Unequal load sharing on T1's

bhatia.sachin · ‎09-29-2005

Hi All,

Im having a 2mb MPLS link on the local end and on the remote end,I have 2 T1's (1.5mb each).Im having issues with the T1's at the remote end.The incoming traffic on the T1's appears to be normal but the outgoing traffic is not load balanced and one of the T1's is getting choked and the other remains virtually idle.Moreover im running voice traffic over the MPLS link,so im facing voice breakage issues.

The service provider was informed of this issue and he has come with a simple solution of enabling per-packet load balancing on the T1's but it has not been much of a help.No routing issues are suspected as it has been done maually.Im pasting the config at both ends.

remote end:

interface Serial0/1/0

description ***MPLS***

bandwidth 1536

ip address 10.10.27.1 255.255.255.252

no ip proxy-arp

ip load-sharing per-packet

load-interval 30

serial restart-delay 0

interface Serial0/1/1

description ***MPLS***

bandwidth 1536

ip address 10.10.28.1 255.255.255.252

no ip proxy-arp

ip load-sharing per-packet

load-interval 30

serial restart-delay 0

local end:

interface Serial0/1/1

description ***MPLS***

bandwidth 2048

ip address 10.10.20.1 255.255.255.252

no ip redirects

no ip proxy-arp

load-interval 30

fair-queue

I would appreciate if someone can provide me a reason and a resolution at to why one of the T1's becomes the active forwarder while the other stays passive.Is this a known issue with MPLS ??

michael_davis · ‎09-29-2005

MPLS vpns use iBGP to distribute customer routes throughout their backbone to routers where your equipment connects. Given BGP behavior, it's pretty easy to have this issue occur. By default, BGP will choose one next-hop for a given prefix. This is true whether the prefix is for a carrier backbone or a customer's rd+prefix.

It's hard to say precisely what the cause here is without knowing the internals of your provider's MPLS / BGP cloud. But I would hazard an educated guess that they a) have not made any provisions for load balanced eBGP with you, or b) have not made any provisions for iBGP multipath in their backbone. Or c) you don't have any bgp peering and they are filtering out your redundant path.

Whatever the cause, one simple solution may be to "bond" your two remote T1s together. One approach is MLPPP, although this may not be supported by your carrier or supported in this instance. Another approach is to configure a eBGP session accross the two T1s from your loopback to their loopback, rather than interface to interface. Obviously, eBGP multi-hop, and specific loopback to loopback static route pairs must be added such that there are two paths (one for each T1) to the other loopback. Traffic is load balanced between the two loopbacks "eBGP next-hops" according to the static routes.

Let me know if this helps by rating the post.

Michael

bhatia.sachin · ‎09-29-2005

Hi Michael,

Thx for your reply.But im not using BGP in my network though im not sure if the service providers are using it amongselves as this is an International circuit.I have defined static routing for the same.MLPPP sounds good but ill have to check with the service-provider again.Is there anything else i can do from my side..??

regards,

Sachin

michael_davis · ‎09-29-2005

I understand. The service provider will _definitely_ be using BGP, even though they are not using it for peering with you. As I said before, MPLS VPNs use BGP to carry customer routes across their network.

From your side, cef per-packet load-balancing with equal cost (default) routes out of the remote site is about as good as you can do by yourself. This will get traffic leaving your remote site to balance pretty evenly. However, you have no control over how traffic is balanced by the provider when returning to you. The provider must work with you to correct this.

attrgautam · ‎09-29-2005

Another shot i can take at this is that probably your provider is doing per destination load sharing ( Some providers dont support per packet load sharing) so it is quite possible if you are reaching a single destination traffic may flow through a single link.