Load Sharing with Static Routes

Unanswered Question
Oct 16th, 2007

Hi,

I have two cisco 871 routers configured with 4 vlans, having each Fa Interface assigned to a different vlan (ex. Fa0=Vlan1,Fa1=Vlan2,Fa2=Vlan3,etc..)

I have all of the vlan interfaces connected to different wireless access-points acting as a wireless bridges. I am using dual links between the routers so that I have some level of link redundancy. I want to be able to use the dual links to load share between them. What is the exact configuration required?

I read that cisco routers do this by default when they find two routes to the same destination but when I try traceroute from a PC connected to a router, to another PC connected to the other router, the packet always passes from the same path.

An example of the routes is as follows:

0.0.0.0 0.0.0.0 192.168.100.1

0.0.0.0 0.0.0.0 192.168.101.1


I tried to issue the no ip route-cache on all the interfaces of the router but still the packet is passing through the same route. Am I missing something?


Thanks,

Alex

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Kevin Dorrell Tue, 10/16/2007 - 13:33

By default, the load is shared by distributing according to source and destination IP addresses, so called "per-destination". So the ping will always take the same link.


To share the load per packet, put ip load-sharing per-packet on both outgoing interfaces.


http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cr/hisw_r/ips_a1h.htm#wp1153558


The other thing you could do it to bundle the interfaces together as a snigle IP address and run PPP multilink. That will even split up large packets and send the chunks over alternate links.


Kevin Dorrell

Luxembourg


Richard Burts Tue, 10/16/2007 - 18:03

Kevin


Your description is exactly right when describing transit traffic (traffic which transits/goes through the router) but is not right for traffic which is originated by the router. When the router originates traffic it will use all available (equal cost) routes in a round robin fashion. There is not anything that can be configured to change this behavior.


Which makes it all the more strange that the original post says that trace always uses a single path. Perhaps the original poster could post the output of show ip rote. I would be interested in seeing that both static routes are installed in the routing table.


HTH


Rick

guruprasadr Tue, 10/16/2007 - 21:04

Hello, [DO RATE ALL HELPFUL POSTS]


Rather using the floating static routes (of equal costs) and with per-packet or per-destination load balancing, its advised to use Multi Link PPP which bundles the link.


Sample Multi-Link PPP Config:

--------------------------------------


interface Multilink1

description *****MULTILINK in India*****

ip address x.x.x.x

ppp multilink

ppp multilink group 1

!

!

interface Serial0/3/0

description **CIR 1**

bandwidth 2048

ip address x.x.x.x

encapsulation ppp

ppp multilink

ppp multilink group 1

!

interface Serial0/3/1

description **CIR 2**

bandwidth 2048

ip address x.x.x.x

encapsulation ppp

ppp multilink

ppp multilink group 1



interface Multilink1

description *****MULTILINK in Europe*****

ip address x.x.x.x

ppp multilink

ppp multilink group 1

!

!

interface Serial0/3/0

description **CIR 1**

bandwidth 2048

ip address x.x.x.x

encapsulation ppp

ppp multilink

ppp multilink group 1

!

interface Serial0/3/1

description **CIR 2**

bandwidth 2048

ip address x.x.x.x

encapsulation ppp

ppp multilink

ppp multilink group 1

!



Also, as told by Rick, can you post the "show ip route" output.



DO RATE ALL HELPFUL POSTS



Best Regards,


Guru Prasad R

Kevin Dorrell Tue, 10/16/2007 - 22:31

Rick,


If I understand his architecture correctly, I think he was actually pinging from a PC in one VLAN to a PC in a different VLAN on the other router, with a layer-3 link between the routers, so it should be CEF switched. I'm not sure. Are the routers working HSRP, or what?


Your point about generating ping from within a router begs a lot of interesting quetsions that I have not had time to look at yet. For example, if you do have two static routes and you ping from the router, does the ping pick an outgoing interface and generate 5 pings to it, or does it generate each ping individually and round-robin them. If it is the latter, does that mean that alternate pings have alternate IP source addresses?


Where is the round-robin control? Do the pings take their place in the round-robin with the rest of the internally generated traffic - routing protocols etc - in that case, the choice of egress interface will be apparently random. Or is it the ping process that has its own round-robin distribution method, in which case they really will be out alternate links?


Kevin Dorrell

Luxembourg


aseychell Tue, 10/16/2007 - 22:45

Hi,

Kevin is right. I am trying to ping from a PC conected to a Vlan on RouterA to a PC connected to a vlan on Router B.

The PPP multilink option seems interesting, however I am concerned in loading up the router too much and also I have the whole IP addressing scheme in place and do not wish to change it now that the router is in production.

I have a small question. If you have 4 interfaces, grouped 2 and 2 with same cost to the same destination, on which interface should I place the ip load-sharing per-packet command. I mean there is no incoming and outgoing interface as traffic is bi-directional.

Also I do not have access to the router now but I checked show ip route and both 0.0.0.0 0.0.0.0 routes are installed.


Thanks very much for the feedback.

guruprasadr Tue, 10/16/2007 - 23:05

HI,


Thanks for confirming about show ip route.


Normally the command the ip load-sharing per-packet / per-destination,


should be placed in the outgoing interfaces of the Router (connected to the Service Provider).


If both the links are purchased from the same Service Provider means, the SP can also define the Per-Packet / Per-destination load balancing at his side with your agreement.


Please speak to your Service Provider Technical Team regarding this requirement.



DO RATE ALL HELPFUL POSTS


Best Regards,


Guru Prasad R

aseychell Wed, 10/17/2007 - 07:21

Hi.

First of all thanks for your reply.

I will try and describe my scenario in a better way. There are no ISP's involved in this. They are just wireless links between my own routers.(3 of them)


-----()====()====()----

LAN R P2P R P2P R LAN


- (Single Links)

=(Dual Links)

R (Router)

P2P (Point to Point wireless bridges)

On all R's, on which interfaces should I issue ip load-sharing per-packet?


Thanks

Alex

aseychell Wed, 10/17/2007 - 11:24

No worry.

I will try tomorrow and will let you know. I am asking as I already tried this config but didnt shut / unshut the interface. Will do some further tests and see how it goes.


Thanks for your help

Richard Burts Wed, 10/17/2007 - 13:53

Kevin


My apologies. When I re-read the original post it is clearly stated that the ping source is a PC connected on one side and the destination is a PC connected on the other side. I clearly misread the post :(


HTH


Rick

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