InterVLAN Routing

Design and Implementation

What Routers Do

• Intelligent, dynamic routing protocols for packet transport

• Packet filtering capabilities

• Connectivity enhancements – DHCP, NAT, QoS, policy-based routing

• Connectivity between LAN and WAN

Routing Issues

• Isolated broadcast domains– Linked by routers (Layer 3)

• Sending nonlocal packets – Routing protocols, gateway of last resort,

default routes

• Supporting multiple VLAN packets across VLAN boundaries– ISL solves problem

ISL

• Supported over Fast Ethernet links– Single ISL link (trunk) can carry traffic from

multiple VLANs• Packets encapsulated with ISL VLAN header

appropriate to destination and routes to appropriate interface

Routing in a Switched Network

• interVLAN routing– Add a route processor in switched network to provide layer 3 routing

• Every packet goes through the router

• Layer 3 switching (RSM- route switch module)– Router running IOS software plugs into switch backplane

• Appears as module with single 800 Mb half duplex trunk port and one MAC address

• Multilayer switching (MLS)– Router sees only first packet; sets up switching paths to bypass the

router

• Cisco Express Forwarding (CEF)– Distributed switching mechanism

RSM Details

• Provides multilayer switching and inter-VLAN routing between switched VLANs– Can be installed in slots 2 through 12– Maximum number of modules is 7 for Cat 5500– RSM is mapped through VLAN 0 and VLAN 1

• VLAN 0 to channel 0; VLAN 1 to channel 1• VLAN 0 assigned ROM MAC address• VLAN 1 assigned base MAC address from 512

available MAC addresses

Routing with Multiple Physical Links

• Use one link for each VLAN on switch to connected router– Each packet travels across a link reserved for VLAN

traffic• Useful when switches and router are already available and can be

quickly connected• Not a good idea to use when network grows

• Trunk links– Use only one link to router – router on a stick or one-

armed router• Integrated Router

– Route processor is inside the switch for tighter integration of layer 2 and layer 3 components

Routing Over Trunk Lines

• 802.1q and ISL trunks– Use Fast Ethernet or Gigabit Ethernet– Each frame identified with VLAN number

• Frame encapsulated as it leaves switch

– Offer scalability because single link can transport many VLANs

– Requires link bandwidth overhead• 30-byte header (ISL) or 4-byte header (802.1q)

Routing with an Integrated Router

• Route Switch Module (RSM)– Runs Cisco ISO software – Interfaces to switch through backplane – channel 0 and

channel 1• Route processor is module with route switch in the module-

type field– Use show module command– Use session to telnet from switch to router

• Multilayer Switch Feature Card (MSFC)– Shortcuts switch layer 3 and higher protocols – to be

covered in detail later

VLAN Connectivity and Trunk lines

• (config)# int 2/3.1

• (config-if)#encap isl vlan2

• Ip address n.n.n.n s.s.s.s– Each subinterface is assigned to specific VLAN– VLAN numbers should match on both router

and switch ends

Routing Processes

• Route processor keeps local table of known routes, metrics, interfaces, and neighboring routers– Uses dynamic routing protocols and static routes

– (config)# ip routing

– (config)# router rip

– (config-router)# network n.n.n.n

• Switch (enable) set ip route default gateway