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BGP Routes advertisement within main subnet

CCIE Aspirant

Hi all,

we have one router connected to two ISP. 

we have one /22 subnet.

To ISP 1 , we have advertised /23 (POOL A) and summarized /22

To ISP 2 , we have advertised remaining /23 ( POOL B) and summarized /22.

this helps us achieve the failover in case link to one ISP goes down.

now we wants to send and receive one /30 subnet from POOL B through ISP 1. is it possible? if yes kindly help me to achieve this task. if we policy route that subnet then it exit from our router to ISP 1 but return is still from ISP 2. so we need to route that subnet in way that we send that subnet from ISP 1 and receive the traffic for it from ISP 1 as well.

we are running BGP with both ISP.





6 Replies 6

MHM Cisco World
VIP Mentor VIP Mentor
VIP Mentor

you need to advertise the /30 with AS-Path prepend 

@MHM Cisco World 

thanks for the reply.

I have did for /30 with as-path prepend. But the return is still with isp2.

u mentioned /32. let me know why /32?

Jon Marshall
Hall of Fame Community Legend Hall of Fame Community Legend
Hall of Fame Community Legend


Have you spoken to your ISP because most ISPs won't accept any advertisements more specific than a /24 so they won't accept a /30 unless you have agreed that with them. 




so for any subnet within /23 we have to ask every time our ISP? can't we control it over selves.

and kindly tell me is there any changes we have to do from our RIPE account?


Richard Burts
Hall of Fame Community Legend Hall of Fame Community Legend
Hall of Fame Community Legend

I do not believe that there is anything you can do from your RIPE account. I am not clear why you need to route this /30 differently. But if you say that you need to do this then we will look for some way to accomplish it. The main part of the problem is that in Internet routing most ISP will not accept an advertisement for a prefix that is so short. So if it goes into the Internet with that /30 prefix the only prefix in the Internet routing table for it is the route through ISP2.

I would suggest an alternative that I believe is your best chance to achieve your requirement.

- set up PBR in your network so that traffic to Internet from that /30 is forwarded to the edge router connecting to ISP1.

- on that edge router set up address translation so that the traffic from the /30 is translated to an address in the block of ISP1. 

That traffic will be forwarded to its destination. The response will be sent to the edge router for ISP1, and the address will be translated back to the /30.



it was typo I correct it /30
there are four method to control the inbound traffic, please see below link

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