SAN: storage area network, server connects to storage via FC or iSCSI.

NAS: network adapted storage, server connect to storage over TCP/IP by using NFS or iSCSI.

DAS: Direct attached storage


Access layer design

refer to

1, Looped triangle

The triangle looped topology is currently the most widely implemented in the enterprise data center. This topology provides a deterministic design that makes it easy to troubleshoot while providing a high level of flexibility

2, looped square

The square-based looped topology is not as common today in the enterprise data center but has recently gained more interest. The square looped topology increases the access layer switch density when compared to a triangle loop topology while retaining the same loop topology characteristics. This becomes particularly important when 10GE uplinks are used. This topology is very similar to the triangle loop topology, with differences in where spanning tree blocking occurs

Spanning tree blocks the link between the access layer switches, with the lowest cost path to root being via the uplinks to the aggregation switches, as shown in Figure 6-9. This allows both uplinks to be active to the aggregation layer switches while providing a backup path in the event of an uplink failure. The backup path can also be a lower bandwidth path because it is used only in a backup situation. This might also permit configurations such as 10GE uplinks with GEC backup.

The possible disadvantages of the square loop design relate to inter-switch link use, because 50 percent of access layer traffic might cross the inter-switch link to reach the default gateway/active service module. There can also be degradation in performance in the event of an uplink failure because, in this case, the oversubscription ratio doubles.

3, Loop free U

4, Loop free invented U


STP Logical interfaces limitation

For Cisco 6500 series switches:

1,  HSRP should be limited to 500 per each aggregation switch

2, RSTP has logic interface limitation as 10000 while MTP has limitation as 50000. number of Logical interfaces = number of vlans * number of trunk port (etherchannel ports count individually) + no trunk port interfaces; Verify with “show spanningtree summary total”

The maximum logical interfaces for Per VLAN Spanning Tree Plus (PVST+) is 1800 for each module and 13,000 total for the switch. The show spanning-tree summary totals command displays the number of logical interfaces in the STP Active column.

The only way around this is to run Multiple Spanning Tree (MST) versus PVST, which has different limits:

  • PVST+ 13,000 total 1,800*/slot
  • RPVST+ 10,000 total 1,800*/slot
  • MST 50,000 total 6,000*/slot

Otherwise, pruning unnecessary VLANs from trunks is the best way to reduce the number of logical interfaces on a module or switch. But, regardless of STP mode, 10 Mbps, 10/100 Mbps, and 100 Mbps switching modules support a maximum of 1,200 logical interfaces per module.

For Nexus 7000

  • PVST+ RSTP 13,000 total, No per I/O module limit
  • MST 75000 total; No per I/O module limit



Catalyst vs Nexus

1, Catalyst supports VSS( Virtual Switch System )for combining 2 switches into one logic switch, just like virtual chassis in Juniper EX. While Nexus support VPC (Virtual port-channel) to combine ports from different switches into the same port channel.  However 2 Nexus switches with VPC configured still run independently in control plane level, therefore L3 redundancy need to be realized by enabling hsrp or vrrp. Virtual chassis in Juniper will select one switch running as control chassis and the rest of the chassis running as line card in active/passive mode. VSS is very much likely as Virtual chassis in Juniper.

2, Nexus support VDC(virtual switching context ) to separate one switch into several switch logically.  VDC will actually run separated control plane for each switching context, that means each VDC has its own L2/L3 instances (vrf, hsrp, lacp, etc)

3, Catalyst support wide range of WAN interfaces and extra FW/VPN modules.

4, FEX support: Nexus 7000 supports the use of the Nexus 2200 Series fabric extenders to additionally expand the system and provide a large-scale virtual chassis in the data center. Up to 32 of the fabric extenders can be supported by the Nexus.

5, Difference between Nexus 7000 and Nexus 9000 is that Nexus support ACI (Application Centric Infrastructure) That will facilitate SDN deployment. There are some other differences too which need to be further researched.

Some concept used in SDN


The Network Configuration Protocol (NETCONF) defined in this document provides mechanisms to install, manipulate, and delete the configuration of network devices. It uses an Extensible Markup Language (XML)-based data encoding for the configuration data as well as the protocol messages. The NETCONF protocol operations are realized as remote procedure calls (RPCs).


YANG is a data modeling language used to model configuration and state data manipulated by the Network Configuration Protocol (NETCONF), NETCONF remote procedure calls, and NETCONF notifications.


describes a REST-like protocol that provides a programmatic interface over HTTP for accessing data defined in YANG, using the datastores defined in NETCONF.


Copied from internet

“BGP-LS is an extension to Border Gateway Protocol (BGP) for distributing the network’s link-state (LS) topology model to external entities, such as the SDN controller. It has received a lot of attention because many SDN apps need this model.

The network’s link-state topology model consists of nodes (typically, but not limited to, routers) and links that connect these routers together. For each link, a set of attributes is also contained. These may include interface addresses, various metrics, and each link’s total and available bandwidth. This topology model is distributed among routers using one of the two prominent Interior Gateway Protocols (IGPs): ISIS (Intermediate System to Intermediate System) and OSPF (Open Shortest Path First) protocols.

The detailed link-state models of these two protocols are not identical. As a result, BGP-LS defines its own more abstract topology model and defines how to map these IGP models to its own model. As the network topology is discovered by the IGPs, the changes are reflected in the BGP-LS model as well and are also distributed using BGP-LS messages to any interested party, such as SDN controllers and apps. Note that the network devices themselves are not interested in learning the network topology this way, as they already participate in IGP and learn it firsthand.”


The Cisco Application Policy Infrastructure Controller (Cisco APIC) is the unifying point of automation and management for the Application Centric Infrastructure (ACI) fabric.

It use policy agent to translate polices into instructions.


The EEM(Embedded Event manager is a software component of cisco IOS, XR, and NX-OS makes life easier for administrators by tracking and classifying events that take place on a router and providing notification options for those events. EEM allows you to automate tasks, perform minor enhancements and create workarounds.
There are two independent pieces: Applets and Scripting
-> Applets are a collection of CLI commands
-> Scripts are actions coded up in TCL(interpreter language)

EEM uses event detectors and actions to provide notifications of those events:

EEM detectors can be:
1) SNMP:-Monitoring SNMP objects.
2) Syslog:-Responds to various syslog messages, allowing for matching on regular expressions.
3) Counter: Monitoring and responding to interface counter when cross threshold settings.
4) CLI events: Screening CLI input for a regular expression match.
5) None: This event detector is use to test EEM script/applet using “event manager run” command.
6) Timers :(Countdown, watchdog and CRON)
7) IP SLA and Netflows events.