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BGP Multihoming and load balancing could cause asymmetric routing??

Hello Experts

 

My scenario is very simple. I have two public address space say 1.1.1.0/24 and 2.2.2.0/24. My AS number is public say 234. I have one router with two links. One link is connected to ISP1 and other link is connected to  ISP2. Both ISP are just advertising the default routes to my router and I am advertising 1.1.1.0/24 and 2.2.2.0/24 subnets to boht ISP.

 

Outgoing load balancing 

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- Since I am getting the two default route in routing table, one from ISP1 and other from ISP2 so I have enabled, mulipath with different-as knob and also configured the per-packet loadbalancing policy and applied in forwarding table 

- Now router will do the per-packet (actually per session L3/L4) load balancing across both links

 

- Here is my question: Is it wise and practical to do the load balancing per-packet for internet traffic? because protocol

 

like http is multi-session protocol, when one session go to ISP-1 and other session will go to ISP-2 then it could cause performance issue?? as ISP-1 path to server may be longer than ISP-2 path. Please advise

 

Incoming Loadbalancing

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- I advertise 1.1.1.0/24 to ISP1 also advertise 1.1.1.0/24 to ISP2 with as-path prepending,so 1.1.1.0/24 subnet will be prefered from ISP1

- Similarly I advertise 2.2.2.0/24 subnet to ISP1 with as-path prepending and advertise 2.2.2.0/24 to ISP2 normally, so 2.2.2.0/24 subnet would be prefered from ISP2

 

- Here is my question: It is possible that traffic for subnet 1.1.1.0/24 comes from ISP-1 and return traffic will go through ISP-2, as my both default route are active. This will cause the asymmetric routing. This would have any performance impact?? How I make sure that retrun traffic should also go from ISP-1 as well.

 

Please put some shed on this? 

 

Regards

 

KK

3 REPLIES
Super Bronze

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Posting

Generally you want to always avoid per-packet load balancing.

New Member

Sorry Not sure

Sorry Not sure

Hi,

Hi, ad Outgoing load balancing) I agree with Joseph: Per-packet load balancing can easily come to packets belonging to the same session taking considerably different paths and arriving in an incorrect sequence to the target device. And the target device has to buffer the packets then. And potentially drop or delay the data sent to the application level. So per-packet load balancing should be avoided. ad Incoming Load balancing) Asymmetric routing could have some performance impact in some cases (path MTU detection, etc.) But not necessarily if per-session load balancing is used. You can't make sure that return traffic should go from ISP-1 as well in your topology. You could play with BGP attributes of the advertised prefixes. But in that case ALL incoming traffic would prefer a path through one ISP. And as you are load balancing the outgoing traffic, there would still be some asymmetric routing. So the only way to avoid asymmetric routing would be using one ISP as primary and second ISP as a backup for both outgoing and incoming traffic. Best regards, Milan
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