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In effect, the Layer 3 switches at the server-farm side become a collapsed backbone for any client-to-client traffic.

The Layer 2 switched backbone is appropriate for a larger campus with three or more buildings to be connected. Adding switches in the

backbone reduces the number of connections and makes it easier to add additional modules. Refer to Figure 7. The backbone is actually a

single Layer 2 switched domain VLAN with a star topology. A single IP subnet is used in the backbone and each distribution switch routes

traffic across the backbone subnet. Because there are no loops, spanning-tree protocol does not put any links in blocking mode, and

spanning-tree protocol convergence will not affect the backbone. To prevent spanning-tree protocol loops, the links into the backbone should

be defined as routed interfaces, not as VLAN trunks.

It is easy to avoid spanning-tree protocol loops with just two Layer 2 switches in the backbone as shown in Figure 7. However, this

restriction limits the ultimate scalability of the Layer 2 backbone design. Another limitation is that all broadcasts and multicasts flood the

backbone. Gigabit EtherChannel can be used to scale bandwidth between backbone switches without introducing a loop.

 

 

An alternative design with higher availability and higher capacity is shown in Figure 8. Here, two Layer 2 switched VLANs form two totally

separate redundant backbones. Note that there is no trunk linking the switch marked VLAN A to the switch marked VLAN B. Each Layer 3

switch in the distribution layer now has two distinct equal-cost paths to every other distribution-layer switch, based on entries in the Layer 3

routing table. If the VLAN A path is disconnected, the Layer 3 switch will immediately route all traffic over VLAN B.

The advantage of the Split Layer 2 backbone design is that two equal-cost paths provide fast convergence. This high-availability

advantage is possible because it is not limited by any protocol mechanisms, such as periodic hello packets. The extra cost of the dual-backbone

design is associated with the extra links from each distribution switch to each backbone switch.

 

The most flexible and scalable campus backbone consists of Layer 3 switches, as shown in figure 9. The backbone switches are connected by

routed Gigabit Ethernet or Gigabit EtherChannel links. Layer 3 switched backbones have several advantages:

• Reduced router peering

• Flexible topology with no spanning-tree loops

• Multicast and broadcast control in the backbone

• Scalability to arbitrarily large size

 

Figure 10 shows the Layer 3 switched campus backbone with dual links to the backbone from each distribution-layer switch. The main

advantage of this design is that each distribution-layer switch maintains two equal-cost paths to every destination network, so recovery from

any link failure is fast. This design also provides double the trunking capacity into the backbone.

 

Figure 11 shows the Layer 3 switched campus backbone on a large scale. The Layer 3 switched backbone has the advantage that arbitrary

topologies are supported because a sophisticated routing protocol such as Enhanced IGRP or OSPF is used pervasively. In Figure 11, the

backbone consists of four Layer 3 switches with Gigabit Ethernet or Gigabit EtherChannel links. All links in the backbone are routed links,

so there are no spanning-tree loops. The diagram suggests the actual scale by showing several gigabit links connected to the backbone

switches. Note that a full mesh of connectivity between backbone switches is possible but not required. Consider traffic patterns when

allocating link bandwidth in the backbone.

Scalable Bandwidth

Upgrading to EtherChannel can provide increased bandwidth and redundancy. An EtherChannel bundle is a group of up to eight Fast or

Gigabit Ethernet links. The bundle functions as a single, logical connection between switches and routers. EtherChannel is convenient because

it scales the bandwidth without adding to the complexity of the design. Spanning-Tree Protocol treats the EtherChannel bundle as a single

link, so no spanning-tree loops are introduced. Routing protocols also treat the EtherChannel bundle as a single, routed interface with a

common IP address, so no additional IP subnets are required, and no additional router peering relationships are created. The load balancing

is handled by the interface hardware.

Use EtherChannel to link backbone switches, to connect the backbone to the distribution layer, and to join the distribution layer to the

wiring closet.