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load balacing & no ip classless vs routing protocol

1) For load balancing could be used 2 routes having different administrative distances ? Or for load balancing always the routes must have the same administrative distance?

2) Here's my understanding of the impact the "no ip classless" command has on router behaviour:

"no ip classless" command has no effect on router behaviour when supernets and default route are learned via OSPF or IS-IS, but has effect when defining a static default route and also when supernets and the default route are learned via RIP, RIPv2, IGRP, EIGRP or BGP.

First please confirm if my understanding is correct and second, if the case, what is the explanation for this different behavior, dependent on the routing protocol type?

Ex:

10.1.1.0/24 |-R1-| 10.1.2.0/24 |-R2-| 10.1.3.0/24

If a static default route is configured in R1, then ping between PCs on 10.1.1.0/24 and 10.1.3.0/24 will not work, because R1 will drop the packets.

If default route is learned via OSPF or IS-IS then ping will work.

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Hall of Fame Super Silver

Re: load balacing & no ip classless vs routing protocol

Hello Badalam,

1) EIGRP and older IGRP can perform weighted load balancing using the variance concept:

the second route has to satisfy feasibility condition or it cannot be considered.

If metric of second path is 2 times the feasible distance this link will be used 1/3 and best link 2/3 (for this is weighted).

2)

I'm sorry I haven't one explanation for this.

We can guess that IS-IS being not IP based can behave differently.

Probably it comes from link state properties:

these protocols are inherently classless they haven't an auto-summary at major network boundary and this cannot be enabled : a topological area border is needed to perform summarization.

From this can come the different behaviour.

I agree the behaviour is evident when even if a default route is available: the classful routing device will discard packets for an unknown subnet of a connected major network even if a default route or supernet is available.

Hope to help

Giuseppe

3 REPLIES
Hall of Fame Super Silver

Re: load balacing & no ip classless vs routing protocol

Hello Badalam,

1) to perform load balancing both routes must be installed in the routing table this require same AD and same metric so if AD is different the better one is used.

2) The ip classless command allows a router to use a default static route to foward packets for an unknown subnet of a connected major network.

the old behaviuor before release 12.0 was to discard packets for an unknown subnet of a connected major network and this was classful routing.

I'm not sure that this is routing protocol dependent my understading is that this is a general property of the routing process.

I've checked 12.4 command reference it says:

If the supernet or default route is learned by using Intermediate System-to-Intermediate System (IS-IS) or Open Shortest Path First (OSPF), the no ip classless configuration command is ignored.

see

http://www.cisco.com/en/US/docs/ios/ipaddr/command/reference/iad_ip.html#wp1011379

So your undestanding is correct,

ip classless is enabled by default after 12.0

Hope to help

Giuseppe

New Member

Re: load balacing & no ip classless vs routing protocol

1) So administrative distance has to be the same, otherwise the route will not be installed in the routing table.

But for same metric, from what I read, it is not mandatory to be so.

IGRP and EIGRP support unequal cost, meaning that the metric can be different for those routing protocols, not mandatory the same. Of course if the variance is 1, then even for IGRP and EIGRP it will be required same metric.

2) Do you have any technical explanation why for IS-IS and OSPF this difference?

Or there is no logic behind, just that it was implemented like that by Cisco. Also, after I read the page referred by the link you posted, I have a doubt, as there it is written the following:

"When this feature is disabled, the Cisco IOS software discards the packets when a router receives packets for a subnet that numerically falls within its subnetwork addressing scheme, no such subnet number is in the routing table, and there is no network default route."

I think it is a mistake there: the last condition, i.e "and there is no network default route", is not required. Even with that default route the packets will be discarded. At least this is what is explained in other Cisco books.

Hall of Fame Super Silver

Re: load balacing & no ip classless vs routing protocol

Hello Badalam,

1) EIGRP and older IGRP can perform weighted load balancing using the variance concept:

the second route has to satisfy feasibility condition or it cannot be considered.

If metric of second path is 2 times the feasible distance this link will be used 1/3 and best link 2/3 (for this is weighted).

2)

I'm sorry I haven't one explanation for this.

We can guess that IS-IS being not IP based can behave differently.

Probably it comes from link state properties:

these protocols are inherently classless they haven't an auto-summary at major network boundary and this cannot be enabled : a topological area border is needed to perform summarization.

From this can come the different behaviour.

I agree the behaviour is evident when even if a default route is available: the classful routing device will discard packets for an unknown subnet of a connected major network even if a default route or supernet is available.

Hope to help

Giuseppe

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