manual-simple bgp multihoming

7/29/2019 Manual-Simple BGP Multihoming

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Manual:Simple BGP Multihoming 1

Manual:Simple BGP Multihoming

Applies to RouterOS:all

Setup

Ilustration below shows simple multihomed BGP setup. This setup can be used for load sharing

between ISPs or one ISP as main and other ISP as backup link.

Lets say that local Internet registry assigned to us two /24 networks: 10.1.1.0/24 and 10.1.2.0/24 and our AS is 30

(Private AS cannot be used in such setups). First network entirely is used for workstations in our corporate network.

Part of the other network is also used for workstation and another part is reserved for server. At this point our

company has only one server with address 10.1.2.130

The goal is advertise our assigned networks to BGP peers and use only one provider as main link, ISP2 link is for

backup only.

Note: This example does not show how to provide connectivity between core router, local networks and

servers

BGP Peering

Consider that IP connectivity between ISPs edge routers and Our Core router is already set up and

working properly. So we can start to establish BGP peering to both ISPs.

#set our AS number

/routing bgp instance

set default as=30
http://wiki.mikrotik.com/index.php?title=File%3AIcon-note.pnghttp://wiki.mikrotik.com/index.php?title=File:Bgp-multihoming.pnghttp://wiki.mikrotik.com/index.php?title=File%3AVersion.png


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#add BGP peers

/routing bgp peer

add name=toISP1 remote-address=192.168.1.1 remote-as=10

add name=toISP2 remote-address=192.168.2.1 remote-as=20

If everything is set up properly, peer should have E (established) flag and router should receive bunch of BGP routes

from both ISPs

[admin@RB1100test] /routing bgp peer> print

Flags: X - disabled, E - established

# INSTANCE REMOTE-ADDRESS REMOTE-AS

0 E default 192.168.1.1 10

1 E default 192.168.1.2 20

Network Advertisements and Routing Filters

Now we can start to advertise our networks and filter out all other unnecessary advertisements.

First step is to advertise our networks

/routing bgp network

add network=10.1.1.0/24 synchronize=no

add network=10.1.2.0/24 synchronize=no

Next step is to specify which routing filter chains will be used

/routing bgp peer

set isp1 in-filter=isp1-in out-filter=isp1-out

set isp2 in-filter=isp2-in out-filter=isp2-out

in-filteris for incoming (received) prefixes, out-filteris for advertised prefixes.

Main/Backup link setup

After chains are specified we can accept our networks and drop everything else as we are not transit provider. As

you know one of the BGP attributes that influence best path selection is AS Path length (shorter AS Path is more

preferred). So as we want ISP2 to be backup only, we will useBGP AS prepend(increase length of AS path) to force

incoming traffic through ISP1.

Outgoing filters to ISP1:

/routing filter

#accept our networks

add chain=isp1-out prefix=10.1.1.0/24 action=accept


#discard the rest

add chain=isp1-out action=discard


/routing filter

#accept our networks and prepend AS path three times

add chain=isp2-out prefix=10.1.1.0/24 action=accept set-bgp-prepend=3


#discard the rest


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We also do not need any routes from both ISPs, because default route is used to force outgoing traffic through ISP1

and leave ISP2 as backup.

/routing filter

add chain=isp1-in action=discard

add chain=isp2-in action=discard

/ip route

add gateway=192.168.1.1 check-gateway=ping

add gateway=192.168.2.1 distance=30 check-gateway=ping

Load sharing setup

Using previous setup we are kind of wasting one link. So it is possible to redesign our setup as illustrated below to

utilize both links.

The same as in previous setup BGP AS prepend will be used to achieve our goal. This time we will advertise one of

the netowrks to ISP1 without prepend and another network prepended three times. The opposite for ISP2.


/routing filter

#accept our networks and prepend second network



#discard the rest


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/routing filter

#accept our networks and prepend first network



#discard the rest


Configuration above is only for packets going to our network. There are several options how to deal with packets

going from our network:

leave gateways as in main/backup configuration - this will result in only one link utilized and asymmetric routing

use policy routing to force outgoing packets over the same link as incoming

use BGP to receive full routing tables from both peers and using BGP attributes make part of the routes available

through one link and other part through another link. For example, traffic local to your country is sent over ISP1

the rest is sent over ISP2.

All those methods are covered in other articles and will not be shown here.

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Article Sources and Contributors 5

Article Sources and ContributorsManual:Simple BGP Multihoming Source: http://wiki.mikrotik.com/index.php?oldid=19642 Contributors: Marisb

Image Sources, Licenses and ContributorsImage:Version.png Source: http://wiki.mikrotik.com/index.php?title=File:Version.png License: unknown Contributors: Normis

File:bgp-multihoming.png Source: http://wiki.mikrotik.com/index.php?title=File:Bgp-multihoming.png License: unknown Contributors: Marisb

Image:Icon-note.png Source: http://wiki.mikrotik.com/index.php?title=File:Icon-note.png License: unknown Contributors: Marisb, Route

File:bgp-multihoming-download-sharing.png Source: http://wiki.mikrotik.com/index.php?title=File:Bgp-multihoming-download-sharing.png License: unknown Contributors: Marisb

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