Specifications

Distribution Layer April 2014
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Procedure 7 Connect to access layer
The resilient, single, logical, distribution layer switch design is based on a hub-and-spoke or star design. The
links to access layer switches and connected routers are Layer 2 EtherChannels. Links to other distribution
layers, and the optional core are Layer 3 links or Layer 3 EtherChannels.
When using EtherChannel, the member interfaces should be on different switches in the stack or different
modules in the modular switch for the highest resiliency.
The physical interfaces that are members of a Layer 2 EtherChannel are configured prior to configuring the
logical port-channel interface. This allows for minimal configuration because most of the commands entered to a
port-channel interface are copied to its members’ interfaces and do not require manual replication.
Configure two or more physical interfaces to be members of the EtherChannel. It is recommended that they are
added in multiples of two.
If this distribution layer will be used as a network-services aggregation block, you likely will not have an access
layer to connect.
Step 1: Configure VLANs.
Configure all VLANs for the access layer switches that you are connecting to the distribution switch.
vlan [data vlan]
name Data
exit
vlan [voice vlan]
name Voice
exit
vlan [management vlan]
name Management
exit
Step 2: If there is no external central site DHCP server in the network, you can provide DHCP service in IOS by
configuring the IOS DHCP server. This function can also be useful at a remote-site where you want to provide
local DHCP service and not depend on the WAN link to an external central site DHCP server.
ip dhcp excluded-address 10.4.100.1 10.4.100.10
ip dhcp pool access
network 10.4.100.0 255.255.255.0
default-router 10.4.100.1
domain-name cisco.local
dns-server 10.4.48.10
The example configuration provides IP addresses via the IOS based DHCP service for the subnet 10.4.100.0/24
and prevents the server from assigning reserved addresses .1-.10.
Step 3: Configure EtherChannel member interfaces.
This design uses Layer 2 EtherChannels to connect all access layer switches to the distribution layer and thereby
create the hub-and-spoke resilient design that eliminates spanning-tree loops. Add links in multiples of two and
distribute as much as possible across physical components of the platform. A configuration is shown using four
member links for additional resiliency.