Page 198

Understanding the Output from Diagnostic Commands

161

Let's back up a little here. You just read that the router receives a frame.

Right now we are going to be very specific with our nomenclature. As
you learned in Chapter 2, a frame is a Layer 2 PDU and a packet is a Layer
3 PDU. The router receives a frame because the Layer 3 PDU from the adja-
cent router had to be encapsulated in order to be sent.

Start with Step 1, pictured in Figure 4.2. Step 1 consists of Layer 2 receiv-

ing an incoming frame. Because these are routers, the packet (Layer 3 PDU)
must be extracted from the frame, the frame discarded, and the packet
passed up to Layer 3.

Layer 3 then performs Step 2. It looks at the packet and says, "Is this for me?"

The router then identifies the destination address within the Layer 3 header. If the
packet is destined for the router, Layer 3 strips off the Layer 3 header and passes
the PDU up to the higher layers. If the packet is not destined for the router, it
follows the remaining steps outlined in Figure 4.2.

Assuming that the packet is not intended for the router, it is sent back down

to Layer 2 to be encapsulated (Step 3). At this point, the router has done a route
table look-up and knows which interface it must send the packet to, but it needs
to know the Layer 2 address (MAC address if Ethernet, FDDI, or Token Ring--
LAN Media) of the adjacent interface. In the case of Ethernet, if the MAC
address does not exist in the ARP table, the router will issue an ARP request for
the MAC address for the interface on Router B. Once the MAC address is
known, the frame header can be created. With the frame header created, the
packet is encapsulated and sent to the next hop (Step 4).

The process is repeated at Router B. The router receives a frame, and then gets

the packet and asks, "Is this for me?" Depending on the answer, the packet is for-
warded to the upper layers or to an exiting interface. This same procedure is
followed along every hop en route to the destination router. Remember that the
logical address specified in the packet does not change--only the MAC address
changes. The logical address allows the packet to be routed to its destination,
whereas the MAC address allows the frame to be forwarded to each next-hop. It
seems monotonous, but it is necessary.

We have now discussed generalities concerning packet flow through a

router. You may notice that no reference was made about which paths were
taken inside the router. The switching path taken by a PDU depends
implicitly on the type of router on which it is implemented. Because Cisco
makes such a wide variety of routers, we will look at each type individually.
By covering the different architectures, you can then better understand the
switching types that are described later in this section.

www.sybex.com