>а как делаешь??

Идея такая - делать анонс в определенное время, в остальное время не анонсировать ничего.

router bgp 234
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network 213.1.1.0
---
neighbor 2.2.2.2 prefix-list PR out
neighbor 2.2.2.2 advertise-map RM2 non-exist-map RM1
---

ip prefix-list PR seq 10 permit 213.1.1.0/24

---

ip access-list extended net
deny   ip any any time-range TIME
permit ip any any time-range TIME1
ip access-list extended net1
deny   ip any any time-range TIME1
permit ip any any time-range TIME

route-map RM2 permit 10
match ip address net

route-map RM1 permit 10
match ip address net1

time-range TIME
periodic daily 10:00 to 13:00
!        
time-range TIME1
periodic daily 0:00 to 10:00
periodic daily 13:00 to 23:59