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This lesson, which focuses on understanding the distribution of labels, is third in a series that will help you learn skills required for the MPLS portion of the Cisco CCIP certification. These skills are tested in the CCIP 642-611 -- Implementing Cisco MPLS Exam.
An MPLS router makes forwarding decisions based on the label contained in the packet that it receives. As we discussed in Lesson 2, the control plane is responsible for binding labels to network routes and for advertising those bindings to MPLS peers. Once the label bindings have been assigned and distributed, the MPLS router can make label forwarding decisions. Central to this entire process is the distribution of these labels and assigned routes between the routers.
There are several key concepts related to label distribution that are required knowledge for the CCIP exam. The concepts are related to how MPLS routers update and exchange labels. The different ways that labels are bound to routes must be understood. Be sure to understand the following mechanisms:
- Unsolicited downstream: This occurs when a downstream label switch router (LSR) advertises its label bindings to its neighbours automatically.
- Downstream on demand: This occurs when an upstream LSR requests a label binding from its downstream neighbor.
- Ordered control: This occurs in ATM-LSRs. The upstream LSR must wait on the downstream LSR to receive the label.
- Independent control: This occurs when a new route (or FEC) shows up in the LSR routing table. The LSR will bind a label to the FEC and advertise it to its neighbors at any time.
A tremendous amount of confusion is associated with "upstream" and "downstream" in terms of label distribution. If you think about it, there is a two-way flow of information that makes up routing. When a packet is transmitted, it flows from an upstream sender to a downstream receiver. In other words, the direction from source to destination is downstream.
Independent control is the common mechanism for most label bindings using unsolicited downstream distribution. So if you have four routers in a row as follows...
R1 -------- R2 ----------R3 -----------R4
...and R1 is attached to network 10.10.10..0/24, then R1 is downstream from R2, R3, and R4. Packets destined to 10.10.10.0/24 will flow downstream to R1. Routing updates about network 10.10.10.0/24 flow upstream from R1 to R2 to R3 to R4.
In terms of label binding, as soon as R1 understands that it has 10.10.10./0/24 in its routing table, it will bind a label to it and advertise it upstream to R2. R2 will assign its own local label and advertise to R3, and so on.
NEXT: Label imposition - we will break down label assignments at each LSR to a particular route in order to demonstrate what I just described. We will examine the label bindings at each LSR, then go over the traffic flow from LSR to LSR using label switching.
About the author: Robbie Harrell (CCIE#3873) is the National Practice Lead for Advanced Infrastructure Solutions for SBC Communications. He has over 10 years of experience providing strategic, business, and technical consulting services to clients. Robbie resides in Atlanta, and is a graduate of Clemson University. His background includes positions as a Principal Architect at International Network Services, Lucent, Frontway and Callisma.