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BGP load balance between two ISPs

Hello Experts,

I'm looking for suggestions. In my case study scenario, a company has two internet routers which have bgp peering to separate ISPs.

RTR-A peers to ISP-A
RTR-B peers to ISP-B
The two routers have an IBGP connection together.

The Routers are connected to a Firewall which does the PATing. For simplicity reasons, let's say there are only 2 internal network subnets. All routes from ISP-A has a local preference of 800 and all routes from ISP-B has a local preference of 700 so the internal network is taking ISP A.

If I want subnet 2 to take ISP-B, what are some suggestions to handle this?
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trojan81
Asked:
trojan81
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1 Solution
 
rfc1180Commented:
http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00800945bf.shtml

Take a look at the section: Load Sharing When Multihomed to Two ISPs Through Multiple Local Routers

Load balancing is not possible in a multihomed environment with two ISPs. BGP selects only the single best path to a destination among the BGP paths that are learned from different ASs, which makes load balancing impossible.

Since you want to route via source IP (Second Internet Subnet), you will have to implement Policy Based Routing.

http://www.cisco.com/en/US/products/ps6599/products_white_paper09186a00800a4409.shtml
http://www.cisco.com/en/US/docs/ios/12_2/qos/configuration/guide/qcfpbr_ps1835_TSD_Products_Configuration_Guide_Chapter.html

Billy
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rochey2009Commented:
Hi,

Which firewall do you have?
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SouljaCommented:
Load Balancing is in fact possible with BGP. Check this article out:

http://www.nil.com/ipcorner/LoadBalancingBGP/
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surbabu140977Commented:
I agree with rfc1180. Load balancing is not achievable. Load sharing is, to some extent. His link has got specific config example for load sharing. Assuming trojan81 has got no AS of his own, so he will have no control over incoming traffic. He can only distribute outgoing traffic.

I am not sure if trojan81 has got valid cisco contract, you might want to take a look at Cisco OER (Optimized Edge routing), which is bound to solve his issue.

Best,
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rfc1180Commented:
>Load Balancing is in fact possible with BGP
Soulja, with due all respect, No, you cannot load balance utilizing BGP in his application; BGP selects only the single best path to a destination among the BGP paths that are learned from different ASs. The author's application, the BGP peers are in different ASes, if they were in the same AS, then yes, you could implement maximum-paths so that both routes are injected into the RIB. "If", the author wanted to "load balance"  the only option he would have is to set a better metric for the routes in the range 1.0.0.0 to 128.0.0.0 (More load sharing than load balancing) that are learned from ISP(A) and a better metric for the rest of the routes that are learned from ISP(B). However, he wants all traffic from one subnet (The second) to be routed via ISP-B, this is source routing and Policy Based Routing is required.
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trojan81Author Commented:
i appreciate everyone's input. I have been out sick the past couple days and will look into the suggestions tomorrow.
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SouljaCommented:
Hi rfc,

I stand corrected.  :-)
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trojan81Author Commented:
RFC,

great link you gave me about load sharing.

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00800945bf.shtml


In this last example: Load Sharing When Multihomed to Two ISPs Through Multiple Local Routers

R6# show ip bgp
BGP table version is 15, local router ID is 192.168.64.6
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete
   Network          Next Hop            Metric LocPrf Weight Path
*> 10.10.10.0/24    192.168.63.3                           0 300 100 100 i
!--- This line shows that network 10.10.10.0/24 is routed through AS 300
!--- with the ISP(A)-R1 link.

Why is it that R6 sees "300, 100, 100, i" as the best path to get to 10.10.10.0/24?  Prepending the AS towards ISP-A should have made the 10.10.10.0/24 network less desirable. So shouldnt R6 best way to get to 10.10.10.0/24 be "400, 100, I" through ISP-B?

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trojan81Author Commented:
RFC,

I re-read the example and see Both R1 and R2 are prepending AS 100 one time for network 10.10.10.0/24.
If both routers are prepending one time, what makes ISP-A the path to take from R6's perspective for network 10.10.10.0/24?
It sounds like the prepending cancels out
 
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rfc1180Commented:
>It sounds like the prepending cancels out
not really, the paths are equal (same amount of hops), move to the next decicion step

BGP has several best path algorithms:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

It is more than likely choosing the path based on step 6 or  10; very rarely do you see the best path selected on 11, 12, or 13.

Billy
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trojan81Author Commented:
Thank you.
I am still undecided on which bgp path selection criteria to use for load sharing, but you have given me a better understanding.  
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rfc1180Commented:
BGP path selection is not a criteria to use for load sharing; you are NOT able to do anyoutbound  BGP load sharing based on a source prefix from your network. You typically can influence on what transit circuits are to be used for inbound to your network by prepending, utilizing Metrics, or the more perferred way is to contact your ISP and ask the for their communities for changing the local_pref for the prefixes that you are advertising. Most ISPs will have this routing policy in affect and operational.

Billy
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