Layer 2 adjacency refers to the ability of devices to communicate directly at the data link layer (Layer 2) without requiring routing (Layer 3) intervention. In data center networks, this means servers can exchange traffic while remaining in the same broadcast domain, typically via VLANs or VXLANs.

Traditional looped designs (like STP-based topologies) maintain Layer 2 adjacency across multiple switches, while modern loop-free designs (such as spine-leaf) limit adjacency to directly connected switch pairs. Consider this network simulation:

# Python network simulation snippet
class Switch:
    def __init__(self, name):
        self.name = name
        self.adjacent_switches = []
    
    def add_adjacent(self, switch):
        self.adjacent_switches.append(switch)

# Looped topology (spanning multiple switches)
core_switch = Switch("Core")
agg_switch1 = Switch("Agg1")
agg_switch2 = Switch("Agg2")
access_switch = Switch("Access1")

core_switch.add_adjacent(agg_switch1)
core_switch.add_adjacent(agg_switch2)
agg_switch1.add_adjacent(access_switch)
agg_switch2.add_adjacent(access_switch)  # Creates loop

When servers requiring Layer 2 adjacency (like VMware clusters or legacy applications) are placed in loop-free topologies, their communication gets restricted to single switch pairs. This becomes evident in VXLAN implementations:

// Sample VXLAN configuration limiting L2 adjacency
interface nve1
  source-interface loopback0
  member vni 10010
    ingress-replication protocol bgp
    suppress-arp  // Limits L2 adjacency to local switch

The apparent paradox in your observation stems from the difference between:

• Looped topologies: Enable VLAN stretching across multiple switches but introduce potential broadcast storms
• Loop-free designs: Contain failure domains but require careful VLAN placement planning

Contemporary data centers often use overlay technologies to simulate Layer 2 adjacency while maintaining loop-free underlays. Here's a common EVPN configuration:

router bgp 65001
  neighbor 192.0.2.1 remote-as 65001
  address-family l2vpn evpn
    neighbor 192.0.2.1 activate
    exit-address-family
!
vrf definition PROD
  rd 65001:100
  route-target export 65001:100
  route-target import 65001:100
  address-family ipv4
    exit-address-family

Layer 2 adjacency refers to the direct connectivity between network devices within the same broadcast domain (VLAN) without requiring Layer 3 routing. In data center designs, this becomes crucial for:

• Server clustering requiring low-latency communication
• VM mobility across physical hosts
• Applications needing bare metal performance

The Cisco design guide mentions two approaches with different adjacency characteristics:

// Example of logical topology representations
# Looped Design (STP-based)
Switch1 -- Switch2 -- Switch3
  |_____________|

# Loop-Free Design (FabricPath/VXLAN)
Leaf1 -- Spine -- Leaf2
Leaf3 -- Spine -- Leaf4

Consider these implementation patterns for VMware environments:

// Traditional STP-based VLAN stretching
interface Vlan100
 description VMotion Network
 spanning-tree portfast trunk

// VXLAN-based overlay (modern approach)
interface nve1
 source-interface loopback0
 member vni 10010
  ingress-replication protocol bgp

While L2 adjacency seems desirable, it introduces:

• Broadcast storm risks requiring STP
• VLAN sprawl management overhead
• Limited failure domains

Newer data center designs often implement:

# EVPN Configuration Snippet
router bgp 65001
 address-family l2vpn evpn
  neighbor EVPN_PEER activate
  advertise-all-vni

The key is balancing adjacency requirements with operational simplicity - most modern designs prefer limited L2 domains with overlay networks providing logical adjacency.