Router-on-a-stick diagram

Router-on-a-stick diagram
Now back to our original question. Why use layer 3 switching? You can see in Figure 18.1
that it is very inefficient to have to use a router to move a packet from Host A to Host D when
they are connected to the same switch. MLS is used to bypass the router on subsequent packets
of the same flow. A
flow
is a table entry for a specific conversation, created by using source and
destination header information for layers 3 and 4. The switch caches the routing information
for that particular flow to make changes to future frames. Several fields within a frame make
it unique:

Source and destination IP addresses

Source and destination MAC addresses

Type of service (ToS)

Protocol type (for example, HTTP, FTP, ICMP, and so on)
These are just some of the characteristics of a frame that can be used to establish a flow.
A switch can be configured to support simple flows, such as IP address to IP address, or the
switch can support complex flows dealing with port and protocol information.
Don’t allow the regular changing of descriptions between packets and frames
confuse you. Remember first that packets are what makes an IP flow, and in
general, flows are described using layer 3 and above information. But remember
also that packets are encapsulated inside frames and that local delivery
across a switch is carried out using the MAC addresses. So, while packets are
delivered end-to-end without changing, it is common for frames to be modified
by routers when the source and destination MAC addresses are changed.
VLAN 10 VLAN 50
VLAN 10 VLAN 50
VLAN 10 VLAN 50
1/1
FE0/0.10
FE0/0.50
Host A
Host B
Host C
Host D
Host E
Host F
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Chapter 18 
Multilayer Switching (MLS)
To summarize, we use MLS to enable the switch to forward the first packet in the flow to the
router and then learn what should be done with the rest of the packets in the flow so the router
doesn’t need to route them. In Figure 18.1, the switch makes the necessary VLAN and destination
MAC address changes in the subsequent frames.