SLB

1. Load-balancing
2. Network load balancing (NLB)
3. SLB L2
4. SLB L3

Load-balancing

• DNS
• anycast
• software (e.g., Apache, nginx)
• NLB, SLB
• F5

Network load balancing (NLB)

• Windows Server
• requires support from OS and network
• packet hash determines the server to process it
  • hash(client IP), NIC-level
  • if cluster changes – recalculate hash-tables

SLB L2

• SLB does not perform NAT, just forwards the packets
• IP addresses are preserved, MAC destination – server
  • servers must be in the same L2 segment
  • IP-MAC table is filled using ARP

(config)# ip slb serverfarm <FARM>

; server IP address
(config-slb-sfarm)# real <IP>

; 3 default, count of SYN without response, if exceeded – switch to other server
(config-slb-real)# reassign <THRESHOLD>

; defaults: 8 conn, 2 clients, if both are exceeded – server failed 
(config-slb-real)# faildetect numconn <N> numclients <M>

(config-slb-real)# inservice

; service frontend
(config)# ip slb vserver <SERV>

; client is mapped to server for all sessions during timewindow, GID groups servers
(config-slb-vserver)# sticky <sec> [group <GID>]

(config-slb-vserver)# serverfarm <FARM>
(config-slb-vserver)# virtual <IP> <PROTOCOL> <PORT>
(config-slb-vserver)# inservice

# show ip slb reals
# show ip slb conn

SLB L3

• announces service IP
  • client connects to service by IP
  • SLB selects server and performs destination NAT
  • server reply triggers source NAT on SLB
• load-balancing
  • round robin
  • least connections
  • weighted
• SLB can check server liveness if there are no clients
• traffic must be symmetric: server reply analysis ≡ server keepalive

; SLB L3
(config-slb-sfarm)# nat server
(config-slb-sfarm)# real <IP> <PORT>