Design Guide

17
LAN Baseline Architecture Overview—Branch Office Network
OL-11333-01
Multilayered Branch Architecture
• Layer 3 failure can take up to three seconds to converge under failure conditions. If the applications
used need significantly less convergence times, then other options (option 1) have to be considered.
In addition to high availability, it is possible to configure the switches to load balance the traffic on the
EtherChannels. Contrary to an L3 design where per packet load balancing can be achieved on equal cost
multiple paths, load sharing is done based on source and destination MAC addresses. This implies that
for a specific host, the traffic always uses the same link of the EtherChannel.
When using the Cisco EtherSwitch Services Module that fits into a slot of the Cisco ISR, the topology
looks similar to Option 1 in
Figure 10 if a single EtherSwitch Services Module is used. The main reason
that the topology looks similar is because the ISR interfaces with the EtherSwitch Services Module
through a gigabit Ethernet interface. If high availability is desired for the distribution layer, the following
two options can be used:
• Stack the services module with an external switch (topology similar to Option 2 in Figure 10).
• Use a dual services module with a trunk in between for high availability.
These two topologies are shown in Figure 11.
Figure 11 Using EtherSwitch Services Module(s) at the Distribution Layer
Scalability
In terms of the number of LAN/switch ports supported, scalability is achieved by putting the distribution
switch. The distribution switch has the required port density to fan out more access switches without
compromising high availability. Without the distribution switch, scalability is limited to the number of
180064
Option 1 Option 2
ISR at the edge with
services module(s)
Access
Distribution
29xx or 35xx
29xx or 35xx
Access Switches
Edge
WAN
AP
Access Point Access Point
Access Switches
WAN
Access Point Access Point
Stackable Etherswitch
Services Module
Cross Stack
Ether-channel
Cross Stack
connection