layer 2(primary source cisco network professional's advanced internet working guide, patrick j...

7/31/2019 Layer 2(Primary Source Cisco Network Professional's Advanced Internet Working Guide, Patrick J Conlan, Jhon Wile

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If ever the nature of frame changes, FCS has to be recalculated. When 802.1Qadds new field in name of Tag, FCS naturally has to be recalculated.

The priority field in dot1Q, 802.1P, helps in getting a richer QoS. A QoS is the reason why dot1q has become sort of a standard for IP Telephony.

Dot1q supports ethernet, token-ring, Common Spanning tree(CST), Multiple Spanning Tree(MST), Rapid Spanning Tree(RST).

Native VLAN support in it allowspoint-to-multipoint topologyand transference of untagged traffic on trunk link.

On dot1q all the trunk ports must belong to the same native VLAN to be able to communicate. Otherwise, there is a VLAN mismatch.Dynamic Trunking Protocol(DTP) achieves the configuration of links as trunks dynamically rather than manually. This is Cisco proprietary. This is enabled on e

by default and frames are sent between devices to negotiate the port status by themselves.

Although, once configured a port manually as access or trunk, DTP negotiation won't have any effect. A manually configured access port if connected to a DT

there will be a lot of drops.

Dynamic Operating modes of DTP: There are 3--dynamic desirable, dynamic auto and dynamic Negotiate. Dynamic desirable is default mode in which the po

sends and receives DTP frames and solicits the connected port with a trunk link. In Dynamic Auto, the port doesn't send DTP frames, but upon receiving it do

respond by configuring itself likewise. In Dynamic Negotiate port neither sends nor receives DTP frames.

End-to-end VLAN: when a VLAN extends PHYSICALLY to the switch which connects the user who needs the resources of that LAN.

VTP Pruning: It uses advertisements to determine whether the data is being flooded to the switches which don't need it, hence more efficient use of bandwi

switch might have the VLAN in its database but no ports assigned or no device connected; such a switch doesn't need those VLAN broadcasts. It has to be ena

the switches. The switch which doesn't have any assigned ports tells it to all that it doesn't need broadcasts.

Configuring VLAN--Commands: 1. sh vlan; 2. name ; 3. vlan (run in config term); 4. no vlan ; 5. switchport access vlan (run in co

6. switchport mode access; 7. switchport voice vlan ; 8. switchport trunk encapsulation dot1q (run in config-if#); 9. switchport mode trunk (after confi

encapsulation); 10. sh int trunk; 11. switchport mode ?; for DTP: switchport mode dynamic ?

Configuring VTP--commands: (config)# vtp ? : vtp domain ; vtp password ; vtp mode ; vtp pruni

pruning

Multilayer Switching: layer 3 switch forward traffic with really small delay(at line speed, one can say). Forwarding lookups are done in hardware using an ASI

Layer 2 switch]. The routing and required forwarding information are held in the hardware. The information is built into tables that are stored in content-add

memory (CAM) and ternary content-addressable memory (TCAM). Now this is quite confusing to say that 'something is in hardware' because everything even

What he must mean here is that there is separate set of gates which have pattern to produce the same effect which processor would have produced through

of commands--like putting a NOR gate to perform what one would have accomplished through a series of register values and instructions to the processor. B

a black-box is still keeping it quite confusing.

Cisco Express Forwarding(CEF) is the mechanism which accomplishes the lookup and required processing in these tables. In order for a multilayer switch to h

ability to route, it needs a route processor. However, remember that on routers each interface is a separate subnet; this is a fundamental

part of its functionality. A router routes traffic from one network or subnet to another. On a switch, when we think about the physical ports, they are Layer 2

cannot have an IP address configured on them.

Switched Virtual Terminal(SVI) is the terminal--VLAN 1--we use on L2 switch to provide it an IP, layer 3 Pathway and a gateway. The so called route processo

these SVIs for layer3 tasks. If you are seeing it here, you should realize that L3 switch still doesn't understand host IPs. Although the router doesn't either, I m

forwards according which network must take it on some interface; it is the L2 switch's MAC table which sorts it out further.

Inter-VLAN Routing

Layer 2(Primary Source: Cisco Network Professional's Advanced

Internetworking Guide, Patrick J Conlan, Jhon Wiley Publications;

plus editorialization). Part I: VLAN and STPSaturday, April 21, 2012

7:08 PM

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How many SVIs can be stored/how many VLANs can be stuffed depends upon three things, usually: 1. Switch Platform; 2. Number of supported VLANs; 3. Pro

overhead.

Multilayer switches can also be configured with the port to act just as a port on a real router would. You can place the IP a ddress and router interface comm

directly on the interface. The port when configured this way is no longer part of any VLAN; it represents a subnet just as a router interface would.

Switching Types: Well, there have been many, but can still be broadly categorized in: 1. Process Switching in which all packets have to processed by central p

2. Cache-based is the one in which only the first arriving packet is processed and the info from it is stored in tables for following packets to be switched quick

looking up RAM; 3. Cisco Express Forwarding is yet another improvement for it is topologybased and uses new tables to better use new hardware. One popu

is Forward Information Base(FIB) which is only a renamed routing table. However, CEF is NOT a routing protocol. If a destination isn't in FIB and packet arriv

for it, gates are closed for this poor guy in British manner--it is dropped! Adjacency Table keeps the L2 information and is stored in CAM where 'algorithms' a

for fast lookup: A key is created to compare the frame to the table content. For example, the destination MAC address and VLAN ID (VID) of a frame constitu

for Layer 2 table lookup. This key is fed into a hashing algorithm, which produces a pointer into the table. The system uses the pointer to access a smaller spe

of the table without requiring a search of the entire table.

Preparing to Transmit: a packet needs to be re-written on the output interface once routing decision has been made and everything is set for forwarding. Fir

device must change the source MAC address from the senders to its own. Second, the device must change the destination MAC address from its address to t

hops address. Third, the time to live (TTL) must be decremented by one, which means that the IP header checksum must be reca lculated. Last, the frame che

must be recalculated.

If you have not created the VLAN in the VLAN database, creating an SVI for a VLAN will not create the VLAN in the VLAN database. The VLAN and the SVI for a

completely separate items.

Creating a routed port, which is not a part of any VLAN already there, gives one freedom to connect something like a server or a firewall to it. The command fconfiguration is just no switchport. The switchportcommand is used to configure most of the Layer 2 functions for the switch, so by saying no switchportyou

the Layer 2 functions and leave a Layer 3 port.

Ether-Channel is a switch-to-switch technique that inversely multiplexes multiple Fast or Gigabit Ethernet switch ports into one logical channel. Its major adv

being cheaper than higher-speed media while utilizing existing switch ports. Also, it is used to increase bandwidth to single server. EtherChannel is a cross-pla

methodof load balancing between servers, switches, and routers; can combine two, four, or eight ports (depending on the switch platform) into one logical c

that can deliver redundancy.

Doesn't just do round-robin; uses techniques which are platform dependent e.g. X-OR of two lowest order bits of source and destination which uses a specifi

to load-balance.

An EtherChannel will NOT form if one of the interfaces is a SPAN destination port. If this is to be a Layer 3 EtherChannel, then the Layer 3 addresses must be

on the port-channel logical interface, not to the physical interfaces in the channel.

All ports in the EtherChannel bundle must be assigned to the same VLAN or be configured as a trunk. Must have the same allowed range of VLANs on all the

in a trunked Layer 2 EtherChannel. If the allowed range of VLANs is not the same, the interfaces do not form an EtherChannel.

Automatic Bundling Protocols: The protocol for EtherChannel is called Port Aggregation Protocol (PAgP). PAgP packets are sent between switches on EtherC

capable ports. These packets are used to negotiate the forming of a channel. When PAgP finds Ethernet links that match all their settings, it groups or bundleinto an EtherChannel. The EtherChannel is then added to the spanning tree topology as a single bridge port.

Then PAgP creates virtual interface for EtherChannel, the one with which processor identifies link bundle. The configuration changes are also made in this on

Link Aggregation Control Protocol(LACP)is open standard bundling protocol, part of 802.3ad. A device running LACP needs to be set with system prioritywhto determine system ID automatically with MAC address.

EtherChannel

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Configuring EhterChannel: commands start with creating virtual terminals first, and then physical ports are assigned to them. (config)#interface port-channe

creates EtherChannel like VLANs are. (config)#interface port-channel fa0/19 - 20 is used to assign many physical slots at once. But

many slots can be assigned this way the many belong to same module/blade actually. Then go into the interface of the post an d assign it the channel-group

if)#channel-group ; (config-if)#channel-protocol ; (config-if)#channel-group


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Classification of Ports depending upon the roles they play: 1. Root Port: That port of the switch which has a direct shortest path to the root bridge. Each sw

except the root switch has one and only one root port. 2. Designated Port: The port on the switch which is inforwarding mode. Each network segment, exce

ofcourse the root bridge, must have one--and the only one. 3. Non-designated Port: the port which is in Blocking state. This isn't always needed though on a

but you can have one.

BID is made up of switch priority and its MAC address. The complete BID looks like 32768.00c0.004c.3451, where 32768 is priority.

In order to figure out the root ports, you have to determine the path costs for each of the non root bridges. Or, each port in use has a path cost associated w

path eventually leads to the root bridge.

When path costs are same, port with the lower port ID becomes designated, e.g . Fa0/7 will become designated in place of fa0/8 or any higher one. Otherwis

with lower cost is designated. It is always path cost, BID and port ID which are primary criteria to determine port state in a particular set-up.

The root bridge must be fast enough to handle the traffic loads of all probable sizes.

Setting priorities Manually: Switch1(config)#spanning-tree [vlan number | vlan list] priority value. Now the command implies that you can have different r

switches for different VLANs. Needless to say, the switch must have that VLAN in its database. Here the role of VTP is critical as it tries to make sure that each

have idea of VLANs configured.

A backup switch might be desired in case of root going down--I mean do you want a scuffle between switches after daddy is dead? Simply assign a priority va

to present root bridge to the switch you want to be heir.

Macros might be used quite frequently to configure IOS in place of a series of commands.

Before putting a switch in the network ONE MUST CHECK THE SET PRIORITY, VTP STATUS, DOMAIN CONFIGURED, ETHERCHANNEL ETC. If ever you run a m

before putting a switch with lower priority in the network, the macro is negated because it WON'T RUN ITSELF AGAIN! Also, if a switch with priority 4096 is th

macro won't make primary of priority 0 because it just doesn't.

STP timer settings: 1. hello timer, 2. forward delay; 3. max age. The t imers signify how long it will take for the STP topology to converge from a change. Th

DIAMETER setting is a way for the STP to recalculate its timers using size of network as reference. It is the number of switches traffic has to travel through fo

from one end of fabric to another; can be anything from 2 to 7. By default STP timers assume diameter to be 7. Configuring diameters is the best way set ST

at once and the most reliably.

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