Load balancing by equal cost Static Routes

[rmujeeb81]

Hello All,

I have 2 WAN links for Internet connectivity and I want to load balance IP traffic on both links. If I use 2 default routes like this,

ip route 0.0.0.0 0.0.0.0 serial 0

ip route 0.0.0.0 0.0.0.0 serial 1

then its enough to achieve load balancing or I have to configure following interface configuration command.

(config-int)# ip load-sharing per-packet

Kindly advice.

Regards,

Mujeeb

[ankurbhasin]

Hi Mujeeb,

Bydefault router is configured for fast switching which means per destination load balancing.

So suppose if you waana reach 1.1.1.1 and you send 10 packets for same destination it will not load balance but will pass via 1 link only.

But if you enable per packet load balancing 5 packets for same destination will pass via serial 0 and 5 will pass via serial 1.

HTH, if yes please rate the post.

Ankur

[rmujeeb81]

Hello Ankur,

ok now I have very clear view about per-destination and per-packet load balancing but can you tell me about equal cost default routes, Is it must to use ip load-sharing per-packet interface conf command or just put 2 default routes.

Regards,

Mujeeb

[ankurbhasin · Accepted solution]

Hi Mujeeb,

Yes the routes which you have configured will give you equal cost as they are configuerd with same admin distance and you need not enable ip load-sharing per-packet if you want per destination load balancing.

But again as I mentioned it will not give you equal packet load balancing till the time you configure ip load-sharing per-packet.

HTH, if yes please rate the post.

Ankur

[rmujeeb81]

Hello Ankur,

Thanks I got my answer.

Regards,

Mujeeb

[24madaeve1]

hi ankurbhasin. I have one doubt pertaining to per-packet load-sharing. In order to connect my two remote sites- A & B, Site A is having two WAN links and Site B is having two WAN links - one from ISP1 (30Mbps link) and the other from ISP2 (50Mbps link). I am doing static route load balancing using same AD values for both the ISPs. I have configured "ip load-sharing per-packet" on both the outgoing interfaces.

The load is getting distributed equally across both the links but total bandwidth utilization across both the links is not going beyond 30Mbps. The combined bandwidth of both links is 80Mbps (50+30). However links are not getting fully utilized even though heavy load is there on the links. Can you please tell me how to make full use of both the wan links at both the ends?

[Richard Burts]

Consider this aspect of per packet load sharing. For every packet that you send through ISP 1 you will send one packet through ISP 2. And consider what will happen as traffic builds up on the link that is at 30M. As it approaches capacity it will slow down. And as it slows down it will slow down the link that is at 50M.

If you have a link at 50M and a link at 30M and you enforce per packet load share then the 50M link can go no faster than the 30M link.

HTH

Rick

[24madaeve1]

I agree with you richard but the weird thing is that my 30Mbps link is also not getting fully utilized. BW utilization is going uptil 15Mbps maximum across both the links. it should go atleast 30Mbps in both the links.

[Fedor Travinsky]

Unless it's a wired one-hop solution with equal equipment and cable length, your channels will have different latency and jitter.

The problem with per-packet balancing is packets in TCP-session will certainly reach the destination out of order. TCP-session will slow down as soon as out of order occurs.

If such jitter between two channels is small enough, it's not a problem for UDP traffic (Torrent or even VoIP) - implementations in these apps got embedded buffers for ordering packets back to normal sequence.

That's why "by destination" is the default balancing method. It allows to utilize summarized bandwidth by using multiple TCP sessions.

Not so fast for a single download, but overall site's performance will reach total maximum.