Practical Applications

The next useful practical command involves seeing the forwarding tables. Cisco’s example is shown in Figure 1.11.

Figure 1.11: MPLS Forwarding Table Commands

Exercise 1.2: MPLS Data Flow

We find in an MPLS network that data moves from switch to switch using link-specific labels. Switches perform functions based on their switching or cross-connect tables.

These tables contain information such as port in, label in, port out, label out, next router, and instructions. The instructions are simple: 'push' (insert a label), 'swap' (change labels), and 'pop' (remove label).

In this exercise, sample tracing of a packet through an MPLS network, five routers R1-R5 connect networks X and Z. Tables 1.4-1.8 are used to discover the LSPs. Table 1.4 is used for Router 1, Table 1.5 is used for Router 2, Table 1.6 is used for Router 3, Table 1.7 is used for Router 4, and Table 1.8 is used for Router 5. Each table is different and represents the MPLS routers internal switching table.

In Figure 1.12, we have an example of how data would move in this situation.

In Table 1.4, the packet (being HTTP port 80) enters as native IP/80 where a label (20) is pushed and the packet is sent out of port D. Notice that as the packet traverses the network, it exits Router 1 at port D and enters Router 3 at port B.

In Table 1.6, the label (20) is swapped for label 600, and the packet exits the router at port D, where it is hardwired to port B of R5.

In Table 1.8 (R5), the packet label 600 is popped to deliver a native packet to network Z.

Note that Figure 1.11 reflects the correct labels.

In this exercise, use the switching tables for Routers 1 through 5 and Figures 1.12 and 1.13 to map data flow and labeling across the network. Of course, the tables contain data that is not used for your packet, but they also contain switching data needed for other packets. Use only the data that you need to move your packets. Follow these instructions:

1. Always start with Table 1.4 and follow applications that enter through Interface A.

   Table 1.4: Switching Table for Router 1

   P_In	Label In	Label Out	Port Out	Instruction	Next Router
   IP/80	None	20	D	Push	R3
   IP/25	None	95	B	Push	R4
   IP/20	None	500	C	Push	R2

2. The decision made by Table 1.4 will lead you to another switching table, depending on the application, port out, and the router out.

3. In Figure 1.12, note that the packet label numbers appear on the drawings. Use Figures 1.13 and 1.14 to indicate the correct label number.

   
   Figure 1.12: Network Trace for HTTP Port Number 80

4. Use Figure 1.13 and Tables 1.4-1.8 to trace e-mail (port 25) through the network, and note the trace on the drawing.

   
   Figure 1.13: Network Trace for Port 25 E-Mail

   Table 1.5: Switching Table for Router 2

   P_In	Label In	Label Out	Port Out	Instruction	Next Router
   B	499	700	D	Swap	R5
   B	500	65	C	Swap	R3
   B	501	700	A	Swap	R9

   Table 1.6: Switching Table for Router 3

   P_In	Label In	Label Out	Port Out	Instruction	Next Router
   B	20	600	D	Swap	R5
   A	65	650	D	Swap	R5
   B	501	700	A	Swap	R9

5. Using Figure 1.14 and Tables 1.4-1.8 to trace FTP (port 20) through the network, and note the trace on the drawing.

   
   Figure 1.14: Network Trace for Port 20 FTP

   Table 1.7: Switching Table for Router 4

   P_In	Label In	Label Out	Port Out	Instruction	Next Router
   B	95	710	D	Push	R5
   A	500	650	D	Push	R5
   B	515	700	D	Push	R5

   Table 1.8: Switching Table for Router 5

   P_In	Label In	Label Out	Port Out	Instruction	Next Router
   A	500	None	D	Pop	CR
   B	600	None	D	Pop	CR
   B	650	None	D	Pop	CR
   C	710	None	D	Pop	CR