Mpls intro from cicso

Here you will find the configuration of each device. Let me explain how that works. Thus, a frame received on Serial 1 with label 5 is to be forwarded out Serial 0 with no label.

When LDP information is received, it records use of label 5 on outgoing interface Serial 0 for the two prefixes we're tracking. Why do we need MPLS?

For MPLS, the labels are the mysterious part. Using LDP, the router selects a free unused label, 5, and advertises it to the upstream neighbor.

By looking up the inbound interface and label in the LIB, the outbound interface and label are determined. This might be all packets bound for the same egress LSR. Now you might be thinking…so what?

I'm just trying to show how routing and labels get propagated, and how the hop-by-hop behavior of LDP can still result in a Label Switch Path being established.

This does create some interesting buffering and store-and-forward issues. The light blue arrow suggests that an ordinary routing update you pick the protocol advertises the routes to the Edge LSR above.

Hold this thought while you read the next section of this lesson where we finally start talking about MPLS! The label table in a LSR may cause the outermost label to be removed.

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Introduction to MPLS