Cisco Catalyst 6880-X Specifications download pdf (Page 45)

Distribution Layer April 2014

Figure 18 - Simplified design with VLANs spanning access switches

2106

VLAN 30

EtherChannel is a logical interface that can use a control plane protocol to manage the physical members of

the bundle. It is better to run a channel protocol instead of using forced-on mode because a channel protocol

performs consistency checks for interfaces programmed to be in the channel and provides protection to the

system from inconsistent configurations.

Cisco Catalyst switches provide both Port Aggregation Protocol (PAgP), which is a widely deployed Cisco

designed protocol, and Link Aggregation Protocol (LACP) based on IEEE 802.3ad. This design uses LACP for

EtherChannel because it is the only protocol supported in a Cisco Catalyst 3750 cross-stack configuration and

can be used in all configurations in this design.

There are several other advantages to the simplified distribution layer design. You no longer need IP gateway

redundancy protocols like HSRP, VRRP, and GLBP because the default IP gateway is now on a single logical

interface and resiliency is provided by the distribution layer switch or switches. Also, the network will converge

faster now that it is not depending on spanning tree to unblock links when a failure occurs because EtherChannel

provides fast subsecond failover between links in an uplink bundle.

The topology of the network from the distribution layer to the access layer is logically a hub-and-spoke topology,

which reduces complexity of design and troubleshooting. The hub-and-spoke topology design provides a more

efficient operation for IP Multicast in the distribution layer because there is now a single logical designated router

to forward IP Multicast packets to a given VLAN in the access layer.

Finally, by using the single logical distribution layer design, there are fewer boxes to manage, which reduces the

amount of time spent on ongoing provisioning and maintenance.

Distribution Layer Roles

Much emphasis has been placed on the distribution layer as the access layer aggregation point because this is

the most common role. The distribution layer serves other roles in LAN designs.

In many smaller locations, the WAN head end and Internet edge terminate at the headquarters location, along

with a server farm or small data center and the LAN access for user connectivity. In these situations a single

distribution layer or collapsed core design may be appropriate to allow the network to stay within budget limits

while serving a smaller LAN access environment. Although the port density and configuration complexity may

not be an issue, operational complexity of supporting many functions on one device must be monitored as the

organization grows.

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Cisco Catalyst 6880-X Specifications Page 45