InterVLAN Routing

1
InterVLAN Routing

• Design and Implementation

2
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

3
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

4
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

5
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

6
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

7
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

8
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)

9
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

10
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

11
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