Information Technology Reference
In-Depth Information
Load Balancing
The biggest challenge with Internet connectivity is making optimal use of the available
bandwidth. The fact that BGP always chooses a single path instead of load balancing like
an IGP can result in very uneven traffic patterns. When optimizing traffic flow, inbound
traffic flow is independent of outbound traffic flow. The mechanisms used to control traffic
flow in each direction are independent of each other, as discussed in the next sections.
Inbound Traffic Load Balancing
There are a few tactics for controlling traffic flow inbound. The methods available depend
on the form of multihoming that is being used:
•
If an enterprise is multihomed to the same provider, it has the most options available
for controlling traffic flow inbound.
•
If there are two connections to the upstream provider, the address space is divided in
half.
•
If the enterprise has a /21 that is assigned by the provider, the enterprise divides it into
two /22 networks. One /22 is announced on one link, and the other /22 is announced
on the other link. Additionally, the /21 is announced on both links. If this does not
provide the desired balance, the network can be further broken down, with different
networks being advertised out each link until the desired traffic flow is achieved. It is
also important to note that if the circuits being load-balanced over are unequal in size,
splitting the prefix in half might not be the most optimal division.
The real difficulty in balancing traffic comes when an enterprise network is multihomed to
two different providers. Announcing more-specific prefixes, as discussed in the previous
example, is not always an option. Obtaining a balanced traffic flow in this environment is
an iterative process.
The summary prefix is announced out both providers, and link utilization is monitored. It
is important to keep in mind that perfect balancing is not a reasonable goal; the goal is fairly
close load sharing. If traffic heavily favors one of the links, prepend your ASN to the path
with the heaviest utilization. Continue to monitor link utilization. If traffic is still favoring
the same link, you can perform additional AS_PATH prepending. Remember, increasing
the AS_PATH length by even one ASN can have a drastic impact on traffic flow, so
increase the AS_PATH a single ASN at a time.
As soon as traffic has been split between the two links as closely as possible with AS_PATH
prepending, very few options are left to further balance traffic. The only remaining option
is to make use of any communities that the upstream provider has available. The commu-
nities that are available vary from provider to provider. Chapter 9, “Service Provider Archi-
tecture,” provides a detailed discussion of how service providers set up their community
policies.
