L3 Routing Notes

Layer 3 routing process of the Switches :

Switched virtual interfaces(SVI)

-interfaces vlan [1-4094]

-vlan must exist in the database first

Native routed interface

- no switchport

-same as ethernet interface on a router

Sw1# show ip route

Default gateway is not set

ICMP redirect cache is empty == means ip routing is not set in switch.

If gateway is not set, switch will try to do arp for all destination ips.

To turn on ip routing in switch

switch(config)# ip routing

Routing can be used over trunk interface. It is advisable to do so since it does not use STP and hence will not have convergence issues.

Layer 3 Routing (Contd)

Router on a Stick

-Layer 2 switch trunks traffic to external L3 router

-legacy version of SVI

Router usually does not support DTP and VTP

-switchport mode trunk

-switchport trunk allowed vlan

Router encapsulated ISL or 802.1q traffic using sub-interfaces

-encapsulation [isl | dot1q] {vlan} {native}

Native vlan must match

-Can be on the main interface or subinterface with native keyword.

Note: sub interface and vlan number need not to be matched. We are doing so just for clarity.

There will be only one native vlan on a trunk