Day18: VLANs Part 3

Native VLAN Configuration on Routers

When using Router-on-a-Stick (ROAS), you can configure the router to participate in native VLAN functionality. Native VLAN frames are not tagged with 802.1Q headers, making transmission slightly more efficient because frames are smaller.

Two methods to configure native VLAN on a router:

1. Subinterface Method: Use the command encapsulation dot1q [vlan-id] native on the subinterface

2. Physical Interface Method: Configure the IP address directly on the physical interface (no subinterface needed, no encapsulation command needed)

802.1Q Tag Structure (Wireshark View)

The dot1q tag is inserted after the source MAC address field and contains:

• TPID (Tag Protocol Identifier): 0x8100 - identifies frame as 802.1Q tagged

• PCP (Priority Code Point): 3 bits for QoS priority (0-7)

• DEI (Drop Eligible Indicator): 1 bit indicating if frame can be dropped during congestion

• VLAN ID: 12 bits identifying the VLAN (1-4094)

Layer 3 (Multilayer) Switching

A multilayer switch can perform both Layer 2 switching AND Layer 3 routing. This is the preferred method for inter-VLAN routing in large networks.

Capabilities of Layer 3 switches:

• Switching and routing in one device

• IP address assignment to interfaces

• Routed ports (interfaces that act like router interfaces)

• Switch Virtual Interfaces (SVIs) for each VLAN

• Routing table with static/dynamic routes

Switch Virtual Interfaces (SVIs)

SVIs are virtual interfaces created in software that can be assigned IP addresses and used as default gateways for VLANs. They enable inter-VLAN routing without sending traffic to an external router.

1. Forgetting ip routing - Without this command, the multilayer switch cannot route traffic between VLANs

2. Forgetting no switchport - You cannot assign an IP address to a switch interface until you convert it to a routed port

3. Forgetting no shutdown on SVIs - SVIs are shutdown by default, unlike physical interfaces

4. SVI won't come up - Students forget the requirements:

   • VLAN must exist on the switch

   • At least one UP/UP access port in that VLAN OR one UP/UP trunk allowing that VLAN

   • VLAN must not be shutdown

   • SVI must not be shutdown

5. Confusing native VLAN methods on router - Remember: physical interface = no encapsulation command needed; subinterface = need encapsulation dot1q [vlan] native

6. Creating SVI doesn't create VLAN - Unlike access port assignment, creating an SVI does NOT automatically create the VLAN

"NO-NO-IP for Layer 3 Switching"

• NO switchport (make it a routed port)

• NO shutdown (enable the SVI)

• IP routing (enable routing on switch)

SVI Up/Up Requirements - "VLAN EATS"

• VLAN must exist

• Enabled (not shutdown)

• Access port up OR Trunk allowing VLAN

• SVI enabled (no shutdown)

Native VLAN on Router:

• Subinterface = Submit the word "native" (encapsulation dot1q [vlan] native)

• Physical = Physically just put the IP address (no encapsulation needed)

802.1Q Tag - "TPC-DV" (Think: "The PC's DeVice")

• TPID (0x8100)

• PCP (priority)

• DEI (drop eligible)

• VLAN ID

Configuring Native VLAN on Router (Method 1 - Subinterface)

Router(config)# interface g0/0.10

Router(config-subif)# encapsulation dot1q 10 native

Router(config-subif)# ip address 192.168.1.62 255.255.255.192

Configuring Native VLAN on Router (Method 2 - Physical Interface)

Router(config)# interface g0/0

Router(config-if)# ip address 192.168.1.62 255.255.255.192

Converting Switch Port to Routed Port

Switch(config)# ip routing

Switch(config)# interface g0/1

Switch(config-if)# no switchport

Switch(config-if)# ip address 192.168.1.193 255.255.255.252

Configuring SVIs

Switch(config)# interface vlan 10

Switch(config-if)# ip address 192.168.1.62 255.255.255.192

Switch(config-if)# no shutdown

Switch(config)# interface vlan 20

Switch(config-if)# ip address 192.168.1.126 255.255.255.192

Switch(config-if)# no shutdown

Configuring Default Route on Multilayer Switch

Switch(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.194

Verification Commands

show ip interface brief

show ip route

show interfaces status

show running-config