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
