Specifications

Copyright © 2009, Juniper Networks, Inc. 17
DESIGN GUIDE - Data Center LAN Connectivity Design Guide
Device-level HA 1.
Most device failures are due to power supply failures or mechanical cooling problems. It is important to always
support business processes with high-performance, carrier-class network switching devices such as the
Juniper Networks EX Series Ethernet Switches or MX Series Ethernet Services Routers. Purchasing equipment
with internal dual load-sharing power supplies and redundant fans or blowers to minimize equipment failure
is always recommended and raises the mean time to repair (MTTR). Additional device-level HA can be provided
by doubling up on key devices to assure that there is a backup device to pick up in the event of a failed device. If
budget doesn’t support a full set of backup devices, purchasing extra key device components such as a backup
set of field-serviceable or hot-swappable power supplies or fan trays, helps mitigate the impact of a component
failure.
Link-level HA2.
Ensuring that the data center maintains the data flow vital to business processes through internal and external
resources is achieved through link-level HA. At the data center, link-level HA requires that two links operate in
an active/backup configuration, such that if one link fails, the other can take over or reinstate the forwarding
of traffic that had been previously forwarded over the failed link. Other technologies such as Link Aggregation
(LAG) can be utilized to bond multiple uplinks and load balance across them.
Network Software HA3.
Juniper Networks JUNOS
®
Software is the consistent operating system software that powers all of Juniper
Networks’ switch, router and high-end firewall products. It provides carrier-class network software to highly
available data centers of all sizes. JUNOS Software supports features like nonstop forwarding (NSF), graceful
protocol restart, in-service software upgrade (ISSU), Bidirectional Forwarding Detection (BFD) and other features
which together make IP networking as failure-safe and reliable as traditional PSTN telephony networks. The
JUNOS Software modularity and uniform implementation of all features enables the smallest data center to
benefit from the same hardened services in their JUNOS Software-based devices as the largest service providers.
VLAN and Spanning Tree Protocol (STP)
Data centers typically use VLANs to group any set of servers or storage devices into logical networks through
software configuration instead of physically relocating devices on the LAN. VLANs help address issues such as
scalability, security and network management, as was introduced in the three-tier application model.
VLANs are Layer 2 broadcast domains that exist only within a defined set of switches. Using the IEEE 802.1Q
standard as an encapsulation protocol, packets are marked with a unique VLAN tag. Tagged packets are then
forwarded and flooded only to stations in the same VLAN. Tagged packets must be forwarded through a routing
device to reach any station not belonging to the same VLAN. Any switch or switch port can be dynamically or
statically grouped into a VLAN. Alternately, traffic may be grouped into a VLAN and forwarded through specific ports
based on the specific data protocol being sent over the LAN. For example, VoIP traffic from a soft phone can be
segmented from other traffic and put into a VLAN that receives a higher QoS.
Spanning Tree Protocol (STP)
VLANs may create multiple active paths between network nodes, resulting in problematic Layer 2 bridge loops. The
loops will cause the same MAC addresses to be seen on multiple ports causing the switch forwarding function to fail.
Also, the loop may cause broadcast packets to be forwarded endlessly between switches, consuming all available
network bandwidth and switch CPU resources.
The IEEE 802.1D STP standard, ensures a loop-free topology for any Layer 2 bridged LAN. STP is designed to leave a
single active path between any two network nodes by first creating a tree within a mesh network of connected LAN
switches and then disabling the links which are not part of that tree. STP thus allows a network design to include
redundant links to provide automatic backup paths if an active link fails, without the danger of bridge loops, or the
need for manual enabling/disabling of these backup links. Each VLAN must run a separate instance of Spanning
Tree Protocol.
Issues with STP
Troubleshooting may be challenging with STP due to complicated routing, incorrect configuration, or mis-cabling.
Since every packet must go through the root bridge of the spanning tree, routing performance with STP can also
be non-optimal. STP often creates underutilized links and lacks a load-balancing mechanism as well. In addition,
STP has a slow convergence of up to 30 to 40 seconds after a topology change. The Rapid Spanning Tree Protocol