A small note on what happens under the hood of Cisco MLS (Multi Layer Switch) when creating routed interfaces.
In MLS, the interface can be in one of two modes:
- "switchport"
- "routed interface" or "no switchport"
When translating an interface to the latter, the switch allows you to assign an ip address directly to the port and use it as a router interface.
However, this is just an abstraction. Here's what happens under the hood of MLS when the interface is set to "no switchport" mode:
- switch creates VLAN
- the switch adds an interface to this VLAN in access mode. This will be the only interface belonging to this VLAN.
- the switch creates an SVI (Switch Virtual Interface) for the selected VLAN and assigns the specified ip address to this SVI
In fact, the following two configurations are almost identical *:
interface Ethernet0/0 no switchport ip address 10.0.0.1 255.255.255.0
interface Ethernet0/0 switchport mode access switchport access vlan 100 interface Vlan 100 ip address 10.0.0.1 255.255.255.0
* if the switch no longer has any access interface belonging to VLAN 100, and VLAN 100 is not allowed on any of the trunk interfaces.
Take a switch with a minimal startup configuration as an example. All interfaces are in switchport mode and belong to VLAN 1:
SW1#sh interfaces status
Port Name Status Vlan Duplex Speed Type
Et0/0 connected 1 auto auto unknown
Et0/1 connected 1 auto auto unknown
Et0/2 connected 1 auto auto unknown
Et0/3 connected 1 auto auto unknown
SW1#sho vlan brief
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Et0/0, Et0/1, Et0/2, Et0/3
1002 fddi-default act/unsup
1003 token-ring-default act/unsup
1004 fddinet-default act/unsup
1005 trnet-default act/unsup
Let's put the Ethernet 0/0 interface into routed mode:
SW1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)#interface ethernet 0/0
SW1(config-if)#no switchport
SW1(config-if)#end
SW1#show running-config interface ethernet 0/0
Building configuration...
Current configuration : 59 bytes
!
interface Ethernet0/0
no switchport
no ip address
end
The VLAN that is created when the interface is switched to routed mode is not visible in the usual VLAN database:
SW1#sho vlan brief
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Et0/0, Et0/1, Et0/2, Et0/3
1002 fddi-default act/unsup
1003 token-ring-default act/unsup
1004 fddinet-default act/unsup
1005 trnet-default act/unsup
But it can be seen among the "Internal VLANs":
SW1#show vlan internal usage
VLAN Usage
---- --------------------
1006 Ethernet0/0
Potential problem with this switch behavior
VLAN ID 1006 Internal VLAN Ethernet0/0, VLAN ID, SW1 VLAN:
SW1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)#vlan 1006
SW1(config-vlan)#
SW1(config-vlan)#exit
% Failed to create VLANs 1006
VLAN(s) not available in Port Manager.
%Failed to commit extended VLAN(s) changes.
*Jan 5 17:34:31.407: %PM-4-EXT_VLAN_INUSE: VLAN 1006 currently in use by Ethernet0/0
*Jan 5 17:34:31.407: %SW_VLAN-4-VLAN_CREATE_FAIL: Failed to create VLANs 1006: VLAN(s) not available in Port Manager
:
- L3- (shutdown), Internal VLAN ID
- VLAN ID
- L3- ( no shutdown), VLAN ID Extended
SW1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)#inter ethernet 0/0
SW1(config-if)#shutdown <- , VLAN ID 1006
SW1(config-if)#do show vlan internal usage
VLAN Usage
---- --------------------
SW1(config-if)#
SW1(config-if)#vlan 1006 <- VLAN ID 1006
SW1(config-vlan)#exit
SW1(config)#interface ethernet 0/0
SW1(config-if)#no shutdown <- routed
SW1(config-if)#end
SW1#sho vlan brief
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active
1002 fddi-default act/unsup
1003 token-ring-default act/unsup
1004 fddinet-default act/unsup
1005 trnet-default act/unsup
1006 VLAN1006 active
SW1#show vlan internal usage
VLAN Usage
---- --------------------
1007 Ethernet0/0
VLAN ID L3
:
! vlan internal allocation policy ascending !
, VLAN ID Extended : 1006 — 4094.
VLAN ID 1006 , VLAN 1007 L3 Ethernet0/0, routed, VLAN ID 1008:
SW1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)#inter ethernet 0/3
SW1(config-if)#no switchport
SW1(config-if)#end
SW1#show vlan internal usage
VLAN Usage
---- --------------------
1007 Ethernet0/0
1008 Ethernet0/3
VLAN 1009, routed , VLAN 1009 , VLAN L3 — 1010:
SW1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)#vlan 1009
SW1(config-vlan)#end
W1#sho vlan brief
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Et0/2
1002 fddi-default act/unsup
1003 token-ring-default act/unsup
1004 fddinet-default act/unsup
1005 trnet-default act/unsup
1006 VLAN1006 active
1009 VLAN1009 active
SW1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)#inter ethernet 0/2
SW1(config-if)#no sw
SW1(config-if)#no switchport
SW1(config-if)#end
SW1#sho vlan internal usage
VLAN Usage
---- --------------------
1007 Ethernet0/0
1008 Ethernet0/3
1010 Ethernet0/2
Internal VLAN, — ascending:
SW1(config)#vlan internal allocation policy ?
ascending Allocate internal VLAN in ascending order <- 1006
descending Allocate internal VLAN in descending order <- 4094
L3 Port-channel
member Internal VLAN ID, Port-channel Internal VLAN ID:
:
SW1#sho inter status
Port Name Status Vlan Duplex Speed Type
Et0/0 connected 1 auto auto unknown
Et0/1 connected 1 auto auto unknown
Et0/2 connected 1 auto auto unknown
Et0/3 connected 1 auto auto unknown
SW1#sho vlan inter
SW1#sho vlan internal usa
SW1#sho vlan internal usage
VLAN Usage
---- --------------------
SW1#
SW1(config)#inter range ethernet 0/0 - 1
SW1(config-if-range)#no switchport
SW1(config-if-range)#do show vlan internal usage
VLAN Usage
---- --------------------
1006 Ethernet0/0
1007 Ethernet0/1
SW1(config-if-range)#channel-group 1 mode active
Creating a port-channel interface Port-channel 1
SW1(config-if-range)#do show vlan internal usage
VLAN Usage
---- --------------------
1006 Ethernet0/0
1007 Ethernet0/1
1008 Port-channel1
Port-channel ( member ), Internal VLAN ID :
SW1(config-if-range)#inter po1
SW1(config-if)#shutdown
SW1(config-if)#do show vlan internal usage
VLAN Usage
---- --------------------
SW1(config-if)#no shutdown
SW1(config-if)#do show vlan internal usage
VLAN Usage
---- --------------------
1006 Port-channel1
1007 Ethernet0/0
1008 Ethernet0/1
P.S.
Loopback Internal VLAN .