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multihoming to two providers with two routers

DannyHuston
Level 1
Level 1

Let's say I have Router A thats peering with ISP A and then Router B peering with ISP B (they talk in eBGP).  Both routers take full routes from their respective ISPs and the routers iBGP peer with each other.  This would mean that on Router A,  it would receive the routes learned from Router B as iBGP routes (vice versa) and become least preferred since eBGP is more preferred based on admin weight, correct?  How would outbound traffic be able to choose which ISP is the most preferable path to reach a destination if there are routes from both ISPs?   And my other question would be how would/should traffic reach these two routers if say the hop before it are all firewalls? 

3 Replies 3

milan.kulik
Level 10
Level 10

Hi,

ad "...it would receive the routes learned from Router B as iBGP routes (vice versa) and become least preferred since eBGP is more preferred based on admin weight, correct?"

Not necessarilly, as eBGP/iBGP comparing is Step 7 withing BGP Best Path Selection Algorithm.

See

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

for details to understand how BGP best path selection works.

ad "... how would/should traffic reach these two routers if say the hop before it are all firewalls?"

You can configure  HSRP on the routers, e.g., and let the FWs to forward the data to the HSRP virtual IP address.

Or some FWs can participate in BGP peering (but the security guys don't like this option usually).

HTH,

Milan

oh I see, so if I understand it, if a router receives eBGP and iBGP for the same prefix it will look at best path and install whichever is preferred.  if there is yet ANOTHER route for the same prefix learned from some other protocol llike eigrp or ospf then it uses admin distance?  Is that correct?

Hi,

yes, correct!

BR,

Milan

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