BGP advertising same networks on 2 ISP independently (without iBGP)

[sravani14]

I am new to BGP. We have established BGP neighborship with 2 uplink providers advertising different networks and receiving default routes from both using same AS number. We couldn't establish iBGP as we have a requirement to use both ISP A and ISP B for specific networks to reach to internet.

Requirement:

- Network A should prefer ISP A for incoming and outgoing

- Network B should prefer ISP B for incoming and outgoing

- Network C should prefer ISP B but in case ISP B goes down should prefer ISP A

I have attached the overview network diagram below for reference. Please advise

[paul driver]

Hello

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@sravani14 wrote:

I am new to BGP. We have established BGP neighborship with 2 uplink providers advertising different networks and receiving default routes from both using same AS number. We couldn't establish iBGP as we have a requirement to use both ISP A and ISP B for specific networks to reach to internet.

Requirement:

- Network A should prefer ISP A for incoming and outgoing

- Network B should prefer ISP B for incoming and outgoing

- Network C should prefer ISP B but in case ISP B goes down should prefer ISP A

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With Incorporating a mixture or aspath- prepending ,local -preference and as-path filtering ,You shroud still be able to achieve your desired goal and utilize ibgp between the two sites at the same time - 
I assume both sites are using the same ASN?i
What routing protocol is running between the site A-B? 

[sravani14]

Hi Paul,

Thank you for your response.

• Internal network (connectivity between sites) is mixture of layer 2 (Vlans), static routing and work in progress to migrate to OSPF.
• Yes both the sites are having same AS number.
• Currently the link between the remote sites is work in progress and not reliable at this moment. So we would like to have the network independent but achieve redundancy for certain networks in case of downtime.

Can we implement the above network setup without iBGP. Please advise.

[paul driver]

Hello

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@sravani14 wrote:

Can we implement the above network setup without iBGP. Please advise.

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Yes you can however you would still require a reliable site-site connection for failover, May i ask what prefixes are you receiving from each ISP, would this be partial, full or just defaults?

[sravani14]

Hi Paul,

Thank you for clarifying. Currently we are receiving only defaults from both ISP.

Thank you

[paul driver · Accepted solution]

Hello

Okay based on your received bgp default routes from either isp what you can do it advertise ospf defaults with different metric-types ( ISP1 ospf default metric-type 1) (ISP2 ospf default metric-type 2)

So default traffic will go via ISP1 however you then policy route certain prefixes via ISP2 so even though you have one preferred default route to ISP1certain defined networks will route via to ISP2 and if the reachability is lost from ISP1 then its advertised bgp default will be removed and so will its related ospf default and as such failover to go via ISP2  for all traffic.

[sravani14]

Hi Paul,

Current setup

- A.A.A.0/24 is advertised to ISP A and ISP B. Applied AS Path prepend(4 times) on ISP B to make it less prefer. iBGP is not established between 2 internal routers.

- Manipulated internal traffic to exit desired ISP A for incoming and outgoing traffic with combination of static and OSPF configuration.

- General internet connection is working fine and tracert shows towards ISP A but we are facing VPN issue when network is advertised on both ISP

Please advise what is the alternative to AS path prepend to make ISPB idle for certain networks