Cisco Catalyst 6880-X Specifications download pdf (Page 43)

Distribution Layer April 2014

Traditional Distribution Layer Design

Traditional LAN designs use a multitier approach with Layer 2 from the access layer to the distribution layer,

where the Layer 3 boundary exists. The connectivity from the access layer to the distribution layer can result in

either a loop-free or looped design.

In the traditional network design, the distribution layer has two standalone switches for resiliency. It is

recommended that you restrict a Layer 2 VLAN to a single wiring closet or access uplink pair to reduce or

eliminate topology loops that Spanning Tree Protocol must block and that are a common point of failure in LANs.

Restricting a VLAN to a single switch provides a loop-free design, but it does limit network flexibility.

To create a resilient IP gateway for VLANs in the traditional design, you must use first-hop redundancy protocols,

which provide hosts with a consistent MAC address and gateway IP for a VLAN. Hot Standby Routing Protocol

(HSRP) and Virtual Router Redundancy Protocol (VRRP) are the most common gateway redundancy protocols,

but they only allow hosts to send data out one of the access uplinks to the distribution layer, and require

additional configuration for each aggregation switch to allow you to distribute VLANs across uplinks. Gateway

Load Balancing Protocol (GLBP) does provide greater uplink utilization for traffic exiting the access layer by

balancing load from hosts across multiple uplinks, but you can only use it in a non-looped topology.

All of these redundancy protocols require that you fine-tune the default timer settings to allow for subsecond

network convergence, which can impact switch CPU resources.

Some organizations require the same Layer 2 VLAN be extended to multiple access layer closets to

accommodate an application or service. The looped design causes spanning tree to block links, which reduces

the bandwidth from the rest of the network and can cause slower network convergence.

Figure 15 - Traditional loop-free design with a VLAN per access switch

2103

Layer-3 Link

VLAN 30

VLAN 40

VLAN 30

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Cisco Catalyst 6880-X Specifications Page 43