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)