170 Chapter 5: Designing a Network Topology
Gateway Load Balancing Protocol
To achieve load sharing along with redundancy, Cisco also has a newer protocol called the
Gateway Load Balancing Protocol (GLBP). GLBP is similar, but not identical, to HSRP
and VRRP. With HSRP and VRRP, the standby routers in a group are superfluous until the
active router fails. These standby routers may have access to bandwidth that is wasted until
a problem arises. Although multiple virtual router groups can be configured for the same
set of routers, which is less wasteful, the hosts must be configured for different default
gateways, which results in an extra administrative burden. GLBP provides load balancing
over multiple routers using a single virtual IP address and multiple virtual MAC addresses.
Each host is configured with the same virtual IP address, and all routers in the virtual router
group participate in forwarding packets.
Members of a GLBP group elect one router to be the active virtual gateway (AVG) for that
group. Other group members provide backup for the AVG in the event that the AVG becomes
unavailable. The AVG assigns a virtual MAC address to each member of the GLBP group.
Each gateway assumes responsibility for forwarding packets sent to the virtual MAC
address assigned to it by the AVG. These gateways are known as active virtual forwarders
(AVFs) for their virtual MAC address. The AVG is responsible for answering ARP requests
for the virtual IP address. Load sharing is achieved by the AVG replying to the ARP requests
with different virtual MAC addresses.
Designing the Enterprise Edge Topology
Depending on a customer’s goals for availability, performance, and affordability, enterprise
edge network design should feature redundant WAN segments in the intranet, and multiple
paths to extranets and the Internet. VPNs can also be used to connect private enterprise sites
via a service provider’s public WAN or the Internet. This section covers enterprise edge
topologies that include redundant WAN segments, multihomed connections to the Internet,
and VPNs. The section also includes a few comments about the service provider edge.
Redundant WAN Segments
Because WAN links can be critical pieces of an enterprise internetwork, redundant (backup)
WAN links are often included in an enterprise edge network topology. A WAN network can
be designed as a full mesh or a partial mesh. A full-mesh topology provides complete
redundancy. It also provides good performance because there is just a single-link delay
between any two sites. However, as already discussed in this chapter, a full mesh is costly
to implement, maintain, upgrade, and troubleshoot. A hierarchical partial-mesh topology,
as shown previously in Figure 5-4, is usually sufficient.

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