background image
Configuration of RIP and IGRP 395
Multiple Routes to the Same Subnet
By default, the IOS supports four equal-cost routes to the same IP subnet in the routing table at
the same time. This number can be changed to between 1 and 6 using the ip maximum-paths
x
router configuration subcommand, where x is the maximum number of routes to any subnet.
As mentioned earlier, the packets are balanced on a per-destination address basis by default;
packets also can be balanced on a packet-by-packet basis, but at a performance penalty.
The metric formula used for IGRP (and EIGRP) poses an interesting problem when considering
equal-metric routes. IGRP can learn more than one route to the same subnet, with different
metrics; however, the metrics are very likely to never be exactly equal. The variance router
subcommand is used to define how variable the metrics can be for routes to be considered to
have equal metrics. The parameter to the command (the multiplier) is multiplied by the lowest
of the received metrics for a particular subnet. Any routes with a metric less than the product of
"best metric" times the multiplier are considered to be equal.
Some rather interesting twists in logic must be considered when deciding whether to use one or
multiple equal-cost routes with IGRP. If maximum-paths is set to 1, then the first of these
equal-cost routes learned to each subnet is placed into the routing table. However, these could
be the routes with the largest metric. To avoid that, maximum-paths could be defaulted to 4 or
could be coded as some other number; in addition, the variance command can be used to define
how close the metrics must be in value to be considered equal. However, in that case, some of
the traffic will flow over the routes with the best metric, and some will flow over the route with
the worst metric. Neither situation seems to be optimal.
A different--and possibly better--alternative is to use the traffic-share min router IGRP
subcommand in conjunction with maximum-paths and variance. This command tells the
router to add the multiple routes to the routing table, but to send only traffic using the route with
the smallest metric. This allows all routes to each subnet to be in the routing table, which is an
advantage for faster convergence. However, all traffic goes across the lowest-metric route that
is currently in the routing table. The traffic-share balanced command, which is the default,
tells the router to use all the routes proportionally based on the metrics for each route.
Enhanced IGRP
Yes, by default
Allowed via
configuration
Yes
OSPF
No, but can do
equivalent with
aggregation
Yes
Yes
Table 6-16
Route Aggregation Comparison of Subnet Numbers (Continued)
Routing Protocol
Auto Summary
Enabled?
Auto Summary
Disabled?
Route Aggregation?
ch06.fm Page 395 Monday, March 20, 2000 5:11 PM