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Chapter 28: Internet Protocol Version 4 Packet Routing
318 Section VII: Routing
Equal-cost Multi-path (ECMP) Routing
The routing table uses ECMP to store multiple routes to a remote
destination so that the switch can distribute the traffic load over several
routes. This can improve network performance by increasing the available
bandwidth for traffic flows. It can also provide route redundancy.
The routing table permits up to 32 routes to the same remote destination,
with up to eight of the routes as active at one time. The routes can be all
static routes, RIP routes, or a combination of the two. Routes to the same
destination must have different next hops. The routing table will not permit
two entries to the same remote destination with the same next hop.
When the routing table contains eight or less routes to the same
destination, all the routes can be active and available to route packets.
The distribution of the traffic among the active routes is controlled through
a hash that combines the packet source and destination IP addresses to
select a route for packets from a source node. The traffic from a specific
source and destined for a specific remote destination is assigned a route
and all traffic to that remote destination from that source is forwarded
using that route. The assignment of a route does not change except if the
path is lost (for instance, the status of an interface changes from up to
down), in which case its traffic is redirected to one of the remaining routes.
When there are more than eight routes in the table to the same
destination, the active routes are selected by preference value, metric
value, and age, in that order. The routes with the eight lowest preference
values are selected as the active routes. Where routes have the same
preference value, selection is based on the lowest metric values.
Otherwise, the selections are based on when the routes were added to the
routing table, with older routes given preference over newer routes.
Those routes not selected as active routes are placed in a standby mode.
The selection of the active destination routes by the switch is dynamic and
can change as routes are added and deleted from the routing table, and
when they change status. For instance, if a new static or RIP route is
added to the routing table when there are already eight active routes to the
same destination, the new route will replace an existing active route if it
has a lower preference value, forcing one of the active routes to change to
the standby mode.
Furthermore, an interface must be physically up with at least one active
port in the VLAN for any of its routes to be considered as available for use.
If an interface is down, meaning there are no active ports in the VLAN, the
routes of the interface are considered inactive and are not assigned any
traffic. For example, if there are eight routes to the same destination, but
two of the routes reside in an interface that is down, those routes are not
used, leaving six available routes.