InterVLAN Acquisition Background
Several capacity accept dealt with the foundations of arrangement design, VLANs, and trunking.
However, VLANs accept been presented as abandoned advertisement domains. The abutting two chapters
discuss how to carriage cartage amid VLANs application Layer 3 functions.
VLANs are about configured on Layer 2 switches to anatomy advertisement domains. VLANs can
exist in one or added switches through the use of trunking. As well, VLANs usually represent
subnetworks of Layer 3 protocols. Since Layer 2 switches do not use Layer 3 acclamation to
make switching decisions, a Layer 3 accommodation is bare to move packets amid VLANs.
Traditionally, routers accept performed this function. Routers accept affluent affection sets that include
intelligent, activating acquisition protocols for packet carriage and abounding packet filtering
capabilities. Additionally, abounding types of connectivity enhancements like DHCP relaying,
Network Address Translation, Quality of Service, and policy-based acquisition add to the
functionality. Routers additionally action connectivity amid LAN technologies and Wide Area
Network technologies, abutting a ample ambit of arrangement media.
Routing in a about-face arrangement can booty on three forms: interVLAN routing, multilayer switching
(MLS), and Cisco Express Forwarding (CEF). InterVLAN acquisition is based on abacus a route
processor about in the switched arrangement to accommodate Layer 3 routing. Every packet
destined from one VLAN to addition charge canyon through the router. MLS, on the added hand, is
based on the assumption that the router alone sees the aboriginal packet of a conversation. Switching
paths are again set up so that consecutive packets bypass the router and are switched by a more
efficient “shortcut” path. CEF is a broadcast switching apparatus befitting copies of route
cache advice in several altered forms to be acclimated for able switching. Catalyst switches
can duke off packets to a CEF-capable router for processing, as in interVLAN routing. Some
Catalyst platforms apparatus CEF anon in hardware.
The afterward sections altercate interVLAN acquisition at length, application a array of designs and
configurations. MLS is discussed in Chapter 8, “Multilayer Switching.” Advice about CEF
can be begin at www.cisco.com/univercd/cc/td/doc/product/software/ios121/121cgcr/
switch_c/xcprt2/xcdcef.htm.