Posted on September 15, 2020 9:38 am
 |  Asked by Jacob Harris
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We’re trying to install a new design for our edge environment using vxlan between two datacenters.  For the most part it all works, but we are having an issue with a route advertisement we had working with a former technology using null routes.  The null routes however in an arista environment seem to break routing completely.

Short Story:

Attempting to install both a /24 and a /23 route in each of two datacenters:


Primary path for and also advertises (for backup path to


Primary path for and also advertises (for backup path to


Have attempted to place both and in the BGP config as network statements using the bgp advertise-inactive command, but it only advertises the address as I do not have a connected route in the /24 address space.

int vlan 101

ip address is installed (because the interface is on a /23 vxlan in both datacenters)

How can we force a route to be advertised via BGP if it doesn’t exist (used a null route in the past to accomplish this)




Answered on September 15, 2020 9:57 am

Hi Jacob,

Thanks for reaching out.

Since and are overlapping subnets we will have only one of the addresses present(connected) on the device unless VRF is configured.

The above requirement got accomplished when I configured the following:

Topology:(Refer to BGP_prefix attachment)


Switch1 and switch2 are eBGP peers.

Switch1 is advertising both and prefixes. switch 1 doesn’t have connected on any of the interfaces.

Switch1(conf)#ip route ------>Configure static route for the prefix that needs to be advertised.

Redistribute the above configured static route into BGP:

Switch1(conf)#router bgp 40

switch1(config-router-bgp)# redistribute static ---->When I configured "redistribute static" I could see getting advertised along with from switch1 to switch2.

Output on switch2:

switch2(config-router-bgp)#sh ip bgp

Network                Next Hop              Metric  LocPref Weight  Path

* >             0       100     0       40 i

* >            0       100     0       40 ?

Let me know if the above helps.

Another possible solution I could think of is to have VRF’s on both the switches(If VRF’s exist then we can have overlapping subnets) and then establish BGP neighborship between the VRF’s to advertise the prefixes.(However this is not an apt solution as use case seems to be different here).



Posted by Alexis Dacquay
Answered on September 15, 2020 11:21 am

Hi Jacob,
As you eluded to: tou can install a static route to null0 on each router, then advertise that specific statement in BGP.

! DC1
ip route null0
router bgp X

And do the equivalent in DC2.

You can add some scripts and tracking options to remove that static to null0 if you want to make it more dynamic.


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