Information Technology Reference
In-Depth Information
Ta b l e 2 - 2
Cisco Enterprise Architecture Model
Hierarchical
Layer
Description
Access
Layer 2 switching
High availability
Port security
Broadcast suppression
QoS
Rate limiting
ARP inspection
VA C L s
Spanning tree
Tr u st clas s ific at ion
PoE and auxiliary VLANs for VoIP
Hierarchical Model Examples
Yo u c a n i m p l e m e n t t h e h i e r a r c h i c a l m o d e l b y u s i n g a t r a d i t i o n a l s w i t c h e d c a m p u s d e s i g n
or routed campus network. Figure 2-2 is an example of a switched hierarchical design in
the enterprise campus. In this design, the core provides high-speed transport between the
distribution layers. The building distribution layer provides redundancy and allows poli-
cies to be applied to the building access layer. Layer 3 links between the core and distribu-
tion switches are recommended to allow the routing protocol to take care of load
balancing and fast route redundancy in the event of a link failure. The distribution layer is
the boundary between the Layer 2 domains and the Layer 3 routed network. Inter-VLAN
communications are routed in the distribution layer. Route summarization is configured
on interfaces toward the core layer. The drawback with this design is that Spanning Tree
Protocol (STP) allows only one of the redundant links between the access switch and the
distribution switch to be active. If the event of a failure, the second link becomes active,
but at no point does load balancing occur.
Figure 2-3 shows examples of a routed hierarchical design. In this design, the Layer 3
boundary is pushed toward the access layer. Layer 3 switching occurs in access, distribu-
tion, and core layers. Route filtering is configured on interfaces toward the access layer.
Route summarization is configured on interfaces toward the core layer. The benefit of this
design is that load balancing occurs from the access layer since the links to the distribu-
tion switches are routed.
Another solution for providing redundancy between the access and distribution switching
is the Virtual Switching System (VSS). VSS solves the STP looping problem by converting
the distribution switching pair into a logical single switch. It removes STP, and negates the
need for Hot Standby Router Protocol (HSRP), Virtual Router Redundancy Protocol
(VRRP), or Gateway Load Balancing Protocol (GLBP).
