANs configured throughout a bridged network.
• Configurations made to a single VTP SERVER device  are propagated across trunks to all connected devices in the network.

Benefits of The Cisco's Virtual Trunk Protocol (VTP).

•      Integration of ISL, 802.10, 802.Q and ATM LAN based VLANs
•      Auto intelligence within the switches for configuring VLANs
•      Configuration consistency across the network
•      An auto mapping scheme for going across mixed media backbones
•      Accurate tracking and monitoring of VLANs
•      Dynamic reporting of added VLANs across the network
•      Plug-and-play setup and configuration when adding new VLANs
•      Default mode is SERVER

VTP Modes

Mode	Domain	Comments
Client	YES	Only Listens - Can't Create / Modify
Server	YES	Complete Control
Transparent	NO	Complete control (High VLAN #) Will Relay if in domain

 

IOS	OS	Comments
sw(config)#vtp mode ? client Set the device to client mode. server Set the device to server mode. transparent Set the device to transparent mode	set vtp mode ? client VTP client mode off VTP off server VTP server mode transparent VTP transparent mode	Default is SERVER
(config)#vtp domain test	set vtp domain test	Default is NULL
(config)#vtp password XXX	set vtp passwd XXX	Default is none | Must match | can be authenticated

VTP Pruning

• Disabled by default
• Configured on VTP Server
• VLAN 1 is left alone (Management)

 Cisco's extended VTP provides means whereby areas of the network containing  no members of a VLAN are identified.  This identification is propagated through the network and used to avoid transmitting frames of that VLAN along trunks into those areas.  Limiting such transmissions on a VLAN basis  is termed VLAN pruning.  In a large network with many VLANs, this is an  important network optimization which lightens the load on many of the networks trouncing connections.

Commands:

set vtp pruning enable

vtp pruning

• Defined as a point to point link
• Router - Uses Sub Interface plan.
• Each VLAN is assigned a unique IP network.
• Main Interface has NO IP Address.

!
interface FastEthernet1/0
 no ip address
 full-duplex
! SUB INTERFACE
interface FastEthernet1/0.1
 ip address 207.115.62.1 255.255.255.0
 ip access-group 130 in
 no ip redirects
 encapsulation isl 1
! SUB INTERFACE
interface FastEthernet1/0.2
 ip address 192.168.240.1 255.255.255.0
 ip access-group 140 in
 no ip redirects
 encapsulation isl 100
! SUB INTERFACE
interface FastEthernet1/0.3
 ip address 192.168.241.1 255.255.255.0
 ip access-group 140 in
 no ip redirects
 encapsulation isl 101
!

Switch -  Connects VIA trunks
Under VTP Section - Must be common domain
Use Set VTP, Set VLAN, Set TRUNK commands

set vtp [domain domain_name] [mode {client | server | transparent}] [passwd passwd] [pruning
 {enable | disable}] [v2 {enable | disable}]

• Command - set vtp domain (PRODIGY) mode (SERVER)
• Command - set vlan #  name   XYZ   state    ACTIVE
• Command - set trunk  X/Y  on
• Command - set vlan  #   x/y
• Command on 2900 - under interface - switchport access vlan x
• Command on 1900 - under interface - vlan-membership static x

#vtp Set up DOMAIN
set vtp domain PRODIGY    [Establish Common environment]
set vtp mode server     [Server = Send Changes, Client = Listen for Changes, Transparent = Idle Config]
set vtp passwd P0lCAT   [protects Domain]
set vtp v2 disable
set vtp pruning disable
set vtp pruneeligible 2-1000
clear vtp pruneeligible 1001-1005
# Match to Router ISL numbers
set vlan 1 name default type ethernet mtu 1500 said 100001 state active    [set vlan 1 name default state active]
set vlan 100 name private type ethernet mtu 1500 said 100100 state active
set vlan 101 name cs-mail type ethernet mtu 1500 said 100101 state active
 

# Turn Trunk ON !!!
set trunk 1/1  on 1-1000

~~~~~~~~~~~~~~~~~~~~~~~~
NOTE :

• to change / clear  the VTP password - VTP  passwd 0  - (zero).
• to clear a VLAN  - CLEAR VLAN X - (X = vlan #).

FastEther Channel or  Port Groups
Fast Ether Channel is a trunking technology based on grouping together multiple full duplex  GIG / Fast Ethernets to provide fault-tolerant high-speed links between switches, routers, and servers. EtherChannels can be composed of  at least  two and up to eight (Switch dependent)  industry-standard  links to provide load sharing of traffic..

Fast EtherChannel does not require the use  of 802.1D Spanning-Tree Protocol (STP) to maintain topology state within the channel. Rather it uses a peer-to- peer control protocol that provides autoconfiguration and sub-second convergence times for parallel links, yet allows higher-level protocols such as STP, or existing routing protocols, to maintain topology. The channel ports look like one bundle, it allows a Forward / Forward condition on STP. The MAC address is used to handle a "balance" of traffic.

Set up of Channels or Groups / Bundles to aggregate ports.

Can be used -

• Router (7500 = channel-group x) to Switch (Cats)
• Switch to Switch (Cat5k = set port channel 3/1-2 on | Cat29K / 35K = port group x distribution  )
• Switch to Server (Server = EtherChannel functionality is provided through a LAN driver)

Configuration:  varies by switch type and IOS.

• Set up of Channels or Groups / Bundles to aggregate ports. Ports can be on different cards and not contiguos.
• Distribution can be controlled by "MAC or IP" and "Source or Destination".

Cat5K /  6K
show port channel - Base configuration.
show port channel info - Full display, shows remote connections.
show port channel statistics   - Look for errors.
show spantree - Base conditions.
 

Cat29K / 35K
show etherchannel summary - Base configuration.
show spanning-tree summary - Base conditions.
 

Channel Connections

• There are four user-configurable channel modes: on, off, auto, and desirable.
• PAgP packets are exchanged only between ports in auto and desirable mode.
•  Ports configured in on or off mode do not exchange PAgP packets.
• 6K trunk channels use mode desirable silent -
• Example set port chan 7/1-8/2 mode desirable silent

Performance issues with asymmetric routing and backup segments

To change cam timer on switch:

• Do this on the Core switches
• You can do this to just one vlan - (in this case vlan 24)
• (OS) set cam agingtime 24 14400
• (IOS) mac-address-table aging-time 14400 vlan 24

Verify Change:

Troubleshooting performance issues with spanning tree:

• Look for LAST TOPOLOGY CHANGE
• If all VLANS keep reverting back - usually under 5 Minutes - LOOK for TRUNKS bouncing.
• Trace back by MAC and forwarder port - leads back to ROOT Switches:

OS
sho spantree sta 2/2 67
spanningtree type ieee
spanningtree multicast address 01-80-c2-00-00-00
bridge priority 49152
bridge mac address 00-0b-fc-3f-63-82
bridge hello time 2 sec
bridge forward delay 15(15) sec
topology change initiator: 1/1
last topology change occured: Sun Apr 15 2007, 05:44:36
topology change FALSE
topology change time 35
topology change detected FALSE
topology change count 27
topology change last recvd. from 00-00-00-00-00-00