When a telco sells you a Frame Relay service, in essence the
telco promises to forward Frame Relay frames sent by one router to one of many
other routers. Before a router can send a packet, it must add the correct data
link header and trailer to the packet. In Figure 15-2, R1 starts by putting an IP packet into a
Frame Relay frame, between a Frame Relay header and trailer.
When building the frame, R1 must put the correct address in the
header. Each Frame Relay header holds an address field called a data-link connection identifier
(DLCI). The DLCI is a 10-bit number, usually written as a decimal
number between 0 and 1023. R1 puts a particular DLCI into the Frame Relay
header, expecting that the frame will be forwarded by the Frame Relay network to
the other routerR2 in this case.
Finally, the provider's Frame Relay network forwards the frame
to the other router. To accomplish the task, each Frame Relay switch forwards
the frame, based on the DLCI, through the network, until it gets to the router
on the other side. It's similar in concept to how a PC might send an Ethernet
frame, with a destination MAC address, and the LAN switch forwards the frame to
the right destination.
Although the general ideas behind Frame Relay switching and
Ethernet switching are similar, the processes do differ a lot when you look at
the details. For instance, Frame Relay has a single address field, which is only
10 bits long, as opposed to Ethernet, which has a source and destination address
field, each 48 bits (6 bytes) long. And Frame Relay switches must be configured
to know where to forward frames with particular DLCIs in their headers, instead
of automatically learning addresses and their locations like Ethernet LAN
switches do.
When comparing this Frame Relay example with a serial link
between two routers, the details are different, but the end result is the same.
When R1 wants to forward a packet to R2, regardless of whether you use a leased
line and PPP, or Frame Relay, when R2 gets the frame, it soon discards the data
link header and trailer and leaves them with the packet. The routers are happy
because they can forward packets.
Sections
Comments (0 posted)
Syndication
