As-path prepend not work properly as per our require when Multiple ISP's b/w in Single RTR

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Dear Experts,

We are using multiple ISP's b/w in our single Cisco 3845 RTR ( both are eBGP) , i have configured as-path prepending for inbound traffic manage
as per our required but we are observing as-path prepending not work properly when multiple ISP's b/w in single router.

Herewith attached eBGP diagram.

We have our own ASN with /22 ip pools.

Our ASN- 123456
ISP-A ASN- 10123
ISP-B ASN- 9498

130.25.51.0/24
130.25.52.0/24
130.25.53.0/24
130.25.54.0/24

We have configured as per our require 1st prefere ip pools 130.25.51.0/24 incomming return path from ISP-B  only, but when it goes down then it comes from ISP-A and remaing IP Pools 130.25.52.0/24 , 130.25.53.0/24 & 130.25.54.0/24 first prefere Incomming return path from ISP-A only and visa-versa.

Here is my configuration route-map match with as-path prepend.

ip prefix-list ISP-B seq 5 permit 130.25.51.0/24
ip prefix-list ISP-A seq 10 permit 130.25.52.0/24
ip prefix-list ISP-A seq 15 permit 130.25.53.0/24
ip prefix-list ISP-A seq 20 permit 130.25.54.0/24

route-map ISP-B permit 10
match ip address prefix-list ISP-B

route-map ISP-B permit 20

match ip address prefix-list ISP-A
set as-path prepend 123456 123456 123456

route-map ISP-A permit 10
match ip address prefix-list ISP-A

route-map ISP-A permit 20
match ip address prefix-lsit ISP-B
set as-path prepend 123456 123456 123456

router bgp 123456

nei 59.123.45.33 route-map ISP-B out

nei 115.202.135.77 route-map ISP-A out

clear ip bgp *

Still as-path prepend not working properly we are observing 1st IP pools return traffic  also comming from ISP-A & B and remaing IP pools return trafic aslo
comming from both the ISP it is not like that exact as per our require. but when i show on looking glass it's showing correct as per our require but when i tracer route
any Ip address of 1st pool which are require from ISP-B it's comming from ISP-A So i am comnfuse more why it's heppaned ??

So kinldy help me for the same.

We greatly appreciate your cooperation.

Thanks in ADV,

[Giuseppe Larosa · Accepted solution]

Hello,

generally speaking AS path prepending provide a way to attempt to influence return path but it does not provide guarantees as you cannot control route policies of other autonomous systems

Your ISPs may have a BGP relationship and agreements between them that are not under your control

For the  traceroute a lot depends from where the traceroute to the pool1 IP address is performed.

if you see the expected AS paths on the looking glass your configuration is fine and this is what you can achieve

Hope to help

Giuseppe

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D/Giuseppe,

Thank you very much , accourding to you it is not under my control ( returrn path) that's why we have facing one of the  issue regarding ISP-B b/w full utilized every time bcoz we are using unequal cost b/w in single RTR eBGP.

ISP-A ( 100 mb) it utilized 60 to 70 mb sometimes 55 mb

ISP-B ( 45 mb) it utilized 45 mb full everytime

So in this case we are getting so many speed complains bcoz ISP- B b/w fully utilized everytime.

Kindly suggest what i  have to do

Thanks in ADV,

[vinayk3]

Hi ,

From the scenario i assume that you dont have any control on ISP routers. and if you want to do load balancing you can opt for PFR.

Else if you only want to use the BGP attributes to load balacne the traffic the best way to achieve this in my view is to divide your subents and decide which link you want to use for those subnets. Now use both Local preference and MED to influence the incoming and outgoing traffic.

I think this should resolve your problem.

Thanks

Vinay