Explicit Routing THE MULTI-PROTOCOL LABEL SWITCHING MPLS ARCHITECTURE

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. 7 Label Distribution Protocols MPLS requires a set of procedures for the reliable distribution of label bindings between LSRs. MPLS does not require the use of a single label distribution protocol. In view of this, various schemes have been proposed for the distribution of labels, of which the label distribution protocol LDP and the resource reservation protocol – traffic engineering RSVP–TE are the most popular. LDP is a new signaling protocol. It is used to distribute label bindings for an LSP associated with a FEC. It has been extended to the constraint-based routing label distri- bution protocol CR-LDP , which is used to set up an explicit route i.e. an LSP between two LSRs. LDP and CR-LDP are described below in Sections 7.1 and 7.2. An alternative method to distributing label bindings is to extend an existing IP control protocol, such as BGP, PIM, and RSVP, so that it can carry label bindings. The extended version of RSVP is known as RSVP-TE, and is the most popular protocol of the above three for distributing label bindings. RSVP-TE has functionality for setting up LSPs using the next hop information in the routing table of an LSR and explicitly routed LSPs. RSVP and RSVP-TE are described in Sections 7.3 and 7.4. Typically, an LSR will run both LDP and RSVP-TE. The two label distribution proto- cols are not compatible, however. In order to establish an LSP, either LDP or RSVP-TE has to be used.

7.1 THE LABEL DISTRIBUTION PROTOCOL LDP

LDP is used to establish and maintain label bindings for an LSP associated with a FEC. Two LSRs that use LDP to exchange label bindings are known as LDP peers. LDP provides several LDP messages, which are classified as follows: • Discovery messages : These messages are used to announce and maintain the presence of an LSR in the network. • Session messages : In order for two LDP peers to exchange information, they have to first establish an LDP session. The session messages are used to establish, maintain, and terminate LDP sessions between LDP peers. • Advertisement messages : These messages are used to create, change, and delete label bindings to FECs. • Notification messages : These messages are used to provide advisory information and to signal error information. Connection-oriented Networks Harry Perros  2005 John Wiley Sons, Ltd ISBN: 0-470-02163-2