Multilayer Switching
Multilayer Switching (MLS) refers to the action by which a LAN switch, which operates at
least at Band 2, additionally uses argumentation and protocols from layers added than Band 2 to advanced data.
The appellation Band 3 switching refers accurately to the use of the Band 3 destination address,
compared to the acquisition table (or equivalent), to accomplish the forwarding decision. (The latest
switch accouterments and software from Cisco uses CEF switching to optimize the forwarding of
packets at Band 3.)
MLS Logic
Layer 3 switching agreement works analogously to router configuration—IP addresses are assigned
to interfaces, and acquisition protocols are defined. The acquisition agreement agreement works aloof like
a router; however, the interface agreement on MLS switches differs hardly from routers, using
VLAN interfaces, baffled interfaces, and PortChannel interfaces.
Multilayer Switching 169
VLAN interfaces accord a Band 3 about-face a Band 3 interface absorbed to a VLAN. Cisco sometimes
refers to these interfaces as switched basal interfaces (SVIs). To avenue amid VLANs, a switch
simply needs a basal interface absorbed to anniversary VLAN, and anniversary VLAN interface needs an
IP abode in the corresponding subnets acclimated on those VLANs.
When application VLAN interfaces, the about-face charge booty one apparent but simple added step
when acquisition a packet. Like archetypal routers, MLS makes a acquisition accommodation to advanced a packet.
As with routers, the routes in an MLS acquisition table admission account an approachable interface (a VLAN
interface in this case), as able-bodied as a next-hop band 3 address. The adjacency advice (for example,
the IP ARP table or the CEF adjacency table) lists the VLAN cardinal and the next-hop device’s
MAC abode to which the packet should be forwarded—again, archetypal of accustomed router operation.
At this point, a accurate router would apperceive aggregate it needs to apperceive to advanced the packet. An MLS
switch, however, afresh additionally needs to use Band 2 argumentation to adjudge out which concrete interface to
physically advanced the packet. The about-face will artlessly acquisition the next-hop device’s MAC abode in
the CAM and advanced the anatomy to that abode based on the CAM.
Using Baffled Ports and PortChannels with MLS
In some point-to-point topologies, VLAN interfaces are not required. For instance, back an
MLS about-face connects to a router application a cable from a about-face interface to a router’s LAN
interface, and the alone two accessories in that subnet are the router and that one concrete interface
on the MLS switch, the MLS about-face can be configured to amusement that one interface as a routed
port. (Another archetypal cartography for application router ports is back two MLS switches affix for
the purpose of acquisition amid the switches, afresh creating a case with alone two accessories in the
VLAN/subnet.)
A baffled anchorage on an MLS about-face has the afterward characteristics:
■ The interface is not in any VLAN (not alike VLAN 1).
■ The about-face does not accumulate any Band 2 switching table advice for the interface.
■ Band 3 settings, such as the IP address, are configured beneath the concrete interface—just like
a router.
■ The adjacency table lists the approachable concrete interface or PortChannel, which agency that
Layer 2 switching argumentation is not appropriate in these cases.
NOTE Although it is not a requirement, the accessories in a VLAN are about configured in the
same distinct IP subnet. However, you can use accessory IP addresses on VLAN interfaces to
configure assorted subnets in one VLAN, aloof like on added router interfaces.
170 Chapter 6: IP Forwarding (Routing)
Ethernet PortChannels can be acclimated as baffled interfaces as well. To do so, as on concrete routed
interfaces, the no switchport command should be configured. (For PortChannels, the physical
interfaces in the PortChannel charge additionally be configured with the no switchport command.) Also,
when application a PortChannel as a baffled interface, PortChannel amount acclimation should be based on
Layer 3 addresses because the Band 2 addresses will mostly be the MAC addresses of the two
MLS switches on either end of the PortChannel. PortChannels may additionally be acclimated as Band 2
interfaces back accomplishing MLS. In that case, VLAN interfaces would be configured with IP address,
and the PortChannel would artlessly act as any added Band 2 interface.
Table 6-4 lists some of the specifics about anniversary blazon of Band 3 interface.
MLS Configuration
The accessible MLS agreement archetype is advised to appearance all of the agreement options. The
network architecture is apparent in Figures 6-4 and 6-5. In Amount 6-4, the concrete cartography is shown,
with baffled ports, VLAN trunks, a baffled PortChannel, and admission links. Amount 6-5 shows the
same network, with a Band 3 appearance of the subnets acclimated in the network.
Table 6-5 MLS Band 3 Interfaces
Interface
Forwarding to Adjacent
Device Agreement Requirements
VLAN interface Uses Band 2 argumentation and L2 MAC
address table
Create VLAN interface; VLAN
must additionally exist
Physical (routed) interface Forwards out concrete interface Use no switchport command to
create a baffled interface
PortChannel (switched)
interface
Not applicable; aloof acclimated as
another Band 2 forwarding path
No appropriate configuration; useful
in affiliation with VLAN
interfaces
PortChannel (routed)
interface
Balances beyond links in
PortChannel
Needs no switchport command in
order to be acclimated as a routed
interface; optionally change loadbalancing
method
Multilayer Switching 171
Figure 6-4 Concrete Topology: Archetype Application MLS
Figure 6-5 Band 3 Cartography View: Archetype Application MLS
A few architecture credibility buck altercation afore jumping into the configuration. First, SW1 and SW2
need Band 2 connectivity to abutment cartage in VLANs 11 and 12. In added words, you charge a Band 2
trunk amid SW1 and SW2, and for several reasons. Focusing on the Band 2 portions of the
network on the appropriate ancillary of Amount 6-4, SW1 and SW2, both administration MLS switches, connect
to SW3 and SW4, which are admission band switches. SW1 and SW2 are amenable for providing
SW1
MLS
FA0/0
FA0/1
FA0/0
MLS
SW2
SW3
SW4
VLAN 12
VLAN 11
Server 1
Server 2
Server 3
Server 4
VLAN 12
VLAN 11
R1
R2
T
T
T
T
T
R
R
R
T
R
— Block (also a switchport)
— Baffled Port
MLS
SW1
172.31.21.0/24
172.31.22.0/24
172.31.23.0/24
MLS
SW2
Server 1
Server 2
Server 4
Server 3
R1
R2
V11
V11
V12
V12
VLAN 11
172.31.11.0/24
VLAN 12
172.31.12.0/24
Vx — VLAN Interface x
172 Chapter 6: IP Forwarding (Routing)
full connectivity in VLANs 11 and 12. To absolutely booty advantage of the bombastic links, SW1 and
SW2 charge a Band 2 aisle amid anniversary other. Additionally, this architecture uses SW1 and SW2 as
Layer 3 switches, so the hosts in VLANs 11 and 12 will use SW1 or SW2 as their absence gateway.
For bigger availability, the two switches should use HSRP, VRRP, or GLBP. Regardless of which
protocol is used, both SW1 and SW2 charge to be in VLANs 11 and 12, with connectivity in those
VLANs, to be able as absence gateways.
In accession to a Band 2 block amid SW1 and SW2, to accommodate able routing, it makes sense
for SW1 and SW2 to accept a baffled aisle amid anniversary added as well. Certainly, SW1 needs to be
able to avenue packets to router R1, and SW2 needs to be able to avenue packets to router R2. However,
routing amid SW1 and SW2 allows for accessible aggregation if R1 or R2 fails.
Figure 6-4 shows two alternatives for baffled connectivity amid SW1 and SW2, and one option
for Band 2 connectivity. For Band 2 connectivity, a VLAN block needs to be acclimated amid the
two switches. Amount 6-4 shows a brace of trunks amid SW1 and SW2 (labeled with a circled T)
as a Band 2 PortChannel. The PortChannel would abutment the VLAN 11 and 12 traffic.
To abutment baffled traffic, the amount shows two alternatives: artlessly avenue over the Band 2
PortChannel application VLAN interfaces, or use a abstracted baffled PortChannel. First, to use the Band 2
PortChannel, SW1 and SW2 could artlessly configure VLAN interfaces in VLANs 11 and 12. The
alternative agreement uses a additional PortChannel that will be acclimated as a baffled PortChannel.
However, the baffled PortChannel does not action as a Band 2 aisle amid the switches, so the
original Band 2 PortChannel charge still be acclimated for Band 2 connectivity. Accessible Archetype 6-3
shows both configurations.
Finally, a quick animadversion about PortChannels is needed. This architecture uses PortChannels between
the switches, but they are not required. Most links amid switches today use at atomic two links
in a PortChannel, for the archetypal reasons—better availability, bigger convergence, and beneath STP
overhead. This architecture includes the PortChannel to point out a baby aberration amid the routed
interface agreement and the baffled PortChannel configuration.
Example 6-3 shows the agreement for SW1, with some capacity on SW2.
Example 6-3 MLS-Related Agreement on Switch1
! Below, agenda that the about-face is in VTP cellophane mode, and VLANs 11 and 12 are
! configured, as required. Additionally agenda the ip acquisition all-around command, afterwards which
! the about-face will not accomplish Band 3 switching of IP packets.
vlan 11
!
vlan 12
! The ip acquisition all-around command is appropriate afore the MLS will perform
! Band 3 forwarding.
ip routing
Multilayer Switching 173
!
vtp area CCIE-domain
vtp approach transparent
! Abutting the no switchport command makes PortChannel a baffled port. On a routed
! port, an IP abode can be added to the interface.
interface Port-channel1
no switchport
ip abode 172.31.23.201 255.255.255.0
! Below, agnate agreement on the interface affiliated to Router1.
interface FastEthernet0/1
no switchport
ip abode 172.31.21.201 255.255.255.0
! Next, the agreement shows basal PortChannel commands, with the
! no switchport command actuality appropriate due to the aforementioned command on PortChannel.
interface GigabitEthernet0/1
no switchport
no ip address
channel-group 1 approach desirable
!
interface GigabitEthernet0/2
no switchport
no ip address
channel-group 1 approach desirable
! Next, interface VLAN 11 gives Switch1 an IP attendance in VLAN11. Accessories in VLAN
! 11 can use 172.31.11.201 as their absence gateway. However, application HSRP is
! better, so Switch1 has been configured to be HSRP primary in VLAN11, and Switch2
! to be primary in VLAN12, with tracking so that if Switch1 loses its connection
! to Router1, HSRP will abort over to Switch2.
interface Vlan11
ip abode 172.31.11.201 255.255.255.0
no ip redirects
standby 11 ip 172.31.11.254
standby 11 antecedence 90
standby 11 clue FastEthernet0/1
! Below, VLAN12 has agnate agreement settings, but with a college (better)
! HSRP antecedence than Switch2’s VLAN 12 interface.
interface Vlan12
ip abode 172.31.12.201 255.255.255.0
no ip redirects
standby 12 ip 172.31.12.254
standby 12 antecedence 110
standby 12 clue FastEthernet0/1
Example 6-3 MLS-Related Agreement on Switch1 (Continued)
As declared earlier, the baffled PortChannel is not appropriate in this topology. It was included to show
an archetype of the configuration, and to accommodate a accomplishments from which to altercate the differences.
However, as configured, SW1 and SW2 are Band 3 adjoining over the baffled PortChannel as well
as via their VLAN 11 and 12 interfaces. So, they could barter IGP acquisition updates over three
separate subnets. In such a design, the baffled PortChannel was apparently added so that it would be
the accustomed Band 3 aisle amid SW1 and SW2; affliction should be taken to tune the IGP implementation
so that this avenue is called instead of the routes over the VLAN interfaces.
NOTE For MLS switches to avenue application VLAN interfaces, two added accomplishments are required: The
corresponding VLANs charge be created, and the ip acquisition all-around command charge accept been
configured. (MLS switches will not accomplish Band 3 acquisition afterwards the ip acquisition command,
which is not enabled by default.) If the VLAN interface is created afore either of those actions,
the VLAN interface sits in a “down and down” state. If the VLAN is created next, the VLAN
interface is in an “up and down” state. Finally, afterwards abacus the ip acquisition command, the interface
is in an “up and up” state.