LAYER 3 ROUTING,SVI,ROUTER ON STICK & ETHERCHANNEL

SVI

• Svi is logical interface on layer 3 switch which acts as default gateway for vlans.
• With svi the vlan must be created in the database. If you issue the

                 int vlan 10
                 ip address 10.229.254.8 255.255.255.0
                 no shut

this will not create the vlan in vlan database. The int vlan 10 will no go up up until
a vlan is created.
A quick way to see if you are experencing the issue you can do sh spanning tree vlan
10 you will see it is not in the forwarding state. If there is no port forwarding for
the vlan in spanning  tree the svi will not go up  up this specfically what it looks at.
You simply just need to create the vlan in typical layer 2

router(config)#vlan 10 or int fa0/1 switchport access vlan 10 will create vlan 10 in
databse.

• For routing on l3 switches ip routing  needs to be enabled if you do not enable you will see

          the following

Default gateway is not set
Host               Gateway           Last Use    Total Uses  Interface
ICMP redirect cache is empty

• The svi will not be up up unless a particular  port is in the forwarding in the vlan. If for some reason you want to exclude ports from the calculation maybe ports to traffic analyser or similar you can use

                           autostate exclude commad

• Routing also has to be enabled

                              ip routing

Routed Port


To create a routed port on layer 3 you simply need to go interface mode and issue the
command no switchport. Then you can do the typical layer 3 config it can do
everything apart from sub interfaces which can not be configured.

Router on a stick

• is the legacy version of svi
• layer 2  frames are sent up to tagged (802.1q) to the router port.if it is to be routed to another vlan in the layer 2 domain the router removes tag puts the new destination layer 2 dot1q tag and sends it backdown the line it came
• Inefficent as the trunk to the router acts a bottlekneck

                     *The subinterface does not have to match the vlan but generally for organisation but not
int fa0/0.'subinterfaceno'
encap dot1q vlan number
ip address 10.229.254.1 255.255.255.0
int fa0/0.'subinterface 2'
encap dot1q vlan number 2

the switch side connecting will be a trunk. If you are running vtp pruning you will
need manually prune on this connection as routers done not support vtp
The native vlan must match between switch + routers trunk if you change the native vlan on the switch from 1 you need
to change it on the router or it will cause problems. To change on the router under
interface fa0/.subint
encap dot1q native 'vlan number'

An interesting point is if you do not change the native vlan on the router and it is not the same as the switch the untagged traffic will go to
the primary interface i.e interface f0/0 so you could in actually fact configure 
an ip address on the  main interface and the untagged traffic would go there
You could also set the native vlan to tag in dot1q on the switch.
with the  command vlan dot1q tag native


Etherchannel

• Aggregating between 2-8 links
• Seen as single port in spanning-tree
• You can nott configure 2 protocols on 1 channel
• must be the same speed + duplex, same vlan allowed if trunk,same vlan if access
• the members can have different spanning tree costs
• Pagp-cisco propetiary desirable auto
• Lacp-open standard active passive
• default load balancing is src mac
• load balancing can be configured different depending on direction
• optios are src-dst mac,src-dstip,tcp/udp
• In Lacp switch lowest priority + lowest mac will which ports will in the etherchannel and which will be in standby
• On port you can influence discussion by configuring lacp port priority