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

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

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

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