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BGP Loadbalancing with multihome network

Level 1
Level 1

Hello All,
I have a scenario like this, using bgp in a multihomed network with two different ISP. my intension is to acheive loadbalancing in both inbound&outbound direction. I have subnetted my prefix 82.x.x.0/24(for example) into two 82.x.x.x/25 prefix and I am advertising the first subnet and /24 prefix to ISP1 and 2nd subnet and /24 prefix to ISP2 to acheive inbound loadbalancing with redundancy.Now my question is regarding outbound loadbalancing.,I am only receving default route from both the ISP, how to acheie the outbound loadbalacing now??
Note: Both the link capacities are same!!

  ISP1             ISP2
   |                      |
   |                      |
   |                      |


8 Replies 8

Mohamed Sobair
Level 7
Level 7


BGP is still destination based rotuting Protocol, for outbound loadbalancing from your inside your prefixes, you still need to uses Policy based routing.

You can also achiev loadbalancing by configuring the command (maximum paths 2), although BGP still uses a single best path to reach particular destination, this command allows BGP to install more than one path in the routing table if the network has the same BGP attribute.

So the conclusion is as follows:

1- Policy based routing to perform loadbalncing based on source address.

2- configuring the (Maximum paths 2) command which allows BGP to install more than one path in the routing table for destination based prefixes.



Thanks for the quick reply Sobair!!

I will only receiving default roues from both ISP, so by only specifying maximum paths 2 will work here??..Will the traffic get equally balanced??


Yes, ONLY if the prefix you want to reach have identical AS-Path attributes from both ISPs, example: AS-Path length, Origin, local preference and Weight and other BGP attributes.

Or else, you will have to configure PBR to perform some kind of outgoing loadsharing based on Source.



Hi sobair,

I need your help further

If my traffics are just plain internet traffic(internet browsing) then how can I apply pbr here. My objective is to use both internet links simultaneously for all the outbound as well as inbound traffic. It would be great if you can explain with an example for this scenario, Also, pls let me know how to acheive the outbound loadbalancing if I receive complete internet routing table from both ISP


       1. Maximum path command wont achieve load-lalancing in such scenario where you have dual homed with diffrent ISP's i.e. two diffrent AS.

       2. PBR would help to achieve load balancing but this fails to achieve redundency or auto failover if one link fails. Manual intervetion required to divert traffic on secondary if active link fails.



CCIE 22703


In addition to above, if you have a ASA pointing default route to outside routers (R1&R2), you can use 2 default routes point to same interface with 2 different nexthops.

Or else you can try using oer/pfr.




Here is an example for you,

For outbound loadsharing, the PBR config should look like this,

Since you have devided your internal subnets to two /25 prefixes ie: 82.x.x.0/25 ,,, and 82.x.x.128/25

you just need to have one prefix pointing to 1st ISP next hop and the second prefix pointing to the 2nd ISP next hop, ie:

int e0/0

ip policy-route map ISP1

int e0/1

ip policy route-map ISP2

route-map ISP1 permit 10

match ip address 10

set ip next hop x.x.x.x (The 1st ISP next hop)

route-map ISP2 permit 10

match ip address 20

set ip next hop y.y.y.y  (The 2nd ISP next hop)

access-list 10 permit 82.x.x.0

access-listt 20 permit 82.x.x.128

Note, you can use PBR with multiple tracking option for automated redundancy and fault tolerant in your case, look at the bellow link for more information,

Let me know if you still need some assistance,,


for the Gitendra the CCIE guy who posted a comment here,

1- The Maximum paths command works for both Ebgp and IBGP , however as I said, the prefix to reach sould have identical as path attributes for loadsharing.

2- For PBR, he doesnt need Manual Intervention if his IOS supports (PBR support with Multiple tracking options) , here is the link;



Hi Mohamed,

1. Maximum path feature can support load-balancing on EBGP or IBGP but for which paths that have received through same autonomous system.You cannot do load balancing because BGP selects only a single best path to a destination among the BGP routes that are learned from the different ASs.This feature won't support for path received through diffrent autonomous system that said it will only selece best single path from 2 diffrent AS. You can have a look at page 7 & 15 in attched PDF.

2. I would agree if PBR support tracking feature.

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