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Sunday, May 13, 2012

UCEN 3133 Advanced Computer Networks - Tutorial 2 - Year 2012

Tutorial 2 – MPLS Overview

1. Sketch the diagram of an Edge LSR (Label Switching Router). Identify 3 components that are important with respect to label switching at the periphery of an autonomous MPLS network, and briefly explain how they function.

     (i)            The Forwarding Information Base (FIB) is a standard IP forwarding table extended with labelling information. It allows labelled packets from within the MPLS core network to routed forwards as pure IP packets
   (ii)            Label Information Base (LIB) - holds all labels assigned by this LSR and the  mappings of these labels to labels received from any neighboring LSRs, during the process of label binding exchange.
 (iii)            Label Forwarding Information Base (LFIB) is used during the actual forwarding of labelled packets and holds only labels that are in use currently

2. Explain how labels can be useful as a preliminary measure  towards the enforcement of some form of QOS guarantee in a MPLS, making reference in your answer to Forward Equivalence Class (FEC)

Forwarding Equivalence Class (FECs) is a way of grouping incoming packets in some way so that all packets identified with a particular FEC are treated in the same manner, for example being switched over the same path (LSP). This allows some form of QOS to be enforced on the LSRs of the label switched path (LSP) that the packets flow through. Labels are associated with each particular FEC, and the FEC is established once at the entry of the IP packet into an ingress LSR.

3. Give examples of 3 items that can be used in classifying a Forward Equivalence Class (FEC)

     (i)            A specific source or destination IP network address
   (ii)            A specific class of traffic (e.g. interactive voice, streaming media) data within the packet
 (iii)            The particular interface on a router which the packet arrives on

4. Explain what is meant by label disposition or imposition with respect to an edge LSR.

Label imposition is the act of appending a label, or a stack of labels, to a packet in the ingress point of the MPLS domain. Label disposition is the act of removing the last label from a packet at the egress point before it is forwarded to a neighbor that is outside the MPLS domain.

5. Consider that an egress LSR in autonomous MPLS (AS 1) is communicating with its peer in another autonomous MPLS network (AS 2) using a suitable protocol (BGP for example). The process of negotiation between these peers results in a decision that exiting IP packets with a specific subnet prefix from the egress LSR in AS1 is to be routed along a specific Label Switched Path (LSP) in AS 2. Sketch an outline of how this might be accomplished using labels. What is the advantage of this approach ?

The ingress edge LSR at AS1 negotiates with the egress LSR of AS1, to append a label to all incoming packets into AS1. This label specifies that packets with a specific subnet prefix are meant to be switched immediately onwards to AS2 once they exit  from AS1. The AS1 ingress LSR builds a label stack by pushing another label corresponding to an LSP that terminates at the IP address of the AS1 egress LSR. This packet with two labels is forwarded along a LSP in AS1. All the other LSRs in the LSP in AS1 only perform switching on the top most label in the stack. Finally, at the egress LSR of AS1 this label is popped leaving the bottom label – which is used to forward the packet on to the ingress router of AS2

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