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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!

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    You can certainly configure it, but bear in mind that the behavior should be that only untagged frames would actually be processed by the parent interface (vs the sub-interfaces that receive tagged frames). Also - most of the time we see subinterfaces nowadays it's in the context of Ethernet and thus maps to dot1q values. It's possible for subinterfaces to be used in other contexts, though, where the parent interface configuration may mean something very different. The obvious examples would be FR or ATM, where a subint maps to a PVC or DLCI and the parent wouldn't get an L3 config.
    – rnxrx
    Feb 12, 2017 at 6:03

2 Answers 2

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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.

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  • Thank you Ron, I appreciate you taking the time to answer my question. Have a great weekend!
    – Gregory Bailey
    Feb 11, 2017 at 20:39
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Sub -interfaces are logical interfàces created on layer3 devices or firewalls . The parent interface is logically segregate . But for parent interface no ip address should be configured .

Example

Router(config)#interface fastethernet 0/1 Router(config)#no ip address Router(config)# no shutdown

Further create sub-interfaces

Router(config)# interface fastethernet 0/1.2 Router(config)# ip address X.X.X.X.255.X.X.X X Router(config)# no shutdown.

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