Cisco Eigrp Summary 2
Variance
Every routing protocol supports equal cost path load balancing. In addition,
Interior Gateway Routing Protocol (IGRP) and EIGRP also support unequal cost path load balancing.
Use the variance n command in order to instruct the router to include routes with a metric of less
than n times the minimum metric route for that destination.
The variable n can take a value between 1 and 128.
The default is 1, which means equal cost load balancing.
Traffic is also distributed among the links with unequal costs, proportionately, with respect to
the metric.
router eigrp 1
network x.x.x.x
variance 2 ( load balance over 2 unequal cost links).
Traffic Sharing
EIGRP not only provides unequal cost path load balancing, but also intelligent load balancing,
such as traffic sharing.
In order to control how traffic is distributed among routes when there are multiple routes for
the same destination network that have different costs, use the traffic-share balanced command.
With the keyword balanced, the router distributes traffic proportionately to the ratios of the metrics that
are associated with different routes.
This is the default setting:
router eigrp 1
network x.x.x.x
variance 2
traffic-share balanced
When you use the traffic-share command with the keyword min, the traffic is sent only
across the minimum-cost path, even when there are multiple paths in the routing table.
router eigrp 1
network x.x.x.x
variance 3
traffic-share min across-interfaces
Load Balancing in CEF
Cisco Express Forwarding (CEF) is an advanced Layer 3 switching technology which can be used for
load balancing in routers. By default, CEF uses per-destination load balancing.
If it is enabled on an interface, per-destination load balancing forwards packets based on the
path to reach the destination. If two or more parallel paths exist for a destination,
CEF takes the same path (single path) and avoids the parallel paths.
This is a result of the default behavior of CEF. CEF takes the single path in cases when load sharing is
done simultaneously on interfaces of different physical types, such as serial and tunnel.
In order to utilize all the parallel paths in CEF and load balance the traffic, you must enable
per-packet load balancing.
When you have different physical interfaces like serial and tunnel. So, on the basis of the
configuration and topology (serial or tunnel), load sharing can fail to work correctly with the
default CEF load balancing mode.
Enable these commands for load sharing on a per-packet basis:
interface serial 0
ip load-sharing per-packet
no passive-interface default
passive-interface default to block all Eigrp adjacencies. Using no passive-interface default
will allow EIGRP communication.
router eigrp 80
passive-interface interface fastethernet 0/1 – no hello send but still advertise network.
passive-interface default – turns on passive-interface globally on all interface.
no passive-interface seial 0/1 – turns off passive-interface on these interfaces.
Ip summary-address eigrp
To configure a summary aggregate address for a specified interface, use the
ip summary-address eigrp command in interface configuration mode.
summary 172.16.0.0/16 to 172.16.7.255
int serial 0/1
ip summary-address eigrp 90 172.16.0.0 255.255.248.0
Configured Eigrp Host Address Only.
router eigrp 80
network 10.1.2.2 0.0.0.0 (host address only).
ip default-network
You can use ip default-network when ip routing is enabled on the Cisco router.
When you configure ip default-network the router considers routes to that network for installation
as the gateway of last resort on the router.
For every network configured with, if a router has a route to that network, that route is flagged as
a candidate default route.
ip default-network 192.168.1.0
“show ip route” will see s* static route.