# L3 Switching

> Markdown source of <https://docs.calebsargeant.com/networking/cisco/switching/l3-switching/>.

The Problem:

Clients need routing between VLANs

## Router on a stick (802.1Q/ISL)

- Advantages:
  - Simple to set up
  - Lower cost
- Disadvantages:
  - Congestion on a link
  - Single point of failure
  - Delay of routing
- Setup:
  - (Switch) configure trunk
  - (Router) create sub-interfaces

``` text
int f0/1
switchport trunk encap dot1q
switchport mode trunk
int f0/0.10
encap dot1q 10
ip address 10.1.10.1 255.255.255.0
int f0/0.20
encap dot1q 20
ip add 10.1.20.1 255.255.255.0
```

## Multi-Layer Switching

- Advantages:
  - Routing at wire speed
  - Backplane bandwidth
  - Redundancy-enabled
- Disadvantages:
  - Cost
- Setup:
  - Create SVIs
  - (opt.) create routed ports
  - (opt.) enable routing protocols

``` text
ip routing
interface vlan 10
ip add 10.1.10.1 255.255.255.0
no shut
interface vlan 20
ip add 10.1.20.1 255.255.255.0
no shut
int fa0/24
no switchport
ip add 10.1.24.1 255.255.255.252
```

## Understanding Layer 3 vs Multilayer Switching

- First packet hits router, all future packets go through CEF/ASIC
- L3 Switch is a switch with router inside
- Multilayer is a switch that can cache routing info (CEF)
- All L3 switches are multilayer switches
- But not all multilayer switches are L3

## Layer 3 Routing vs Layer 3 Switching

- Router and L3 switch both have IOS software routing
- Software routing is relatively slow compared to ASICs
- L3 switches can play a little software - hardware trick

## Exceptions to CEF

- Packet with header options
- Packet with TTL expired
- Packets destined to a tunnel interface
- Packets with unsupported encapsulations
- Packets requiring fragmentation (MTU exceeded)

## Verifying CEF Processing

``` text
ip cef
show ip cef summary
show ip cef vlan 200
sh ip arp 172.30.100.11
```