A friend came to me asking a question regarding the output below, but given my novice level I have been unable to help. Below is the sh cdp neighbor command from a Cisco 3550 Switch that is connected to a Cisco 2821 router. The question is what do the decimal point after Gig 0/1 signify? For instance, Gig 0/1.8? Also, why would someone have a setup of using the same interface three times? Thanks for any assistance.
Device ID Local Intrfce Holdtme Capability Platform Port ID
KYX-Admin02 Gig 0/2 176 T S WS-C2924M-Gig 1/1
KCn Fas 0/4 121 R S I Cisco 2821Gig 0/1.68
KCn Fas 0/4 121 R S I Cisco 2821Gig 0/1.8
KCn Fas 0/4 131 R S I Cisco 2821Gig 0/1
KCn Fas 0/2 131 R S I Cisco 2821Gig 0/0
The best way to understand why the same interface is used multiple times is to look at the legacy way of intervlan routing called "Router on a switch"
The link below would help ya..
Just to add to Jerry's post.
The 3550 is a L3 switch so you don't actually need to use subinterfaces on the router. In fact by using subinterfaces on the router you are limiting the bandwidth available between those vlans.
In the majority of cases if you have a L3 capable switch then the inter-vlan routing should be done there and not using subinterfaces on a router.
Subinterfaces and use the router to route between VLAN's.