Frame Relay / Subinterface question

I'm sitting through a discussion of frame relay and more specifically configuration of point to point and multipoint subinterfaces. I'm following the addressing and data handling part of it, but the physical part of it never got explained.

Is there a short explanation that someone could either provide or refer me to that explains how multiple serial connections from different frame relay switches end up being connected to the same physical interface (eg serial0)?

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Comments

ColbyG

Google DLCIs.

Wikipedia wrote:

A data link connection identifier (DLCI) is a channel number which is attached to frame relay data frames to tell the network how to route the data. This 10-bit field defines the destination address of a packet. The address is local on a link-by-link basis.

Frame relay is statistically multiplexed, which means that only one frame can be transmitted at a time but many logical connections can co-exist on a single physical line. The DLCI allows the data to be logically tied to one of the connections, so that once it gets to the network it knows where to send it.

The standard allows the existence of 1024 DLCIs, however only numbers from 16 to 991 are available for end users' equipment. The rest are reserved for various management purposes.

http://en.wikipedia.org/wiki/Data_link_connection_identifier

networker050184

It isn't more than one physical connection its all logical. Just think of it like router on a stick, you use VLANs to logically separate the traffic even though its coming in from one switch on one interface. On the other side of that switch it can be broken out to one VLAN per interface or other trunks that carry multiple VLANs. Same concept with frame relay and DLCIs.

KPWright

networker050184 wrote: »

It isn't more than one physical connection its all logical. Just think of it like router on a stick, you use VLANs to logically separate the traffic even though its coming in from one switch on one interface. On the other side of that switch it can be broken out to one VLAN per interface or other trunks that carry multiple VLANs. Same concept with frame relay and DLCIs.

Thanks Networker,

As you can tell, I don't have physical equipment available to me so I'm sort of dependent on the diagrams provided. On the slides they are using for this subject, they are showing each DLCI like it is a separate physical serial connection going to a separate Frame Relay switch somewhere (they give examples like this goes to New York and this goes to Chicago). The diagrams show a single router with lines going to multiple FR switches at the edge of the cloud. That all made sense when they were talking about separate serial ports making those connections, but didn't make any sense once they started subdividing a single port through configuration.

So now the world makes more sense. One office may have a single physical link to a provider's FR switch. I haven't read through the Wiki reference yet, but while I assumed the "local significance" of the DLCI pertained only to the routers at each end of the link, obviously that designation is also significant to the frame relay port to which it is connected. As a matter of fact the information I'm reading right now around inverse ARP and LMI auto-configuration seems to imply the frame relay switch actually allocates the DLCI designations.

Logic prevails for the moment.

Thanks again

kalebksp

KPWright wrote: »

I haven't read through the Wiki reference yet, but while I assumed the "local significance" of the DLCI pertained only to the routers at each end of the link, obviously that designation is also significant to the frame relay port to which it is connected. As a matter of fact the information I'm reading right now around inverse ARP and LMI auto-configuration seems to imply the frame relay switch actually allocates the DLCI designations.

Local significance means that the DLCI is only used between router and the directly connected frame switch. Routers connected by frame relay don't know or care what DLCIs the other routers are using to communicate.