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Ahmed M Alzaeem

how to control upload & download by BGP attributes ? i have a topology !

hi ,

i want to ask about an issue that im facing from :

i have the topology below :

we have R1 & R2 as gateway routers an working bgp protocol with diffrent two independent isps

no ibgp between R1 & R2

R3 IS the lan route that has the cutomers networks

now have a look about the topology

on R1 im advertising the entire subnet 1.2.3/22  to isp1

on R2 only im advertising the subnet which is part of to isp2

there is a policy based routing on R1 as follow :


if the src ip from ==>send it R2 then isp2 

otherwise ==> send it to isp1

as u see , if the src ip on R3  was  then the upload direction will be from isp2 , otherwise , the upload direction  will be from isp1


if the src ip on R3  was . then the upload direction will be from isp1

note that the advertising was done by defualt Path attributes in  both R1 & R2

now ive discussed the topology and i will begin discuss the poblems :

problem 1 :

im fond of the subnet of

so that the upload and the download to be from isp2 , but the problem is ===> the upload is from isp2  but the download is from isp1

so  how to solve this problem ??

actually i thought with alot of solutions :

a - to part the subnet of into  many subnets parts  and ignore advertising the subnet of  from R1 too isp1  and by this i will solve the problem of the download direction .

but in the same time , i found that parting the subet with alot of subnets will annoy my bgp table and will have alot of individual subnets and may cause an errors , so i began to think that :

on R1 i will still advertsie the entire /22 subnet but i want to play with PA , so that the PA of /27 subnet  that will be advertised on R2 will be better than the subnet /22 that will be advertised on R1 ,

so agian , does my opinion is good ??

and what path attributes i can play with to achive my  goal ??

does using path attribute of set as prepend  can solve the problem ??  i mean i will set prepend to the subnet /22 with  my ASN  many times ,and as a result the /27 will be better than /22 because it has lowe ASN path .

does set metric is applicable ?

plz advice about the best solution to achive my goal  with less configs because im facing production networks with heavy internet traffic and dont want any msikates

i will  discuss the other problesm after solving the 1st problem


Ahmed M Alzaeem

hi ,

i performed a PA on R1  about the subnet /22

i createda route map and set as path prepend x x x x x x x

but the same issue , the downlink is still from isp1 ,

i  typed set origin undeermined  but the same issue ,  the downlink is still from isp1 !!!!!!

here is my config :


route-map zzzzL, permit, sequence 20

  Match clauses:

    ip address prefix-lists: newsubnet

  Set clauses:

    as-path prepend xxxx5 xxxx5 xxxx5 xxxx5 xxxx5 xxxx5 xxxx5

    origin incomplete

  Policy routing matches: 0 packets, 0 bytes


the config was done oN R1 and still facing the same problem



If I understand this your situation properly, you may be facing Asymmetric routing issue, for this is common in BGP, and you can only set it right, if you have agreement with your ISP to help you set their MED attribute value to favor you.