Introduction
The foundation of a modern Datacenter fabric is an Underlay Network and it is crucial
to understand the operation of the Control-Plane protocol solution used in it. The
focus of this chapter is OSPF. The first section starts by introducing the
network topology and AS numbering scheme used throughout this book. The second
section explains how OSPF speakers connected to the same segment become fully
adjacent. The third section discusses the process of how OSPF speakers exchange
Link State information and build a Link-State Database (LSDB) which is used as
an information source for calculating Shortest Path Tree (SPT) towards each
destination using Dijkstra algorithm. The focus of the fourth section is an OSPF
LSA flooding process. It strat by explaining how local OSPF speaker sends Link
State Advertisements wrapped inside a Link-State Update message to its adjacent
router and how receiving OSPF speakers a) installs information into LSDB, b)
Acknowledge the packet, and c) floods it out of OSPF interfaces. The fifth
section discusses of LSA and SPF timers. At the end of this chapter, there are OSPF
related configurations from every device.
Infrastructure AS Numbering and IP
Addressing Scheme
Figure
1-1 illustrates an AS numbering and an IP address scheme used throughout this
book. All Leaf switches have dedicated BGP Private AS number while spine switches
in the same cluster share the same AS number. Inter-Switch links use Unnumbered
IP addressing using (interface Loopback 0) which is also used as OSPF Router-Id.
Loopback 0 is not advertised by any device. OSPF type for Inter-Switch link is
point-to-point so there is no DR/BDR election process. Leaf switches also have
interface Loopback 30 that is used as a VTEP (VXLAN Tunnel End Point) address. Loopback
30 IP addresses are advertised by Leaf switches. All Loopback interfaces are in
OSPF passive interface mode. At this stage, all switches belong to OSPF Area
0.0.0.0.
Figure 1-1: AS Numbering and IP Addressing Scheme.
