BGP Multihoming - Dual ISP

[kramer977]

Hello Experts,

I'm looking for some advice and feedback on a set up we're in the midst of building.

Basically, we're looking at a Multihomed Setup via BGP which include 2 Ethernet dedicated lines from different providers at a single data center (WAN Edge).  Connectivity is direct with the client site, and the same exact routes will be received on both lines.  BGP AS number is the same on both, along with the routes coming in.  The set up includes dedicate routers for each link.

Traffic will be flowing outbound to the client and they want to receive traffic from our end on both lines, equally.  My understanding is that this cannot be done and that we can only load share based on specific IP ranges and subnets broken down, which is not what the client wants.  Is this a correct statement or can we send traffic outbound on both links? 

Another question, once connectivity is up and configured, will the routes not take the path which has the Lowest IP configured as the neighbor address? All other metrics should be equal.  My understanding is that all traffic will traverse the one dedicated line.

Just looking for some clarification.  

Thanks

[Giuseppe Larosa]

Hello Kramer977,

if inside the network on the right of the network diagram you are using iBGP border routers and multilayer switches are in same BGP AS your understanding is correct and a single best path is chosen using best IGP metric to next-hop and then the lowest BGP router-id.

However, BGP provides some means for implementing iBGP multipath if the Layer3 switches support the following command

router bgp 50

address-family ipv4 unicast

maximum-paths ibgp 2

!

or

router bgp 50

maximum-paths ibgp 2

!

You should be fine if IGP metric to the two border routers is the same from the point of view of the Layer 3 switch.

On border routers you may want to use next-hop self towards the two L3 swiches to avoid issues with the original eBGP next-hop.

Hope to help

Giuseppe

[kramer977]

Ciao Giuseppe,

Sorry, I attached the wrong diagram, those are L3 routers which perform the peering with the Edge client routers.

The relationship will be EBGP because we are neighbouring routers with different AS's (Our AS with the clients AS).  Sorry for the confusion.  

The only differentiating factor I see which would manipulate the routes and favor one over the other is the Neighbor addresses.  The client believes traffic should go out of both links equally for all of the same advertised external blocks.

[Giuseppe Larosa]

Hello krame977,

for EBGP use

router bgp 50

maximum-paths 2

to install two eBGP advertisements on each L3 switch. This feature is even older the iBGP multi-path.

Hope to help

Giuseppe