BGP Question

RouterOS Forwarding Protocols
1

Hello guys!

I need help with BGP.
I have simple multihome setup with my two upstream providers, and for now, it works very well.

Problem:

I have /21 prefix that i advertise, i want to split it into 8 /24 blocks, so i can route some IP addresses to ISP1 and other part to ISP2.
Is this possible? i readed very much about BGP on few forums and i didn’t find how to solve this problem, i founded some posts about ‘prepending’ but it’s not very clear to me, either…

Please give me some advice…

Regards!

2

http://wiki.mikrotik.com/wiki/Manual:Simple_BGP_Multihoming

3

I readed that article, i have /21 not two /24 prefixes, like in example…

Nevermind, i found a solution for my problem, thanks anyway! :slight_smile:

4

Hi,

A /21 is simply 8 /24 subnets aggregated. As such splitting this to specific providers is simply a matter of bgp filters for the relevant providers.

  • Setup a BGP session to each provider.
  • Add all the /24’s to your bgp networks.
  • Setup filters for the specific /24’s you want to announce to each of the providers.
  • DONE

Other items to consider

  • if you are in the RIPE region (as example) you’d have to add route entries for all the /24’s otherwise you’re likely to end up having some of your route announcements dropped by BGP filters on a number of larger providers.
  • A number of providers in Europe certainly suggest a minimum announce of a /23
  • By just announcing specific routes to specific providers you don’t have any redundancy - if one provider fails you stop announcing those /24’s and they dissapear off the internet.

A workaround for the last item is to still announce the /21 to both providers. The issue here is that in such a case some upstream networks might still prefer the /21 route rather than the /24’s – it should not happen but I’ve seen it a couple of times.

Hope this helps for future reference.

5

Thank you very much friend, it’s good to know this and yes i’m using this workaround (announcing /21 to both providers) and it’s working good for now… :slight_smile:

Regards!

6

Glad I cloud help mate. Sometimes trying to figure stuff out at odd times of day is a lonely place to be :wink:

7

Hello friend!

I know this is quite old topic but i need your help once again :slight_smile:

Few days ago i realized that my download traffic is going through ISP2 and upload through ISP1, routing filters doesn’t works like earlier too!
Let’s say, when i want to assign one of my /24 blocks to ISP1 not ISP2 it doesn’t work or upload traffic is going through ISP1 and downlad through ISP2…
I didn’t changed anything, and my upstream providers didn’t change anything too… i’m lost for now, i exported routing filters below.

/routing filter
add action=accept chain=ISP1-OUT disabled=no invert-match=no prefix=\
    XXX.XXX.216.0 prefix-length=21 set-bgp-prepend-path=""
add action=accept chain=ISP2-IN disabled=no invert-match=no prefix=0.0.0.0/0 \
    prefix-length=0 set-bgp-prepend-path="" set-out-nexthop=109.198.5.229
add action=accept chain=ISP2-IN disabled=no invert-match=no prefix-length=16 \
    set-bgp-prepend-path=""
add action=discard chain=ISP2-IN disabled=no invert-match=no \
    set-bgp-prepend-path=""
add action=accept chain=ISP2-OUT disabled=no invert-match=no prefix=\
    XXX.XXX.216.0 prefix-length=21 set-bgp-communities="" set-bgp-prepend=4 \
    set-bgp-prepend-path=""
add action=discard chain=ISP1-OUT disabled=no invert-match=no \
    set-bgp-prepend-path=""
add action=accept chain=ISP1-OUT disabled=no invert-match=no prefix=\
    XXX.XXX.221.0/24 prefix-length=24 set-bgp-prepend-path=""
add action=accept chain=ISP1-OUT disabled=no invert-match=no prefix=\
    XXX.XXX.216.0/24 prefix-length=24 set-bgp-prepend-path=""
add action=passthrough chain="" disabled=no invert-match=no prefix=0.0.0.0/0 \
    prefix-length=0-32 set-bgp-prepend-path=""
add action=passthrough chain=ISP2-OUT disabled=no invert-match=no \
    prefix-length=24 set-bgp-prepend-path=""
add action=accept chain=ISP1-OUT disabled=no invert-match=no prefix=\
    XXX.XXX.219.0/24 prefix-length=24 set-bgp-prepend-path=""
add action=accept chain=ISP2-OUT disabled=no invert-match=no prefix=\
    XXX.XXX.222.0/24 prefix-length=24 set-bgp-prepend-path=""
add action=accept chain=ISP2-OUT disabled=no invert-match=no prefix=\
    XXX.XXX.223.0/24 prefix-length=24 set-bgp-prepend-path=""
add action=accept chain=ISP1-OUT disabled=no invert-match=no prefix=\
    XXX.XXX.220.0/24 prefix-length=24 set-bgp-prepend-path=""
add action=accept chain=ISP1-OUT disabled=no invert-match=no prefix=\
    XXX.XXX.217.0/24 prefix-length=24 set-bgp-prepend-path=""
add action=accept chain=ISP2-OUT disabled=no invert-match=no prefix=\
    XXX.XXX.218.0/24 prefix-length=24 set-bgp-prepend-path=""

Best Regards!

8

Hi

Can I ask you how my ISP Source you have ..?

Also See the below link:
http://mum.mikrotik.com/presentations/AE12/ali.pdf

take a look to my diag

Here is BGP config export..



/routing bgp network
add disabled=no network=x.x.224.0/23 synchronize=no
add disabled=no network=x.x.226.0/23 synchronize=no
add disabled=no network=x.x.224.0/22 synchronize=no

…
..
..

/routing bgp peer
add address-families=ip as-override=no default-originate=if-installed disabled=no hold-time=3m in-filter=
IN-Fillter_peer_MAZ instance=default multihop=yes name=peer_MAZ nexthop-choice=default out-filter=
Out-Fillter_peer_MAZ passive=no remote-address=x.x.71.73 remote-as=(AS number) remove-private-as=no
route-reflect=no tcp-md5-key=“” ttl=default use-bfd=no
add address-families=ip as-override=no default-originate=if-installed disabled=no hold-time=3m in-filter=“”
instance=default multihop=yes name=peer_IQ nexthop-choice=default out-filter=Out-Fillter_peer_IQ passive=no
remote-address=x.x.193.3 remote-as=(AS number) remove-private-as=no route-reflect=no tcp-md5-key=“” ttl=default
use-bfd=no

(There is Multihop option its need to set yes if there is indirect connection between you router and remote bgp router , its mean there some router between your router and the router you can to make bgp session with it..)
..
..
..

/routing filter
add action=accept chain=Out-Fillter_peer_IQ comment=“IQ Ann…::::::::::::::::::::::::::::” disabled=no invert-match=no prefix=x.x.226.0/23
prefix-length=23 set-bgp-prepend-path=“” set-bgp-weight=100
add action=accept chain=Out-Fillter_peer_IQ disabled=no invert-match=no prefix=x.x.224.0/22 prefix-length=22 set-bgp-prepend-path=“”
set-bgp-weight=200
add action=discard chain=Out-Fillter_peer_IQ disabled=no invert-match=no set-bgp-prepend-path=“”
add action=accept chain=Out-Fillter_peer_MAZ comment=“MAZ Ann…:::::::::::::::::::::::::::::::::” disabled=no invert-match=no prefix=
x.x.224.0/23 prefix-length=23 set-bgp-prepend-path=“” set-bgp-weight=100
add action=accept chain=Out-Fillter_peer_MAZ disabled=no invert-match=no prefix=x.x.224.0/22 prefix-length=22 set-bgp-prepend-path=“”
set-bgp-weight=200
add action=discard chain=Out-Fillter_peer_MAZ disabled=no invert-match=no set-bgp-prepend-path=“”

..
..
..


advertisement through bgp

admin@route> routing bgp advertisements print

PEER PREFIX NEXTHOP AS-PATH ORIGIN LOCAL-PR
peer_MAZ x.x.224.0/23 x.x.99.2 igp
peer_MAZ x.x.224.0/22 x.x.99.2 igp
peer_IQ x.x.226.0/23 x.x.210.154 igp
peer_IQ x.x.224.0/22 x.x.210.154 igp



Hope fully this sample can be help full

Also there is thing you need to do with RIPE … for IP announcment…


Good Luck..!

Ali Sami
MTCNA,MTCRE,MTCWE,MTCTCE, Mikrotik Trainner
skype: alixon78
BGP.jpg