IOS-XR default settings on route announcement differs from the standard IOS behaviour. IOS defaults to sending updates as long as a network statement has been defined in the router process. IOS-XR has a implicit deny rule instead.

In example below a eBGP neighbour configuration for a IOS-XR router:

configure
!
router bgp 65004
 address-family ipv4 unicast
  network 192.0.2.0/24
 !
 neighbor 10.0.0.9
  remote-as 65000
  password encrypted 0822455D0A16
  update-source GigabitEthernet0/0/0/2
  address-family ipv4 unicast
 end

IOS-XR default setting is to drop eBGP announcements if not explicitely configured to do it. Verifying applied policy rules:

RP/0/0/CPU0:XRV4#sh bgp neighbor 10.0.0.9 | in policy
Wed Mar  6 20:42:54.181 UTC
  eBGP neighbor with no inbound or outbound policy; defaults to 'drop'
    No policy: 5, Failed RT match: 0
    By ORF policy: 0, By policy: 0

To announce networks with eBGP a route-policy needs to be defined and bound to the eBGP neighbor. In example below a pass-all policy is shown:

configure
!
route-policy rp-bgp-allow-all
  pass
end

Having a route policy configured it needs to be applied to the BGP neighbor within the address-family:

configure
!
router bgp 65004
 neighbor 10.0.0.9
  address-family ipv4 unicast
   route-policy rp-bgp-allow-all in
   route-policy rp-bgp-allow-all out
  !
 !
!
end

Verifying the configuration process. Router with applied routing policy on according eBGP neighbor:

RP/0/0/CPU0:XRV4#sh bgp neighbor 10.0.0.9 | i Pol
Wed Mar  6 20:44:52.931 UTC
  Policy for incoming advertisements is rp-bgp-allow-all
  Policy for outgoing advertisements is rp-bgp-allow-all

NX-OS has the same defaults as IOS-XR has.