BGP

Unanswered Question
Apr 22nd, 2010
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Hi,


The current set up is two internet routers with ebgp connection to two different service providers and learning full internet routes. And we would like to make use of both the links and load share traffic.


Can someone share a working configuration for load sharing traffic across two ISP Connections to the internet. Also, any experiences you can share on this topic. And FWIW, I have already looked at cisco.com documentation and need some real world experience and feedback.


Thanks,

Brian

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Giuseppe Larosa Thu, 04/22/2010 - 12:41
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Hello Brian,

have you got your own public IP address block?

Have you got your own BGP AS number?

I suppose yes because you say you receive full internet tables over them


in this case you can achieve outbound load balancing, for inbound direction results cannot be as good as in the opposite direction.


for outbound direction:

internal routers need to see two default routes in the IGP protocol generated by the two edge routers

an iBGP session between the two routers is a wise step.


or if there are clients connected directly to edge BGP routers you can use GLBP for this.


to try to influence return traffic you would need to have at leat two /24 public IP subnets and to use selective prepending in order to send one network net1 not prepended and one net2 prepended and viceversa to ISP2


This doesn't guarantee a fair use of the two links on the return path but just that both will be used.

The reason is that one ISP can be more dominant and this can compensate for your prepending


note: you can prepend only your own AS number to avoid connectivity problems


route-map selective_prepend_toISP1 permit 10

match ip address prefix net2

set aspath prepend yourAS yourAS yourAS

! second block for net1

route-map selective_prepend_toISP1 permit 20

match ip address  prefix net1


router bgp yourAS

neigh ISP1 route-map selective_prepend_toISP1 out


on router R2 the opposite:


route-map selective_prepend_toISP2 permit 10

match ip address  prefix net1

set aspath prepend yourAS yourAS yourAS

!  second block for net1

route-map selective_prepend_toISP2 permit 20

match ip address  prefix net2


router bgp yourAS

neigh ISP2 route-map selective_prepend_toISP2 out




in case you haven't your own AS number and yout own IP public ip address block NAT is involved


http://www.cisco.com/en/US/tech/tk648/tk361/technologies_white_paper09186a0080091c8a.shtml


Hope to help

Giuseppe

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