The Next Hop Label Forwarding Entry NHLFE

PROBLEMS 147 ATM-LSR A ATM-LSR B ATM-LSR C ATM-LSR D 20 15 15 Figure 6.9 VC merging. ATM cell labeled with the value 15. The cells will be transmitted to C, where they will have their label changed to 20 and then they will be forwarded to the buffer of the output port that connects to D. In this buffer, it is possible that these cells will get interleaved with the cells belonging to an IP packet, call it packet 2, associated with same FEC and transmitted from B. That is, as the cells are queued-up into the buffer, packet 1 cells can find themselves between two successive packet 2 cells. Since all of these cells will be sent to D with the label of 20, D will not be able to identify which of these cells belong to packet 1 or packet 2. Consequently, D will be not be able to reconstruct the original AAL 5 PDUs. A simple solution to this problem is to first collect all of the cells belonging to the same IP packet in the buffer of the output port of C. Once all of the cells have arrived, then they can be transmitted out back-to-back to D. To do this, the switch will need to be able to identify the beginning cell and last cell of an AAL 5 PDU. If the switch is set up with the early-packet and partial-packet discard policies, then the mechanism to do this might be in place. Otherwise, multiple labels can be used, so that the path from A to D is associated with a different set of labels than the path from B to D.

6.3.2 Hybrid ATM Switches

It was mentioned above that in MPLS over ATM, the entire ATM signaling functionality is removed from the ATM switches. Instead, each ATM switch runs the MPLS control plane. That is, the MPLS protocol, a label distribution protocol, and IP routing protocols. However, the existence of MPLS on an ATM switch does not preclude the switch from running the ATM signaling protocol. In fact, both schemes can coexist on the same ATM switch and on the same ATM interface. PROBLEMS 1. Make-up an arbitrary IP header and calculate its checksum. Introduce errors in the bit stream i.e. flip single bits so that the checksum when calculated by the receiver will fail to detect that the IP header has been received in error. 2. Consider the IP address: 152.1.213.156. a What class address is it? b What is the net and host address in binary? 148 THE MULTI-PROTOCOL LABEL SWITCHING MPLS ARCHITECTURE 3. A class B IP network has a subnet mask of 255.255.0.0. a What is the maximum number of subnets that can be defined? b What is the maximum number of hosts that can be defined per subnet? 4. Consider the label allocations in Table 6.3. How would the labels change if we unplugged the link between C and D? 5. The MPLS architecture permits an LSR to maintain multiple entries for each incoming label. Give an example where this feature can be used. 6. Describe the difference between an explicit route and a hop-by-hop route. Under what conditions an explicit route between an ingress LSR and an egress LSR coincides with the hop-by- hop route? 7. In MPLS, label allocation can be done using either the unsolicited downstream scheme or the downstream on demand scheme. In Cisco’s tag switching, upstream tag allocation was also possible. Describe how this scheme works. Hint: it is the opposite of the unsolicited downstream scheme. 8. Consider the example of the LSP shown in Figure 6.8. Let us assume that a second LSP has to be set up from LSRs 1 to 6 over the tunnel connecting LSRs 3 and 4. Show the label stack on each hop and the label operation that has to be performed at each LSR.