Building Cisco Multilayer Building Cisco Multilayer Switched Networks (BCMSN) Switched Networks (BCMSN) Inter-VLAN Routing

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Building Cisco Multilayer Building Cisco Multilayer Switched Networks (BCMSN) Switched Networks (BCMSN)

Inter-VLAN Routing

Layer 2

Layer 2 vs Layer 3 Reviewvs Layer 3 Review

• Layer 2 switches do not do frame modification

– i.e. “Transparent” bridging

• Implies hosts in a VLAN can only reach MAC’s directly in the CAM table

– i.e. the local broadcast domain

• Layer 3 routers/switches perform layer 2 packet rewrite

– Remove the layer 2 header and rebuild it

• Implies Inter-VLAN traffic must be routed

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Switch to Router w/ Multiple Links Switch to Router w/ Multiple Links

• One solution for Inter-VLAN routing is to use one physical link per VLAN between the layer 2 switch and layer 3 router

• How it works

– Frames leaves switch on link 1 in VLAN 10 – Router rewrites frame to MAC in VLAN 20 and

sends back on link 2

– Switch uses CAM of VLAN 20 to reach destination

Switch to Router w/ Multiple Links Example Switch to Router w/ Multiple Links Example

R1

SW1

Fa0/1 Fa0/2

A B

Fa0/1

Fa0/4 Fa0/2

Fa0/3

VLAN 10 VLAN 20

10.0.0.1/24 20.0.0.2/24

10.0.0.254/24 20.0.0.254/24

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Router

Router--onon--aa--StickStick

• Multiple interfaces work, but not scalable because of port density and cost

• Alternate solution is to use one physical link between layer 2 switch and layer 3 router running as ISL/802.1Q Trunk

• How it works

– Frame leaves switch on trunk link with VLAN 10 encapsulation

– Router rewrites frame to MAC in VLAN 20 and sends back on the same trunk link with new encapsulation – Switch uses CAM of VLAN 20 to reach destination

Router

Router--onon--aa--Stick ExampleStick Example

R1

SW1

Fa0/0.10

A B

Fa0/1

Fa0/4 Fa0/3

VLAN 10 VLAN 20

10.0.0.1/24 20.0.0.2/24

10.0.0.254/24 20.0.0.254/24 Fa0/0.20

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Switched Virtual Interfaces (SVIs) Switched Virtual Interfaces (SVIs)

• Better solution is to combine the layer 2 switch and layer 3 router together

– i.e. layer 3 switch

• Switch to router communication and rewrite happens on the backplane/fabric

– e.g RSFC/MSFC

• Implemented as interface vlan [vlan] on the layer 3 switch

• Faster, more scalable, and easier to manage

SVIs Example SVIs Example

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Native Layer 3 Routed Ports Native Layer 3 Routed Ports

• In addition to layer 2 switchports, most layer 3 switches can run ports in native layer 3 routed mode

– i.e. no switchport

• Native layer 3 ports treated just like an Ethernet port on a router

– IP address assignment, ACLs, QoS, etc.

• Typically used in designs where uplinks are routed

– Access to distribution layer uplinks – Distribution layer to core layer uplinks

• Eliminates STP convergence time

– Convergence is now a function of layer 3 routing protocol – See High Availability Campus Network Design-Routed Access

Layer using EIGRP or OSPF for more info

Native Layer 3 Routed Ports Example Native Layer 3 Routed Ports Example

A

VLAN10

Fa0/3

VLAN 10

10.0.0.1/24 10.0.0.254/24

B

Fa0/4

VLAN 20

20.0.0.2/24 20.0.0.254/24

VLAN20 100.0.0.0/24

Fa0/13

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Inter

Inter--VLAN Routing Q&AVLAN Routing Q&A