BGP with Maximum-paths

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May 21st, 2009
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Hi Everybody,


I need a clarification on the behaviour of BGP with equal cost path (maximum-paths 2)


I have 1841 Router with Dual leased line named my ISP named ISP-1. I was runnning BGP between my router(AS 500) & ISP-1 (AS100). Since the two links are equal cost, by using maximum-paths command i was loadbalancing the traffic. "show ip route" command was showing that destination was having two paths one via ISP-2 & another via ISP-1


I surrendered one of the link to ISP-1 and procured a new link from ISP-2(200) and running BGP as like previous configuration.


But after this inspite of "maximum-paths 2" command only one route is appearing in the routing table of my router.


Why & What is the meaning of this behaviour of the router when connected to different AS. Why it is not load balancing or taking multiple paths when connected to different AS. But it is working if the BGP peering is same AS


can you please help me in understanding this concept and solution


RBK



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Giuseppe Larosa Thu, 05/21/2009 - 10:58
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Hello R.B: Kumar,

BGP metrics uses different criteria:


BGP maximum-paths 2 works if the first criteria for selecting the BGP best path match:

weight, local preference, AS path length, origin code, MED.

if all these parameters are a match BGP can use two paths.


to understand what is different in advertisement from ISP-1 and ISP-2 do


sh ip bgp


I guess you are not receiving full tables on the C1841.


When the two links are from the same ISP all the above mentioned parameters likely match.


There are other possible variants that involve the MED parameter, however the sh ip bgp should provide your answer.


Hope to help

Giuseppe


hclisschennai Thu, 05/21/2009 - 11:36
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Hi Giuseppe


Thanks for your response


I have just gone thru a document in Cisco. It states that Loadbalancing between two different AS is not possible unlike Loadbalancing between same AS. The solution is to configure weight parameter such a way that routes received from ISP-1 should be higher than 0 for prefixes 0-128 and configure weight parameter such a way that routes received from ISP-2 should be higher than 0 for prefixes 128-254.


I believe my case is also the same.Please check the "Load Sharing with BGP in Single and Multihomed

Environments: Sample Configurations" document. It gives the solution to my problem


RBK


RBK

Giuseppe Larosa Thu, 05/21/2009 - 12:11
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Hello R.B. Kumar,

you're right.


there is a workaround explained by Harold and Narayan

you should set the same MED inbound

+

bgp always-compare-med


+ an ios hidden command

bgp bestpath as-path multipath-relax


as explained here by Harold


http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=WAN%2C%20Routing%20and%20Switching&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40^1%40%40.2cd2b9a8/1#selected_message


bgp bestpath as-path multipath-relax


Hope to help

Giuseppe



jim_berlow Thu, 05/21/2009 - 12:41
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If the parameters are all the same, use this command:


bgp bestpath as-path multipath-relax


This is a hidden command with little documentation, but it allows for 2 "equal" default routes to be entered into the route table.


Good luck,

Jim

jim_berlow Thu, 05/21/2009 - 12:42
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If the parameters are all the same, use this command:


bgp bestpath as-path multipath-relax


This is a hidden command with little documentation, but it allows for 2 "equal" default routes to be entered into the route table.


Good luck,

Jim

hclisschennai Thu, 05/21/2009 - 22:03
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Hi,


Your posts are very useful.


As mentioned i have two ISPs (ISP-A [AS 100] & ISP-B [AS200]) connected to one router. I am accepting some prefixes from both the ISP, say for example i am accepting prefix 128.248.17.0 /24 from both the ISPs.


I noticed that randomly BGP chooses either ISP-1 or ISP-2 as the path to 128.248.17.0


What is the logic behind this best path selection. I donot think is following BGP best path selection ie, Adminweight, Local preference, IGP, MED etc.


Let me know you thoughts


RBK

mohamedtwair Mon, 06/29/2009 - 04:44
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Hi everybody,

can you please send me with this case:

We are trying to enable load balancing on Cisco 1801 across 2 Mbps WAN links. When tested separately, each link is reaching its full capacity. When aggregated, the links are hardly reaching 1.5 Mbps throughput.

please if you need any further info let me know.


Regards

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