How to load divide using BGP with multiple provider based on Source address

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Jan 14th, 2010
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I have two networks ( say 10.20.20.0/24 and 10.20.21.0/24). I use two providers using BGP to provide redundant connections. I have made one of them (Say ISPA ) preferred by setting its local preference to 140 in contrast to ISP B with local preference 120 and longer AS path to influence outbound and inbound paths. What I would like to do is to route 10.20.20.0/24 through ISPA and 10.20.21.0/24 through ISPB. At the same time, if any of the ISP links fail, other should then provide route for both networks. Please look at attached .png image file. Please advise soon.



Brijesh

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Correct Answer by Giuseppe Larosa about 7 years 2 months ago

Hello Brijesh,


you should advertise 10.20.20.0/24 and 10.20.20.0/23 to ISPA, 10.20.21.0/24 and 10.20.20.0/23 to ISPB.


In this way you take advantage of most specific route. A /24 prefix is accepted on the internet.


The /23 aggregate is needed for fault tolerance. You need to verify that ISPs are not filtering the /24. You may need an agreement with them.


I assume that these two IP subnets are placeholders for two public ip addresses blocks otherwise NAT is involved and other features would be necessary.


To send traffic to ISPA based on source IP address you need to add PBR policy based routing to the picture. BGP is not enough alone for this.


PBR = policy based routing


http://www.cisco.com/en/US/docs/ios/12_3t/12_3t4/feature/guide/gtpbrtrk.html



Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Thu, 01/14/2010 - 04:44
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Hello Brijesh,


you should advertise 10.20.20.0/24 and 10.20.20.0/23 to ISPA, 10.20.21.0/24 and 10.20.20.0/23 to ISPB.


In this way you take advantage of most specific route. A /24 prefix is accepted on the internet.


The /23 aggregate is needed for fault tolerance. You need to verify that ISPs are not filtering the /24. You may need an agreement with them.


I assume that these two IP subnets are placeholders for two public ip addresses blocks otherwise NAT is involved and other features would be necessary.


To send traffic to ISPA based on source IP address you need to add PBR policy based routing to the picture. BGP is not enough alone for this.


PBR = policy based routing


http://www.cisco.com/en/US/docs/ios/12_3t/12_3t4/feature/guide/gtpbrtrk.html



Hope to help

Giuseppe

brijeshpradan Thu, 01/14/2010 - 05:08
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Thanks for the reply.


I have another question related to my problem. Can I use send-community to influence outgoing paths to internet? If so, what configurations are needed at my end and at my provider's end ( please refer to diagram attached above).

Giuseppe Larosa Thu, 01/14/2010 - 05:21
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Hello Brijesh,

thanks for your kind remarks.


The use of BGP communities is technically possible when you advertise multiple routes to each ISP.


You should use a route-map to set specific BGP communities on specific routes.

The route map can then be applied outbound towards ISP eBGP neighbor.


inside the route map the set command to be used is:


set community 12:34 ?
  <1-4294967295>  community number
  aa:nn           community number in aa:nn format
  additive        Add to the existing community
  internet        Internet (well-known community)
  local-AS        Do not send outside local AS (well-known community)
  no-advertise    Do not advertise to any peer (well-known community)
  no-export       Do not export to next AS (well-known community)


important additive keyword because actually multiple BGP communities can be associated to a BGP prefix (instead only a single route tag can be associated to an IGP route)


the neigh ... send-community is needed on both sides


What is most important ISP has to agree on using BGP communities and what BGP communiities values to use.


An handy command when working with BGP community is

ip bgp community new-format


that allows to see BGP communities as ASN:value in router show commands


in your case you can use two different BGP communities one for /24 prefix and one for /23 prefix.


the ISP knowing this can perform some actions based on agreed BGP community values.


It can be of limited use in your case.


Hope to help

Giuseppe

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