Hello,
I assume you want to see the network 172.35.20.0/24 on the Branch router.
1.) Check if the network 172.35.20.0 has been properly redistributed from OSPF to BGP on the HQ site by issuing the command show ip bgp and looking for that network. Verify that the BGP reports this route as valid and best in your BGP table. Also, by issuing the command show ip bgp 172.35.20.0/24 make sure that it was announced to your ISP's router: look for the number of the update-group in that command's output and then checking who is a member of that update group by entering show ip bgp update-group
2.) Skip this step if you are running eBGP with your ISP. Otherwise, if you are running iBGP to your ISP, verify what is the next hop address attribute of the network 172.35.20.0/24 in your BGP table. If it is not 0.0.0.0 then it might be possible that the ISP does not know the route to that next hop. In this case, using "next-hop-self" in the BGP neighbor configuration might help.
3.) Assuming that the route has been announced to your ISP with a reachable next hop (you can verify this also by debugging the BGP), the ISP should have received that route from you. On the Branch, verify the BGP table by show ip bgp. If the network 172.35.20.0 is not there, you will probably need to discuss this with your ISP. If it is there but did not make it into your routing table, try to verify if its next hop is reachable.
Also, debugging the BGP on the Branch site might provide useful output - perhaps the AS_PATH already contains the AS number of the Branch site and the router rejects it, or there may be another hint as to what is the problem.
Best regards,
Peter
