Quick question for a network ninja….
Say I have four sub-interfaces on a router (not a layer 3 switch). Each interface has already been configured and each sub-int is a /23. Now, the parent interface isn't configured. There isn't an IP address configured at all. I want to know if I can get away with making the parent int a /24 on a totally different subnet since the sub-ints aren't actually part of the parent int network. Not sure if I'm making sense so I will show…
Say these are the sub-interfaces:
-interface FastEthernet1/0.10
-description FacultyVLAN
-encapsulation dot1Q 10
-ip address 10.10.10.1 255.255.254.
-interface FastEthernet1/0.20
-description AdministrativeVLAN
-encapsulation dot1Q 20
-ip address 10.10.12.1 255.255.254.0
-interface FastEthernet1/0.30
-description InstructionalVLAN
-encapsulation dot1Q 30
-ip address 10.10.14.1 255.255.254.0
-interface FastEthernet1/0.40
-description ServerVLAN
-encapsulation dot1Q 40
-ip address 10.10.16.1 255.255.254.0
Am I ok to set the parent FastEthernet port as 10.10.18.1/24? From my understanding the parent interface at this point is only serving as the gateway for those SVIs to get, correct. Need a little help with understanding. Cheers mates!
Just want to make sure that I'm going about this the right way. Cheers!
Best Answer
Each interface, including logical interfaces, e.g. subinterfaces, is a separate interface, and it is configured independently.
Normally, with subinterfaces for a LAN, you will use 802.1Q encapsulation, and that places each interface in a different VLAN. The physical interface will be in its own VLAN. VLANs can have completely different addresses and masks, independent of any other interface configuration..
If this is a WAN interface with something like frame relay, each interface, or subinterface, will have its own DLCI, and those logical links will each have their own IP addresses and masks, independent of any other interface configuration.
So, yes, you can configure the physical interface with a different address mask, as long as it doesn't overlap with any already configured network.