Consider the Multi-Protocol Label Switching (MPLS) network shown in Figure 3, and suppose that routers R5 and R6 are now MPLS enabled. Suppose that we want to perform traffic engineering so that packets from R6 destined for A are switched to A via R6-R4-R3-R1, and packets from R5 destined for A are switched via R5-R4-R2-R1. Show the MPLS tables in R5 and R6, as well as the modified table in R4, that would make this possible. (Clearly indicate the name of the router and clearly draw the routing table where required to show additions or changes to routing tables. You do not need to reproduce the network)
Figure required: Figure 3 (MPLS Network): routers R1–R6 and destinations A and D. Existing tables — R4: in –, out 10, dest A, iface 0; in –, out 12, dest D, iface 0; in –, out 8, dest A, iface 1. R3: in 10 → out 6, dest A, iface 1; in 12 → out 9, dest D, iface 0. R2: in 8 → out 6, dest A, iface 0. R1: in 6 → out –, dest A, iface 0. R5 and R6 attach to R4; their interface numbers are not labelled.
Try an answer before revealing the guidance below.
Key Concepts
- Label push at ingress
- Label swap
- Incoming label identifies the path
- Distinct labels per path
- Out interface
Answer Approach
- Note which downstream labels R3 and R2 already expect for destination A (10 and 8).
- Make R6 and R5 push different labels so R4 can tell the two flows apart.
- Add R4 entries that map each incoming label to the correct outgoing label and interface.
- Write each table with in label, out label, dest, out interface.
Full Answer
Answer status: Draft answer (unofficial). Revision notes, not an official marking scheme.
Using the Figure 3 labels (R3 expects label 10 for A on R4’s interface 0; R2 expects label 8 for A on R4’s interface 1). R6 table: in label – | out label 10 | dest A | out interface 0 (its only link, to R4). R5 table: in label – | out label 8 | dest A | out interface 0 (its only link, to R4). Modified R4 table: in label 10 | out label 10 | dest A | out interface 0 (to R3); in label 8 | out label 8 | dest A | out interface 1 (to R2); keep the existing entry for D (out 12, interface 0). Any two distinct incoming labels at R4 would work; what matters is that R4 can distinguish R6’s packets from R5’s. (R5’s and R6’s interfaces are not numbered in the figure; each has a single link to R4, assumed to be interface 0.)
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