Information Technology Reference
In-Depth Information
Dual attachment: This solution increases availability by using redundancy network
interface cards (NIC).
Fast EtherChannel (FEC) and Gigabit EtherChannel (GEC) port bundles
Route Redundancy
Designing redundant routes has two purposes: balancing loads and increasing availability.
Load Balancing
Most IP routing protocols can balance loads across parallel links that have equal cost. Use
the maximum-paths command to change the number of links that the router will balance
over for IP; the default is four, and the maximum is six. To support load balancing, keep
the bandwidth consistent within a layer of the hierarchical model so that all paths have the
same cost. (Cisco Enhanced Interior Gateway Routing Protocol [EIGRP] is an exception
because it can load balance traffic across multiple routes that have different metrics by us-
ing a feature called variance .)
A hop-based routing protocol does load balancing over unequal-bandwidth paths as long
as the hop count is equal. After the slower link becomes saturated, packet loss at the satu-
rated link prevents full utilization of the higher-capacity links; this scenario is called pin-
hole congestion. You can avoid pinhole congestion by designing and provisioning
equal-bandwidth links within one layer of the hierarchy or by using a routing protocol
that takes bandwidth into account.
IP load balancing in a Cisco router depends on which switching mode the router uses.
Process switching load balances on a packet-by-packet basis. Fast, autonomous, silicon,
optimum, distributed, and NetFlow switching load balance on a destination-by-destina-
tion basis because the processor caches information used to encapsulate the packets
based on the destination for these types of switching modes.
Increasing Availability
In addition to facilitating load balancing, redundant routes increase network availability.
Yo u s h o u l d k e e p b a n d w i d t h c o n s i s t e n t w i t h i n a g i v e n d e s i g n c o m p o n e n t t o f a c i l i t a t e l o a d
balancing. Another reason to keep bandwidth consistent within a layer of a hierarchy is
that routing protocols converge much faster on multiple equal-cost paths to a destination
network.
By using redundant, meshed network designs, you can minimize the effect of link failures.
Depending on the convergence time of the routing protocols, a single link failure cannot
have a catastrophic effect.
Yo u c a n d e s i g n r e d u n d a n t n e t w o r k l i n k s t o p r o v i d e a f u l l m e s h o r a w e l l - c o n n e c t e d p a r t i a l
mesh. In a full-mesh network, every router has a link to every other router, as shown in
Figure 2-16. A full-mesh network provides complete redundancy and also provides good
performance because there is just a single-hop delay between any two sites. The number
of links in a full mesh is n ( n - 1)/2, where n is the number of routers. Each router is con-
router with links to at least two other routing devices in the network.
 
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