Load Balancing on EIGRP

[amacdos]

Hi there ,

I have a question on how to check on how to load balancing works on EIGRP .

I have three router going to same site , EIGRP running . with 2 mbps line each .

3 routers - all router speed is 2 Mbps one site connected to other site with 3 routers 2 Mbps also .

How do I verify , whether is it doing load balancing or not .

And I am not sure , where is it per packet or per direction .

Anybody can help ...

[pkhatri]

Hi,

A good start would be to look at the 'show interface' counters on each link to see how much traffic is going out it.

To determine the type of load-sharing in effect, view the output of 'sh ip int'. If CEF is enabled, the default load-sharing will be per-destination. The same applies if fast switching is in use. If you want per-packet load-sharing, use the 'ip load-sharing per-packet' command under the interface.

Pls do remember to rate posts that help.

Paresh

[amacdos]

Paresh ,

On the show interface ,the ip cef is enable I am able to see input rate , is 765 kbps

And on the other router , i am able to see 656 kbps .

But at times it goes

First router 650 kbps

Second router 1900 kbps

I thought EIGRP is supposed to do load balancing

50 % one router the other router 50 % of the traffic

In which occasions do we need to use the load-sharing per packet rather than per destination

[pkhatri]

Hi,

What you should understand is that no routing protocol out there professes to do load-balancing; they only support some form of load-sharing. The difference between the two is that load-balancing means an equal distribution between 2 or more links, which is almost impossible to achieve. Load-sharing, on the other hand, means the distribution of traffic between 2 or more links so that some measure of traffic sharing occurs.

The above explains why you are seeing what you are seeing. EIGRP seems to be working okay. However, depending on your traffic patterns, you will get a dis-proportionate sharing of traffic between your links.

Per-packet load-sharing may make the distribution more even. However, per-packet load-sharing is generally not recommended, as it results in out-of-order packet delivery. As a result, protocols like TCP would start spending more time re-transmitting packets etc, which could result in a negation of all the benefits you have gained by using per-packet load-sharing.

The bottom line is that the behaviour you are seeing is okay and a direct consequence of the reasoning I stated above.

Pls do remember to rate posts that help.

Paresh

[kamlesh.sharma]

Hi,

if you didn't configured EIGRP and nothing more than defaultly settings are.

# equal cost load balancing

# per-destination

Now about your link utilization it could be different for different link why so bcos you are doing per-destination. if one source is trying to access one destination and it is reachable via router's 1s link that that source and destination will always be reachable via that link only.

and according to different traffic patterns from that source to destination link utilization will vary.

I hope this will answer your query. Please rate post.

if not please write

[amacdos]

Hi Kamlesh , thanks for your help also . You were saying one source is trying to access one destination and it's reachable via router's 1 link only , how's it being done . I mean , how does it work . It's true in a way because when , one router went down , one particular application did not work , even the other routers are up and suppose to take the traffic .But it didn't .

The application went through , only when I brought back the router which was down . How does it work ....

[kamlesh.sharma]

hi,

I think for further troubleshooting we need to look at the configuration as u are saying if one interface goes down ur application doesn't work.

may be you have configured default gateway of the machine to that router's ip address and when it goes down your application stops working.

hth and waiting for ur config

[michaelc0n]

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 the default)...that is why your application would fail. As an alternative have you tried CLI command under router config# traffic-share balanced (under eigrp). "traffic-share balanced" maybe another option to distribute traffic. Rate this, thanks!

[rmujeeb81]

Dear All,

As we know that EIGRP do load balancing on Equal cost paths and Unequal cost paths as well. By using "variance" we can do load balancing on Unequal cost paths so for above scenario is it possible or not ?

Regards,

Mujeeb

[michaelc0n]

The command("traffic-share balanced") I posted previously works in conjunction with "variance"

Sample Config:

router eigrp 1

network x.x.x.x

variance 2

traffic-share balanced

In setting the above config that will distribute your traffic (per-packet) across the links that meet the variance criteria.

===============================================

This link will assist in troublshooting/verifying CEF load sharing:

http://www.cisco.com/en/US/tech/tk827/tk831/technologies_tech_note09186a0080094806.shtml

Command: "ip load-sharing " gives you the option of per-packet or per-destination when using CEF.

Hope that helped, rate this...thanks!

[amacdos]

Hi guys , sorry for the late reply .

Kamlesh , I did not configure the applications server with the router ip address . Means I didn't configure any of the router to act as a gateway .

Not sure , why it happens . But I did not configure the routers with per packet and it takes by default , it's per destination , the EIGRP that we have configured is on equal cost because it's all 2Mbps leased lines .

Router 1>sh ip pro

Routing Protocol is "eigrp A"

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Default networks flagged in outgoing updates

Default networks accepted from incoming updates

EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0

EIGRP maximum hopcount 100

EIGRP maximum metric variance 1

Redistributing: eigrp A

EIGRP NSF-aware route hold timer is 240s

Automatic network summarization is not in effect

Maximum path: 4

Routing for Networks:

10.1.1.1/27

10.2.2.2/32

10.3.3.3/30

Routing Information Sources:

Gateway Distance Last Update

10.4.4.4 90 01:11:06

10.5.5.5 90 01:11:06

10.6.6.6 90 01:11:06

Distance: internal 90 external 170

I did not configure variance on the routers also .

But we have QOS on the Routers for Voice .

[michaelc0n]

Here are a few commands that you can issue to verify proper load sharing:

#show ip cef (verify cef is enabled)

#show cef interface X (verify that Per packet loadbalancing is disabled)

#show ip route (verify the equal paths to dest.)

#show ip cef X.X.X.X (verify the FIB)

#show ip cef X.X.X.X internal (verify "traffic share 1" ... should be set to 1 for equal cost per-destination load sharing among three equal cost routes)

Will you post a copy of "sh ip rou" & "sh ru" from each router(omit private info.)? Thx

[rmujeeb81]

Dear michaelcon,

Sorry for late reply.Well the link you forwarded in your previous mail is very helpful to understand cef.

I have one more query that if cef is enabled on interface and EIGRP is doing Unequal cost load balancing for a destination than what will happen if a packet arrive on this router for remote detination.

First router do route lookup on this packet than cache this entry in switch cache.If " traffic-share balanced " is not enabled than router forwards subsequent packets to the same interface so I want know that in this case Unequal cost load balancing by using " variance " has no significance.

Regards,

Mujeeb

[michaelc0n]

rmujee81,

Here are couple of links to assist w/ your questions, hope they help you as they have helped me.

Load Balancing with CEF:

http://www.cisco.com/en/US/products/hw/modules/ps2033/prod_technical_reference09186a00800afeb7.html

How Does Unequal Cost Path Load Balancing (Variance) Work in IGRP and EIGRP?:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009437d.shtml#topic2

[kamlesh.sharma]

hi amacdos,

how you doing and what about your problem.

just let us know.