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

#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-filter is for incoming (received) prefixes, out-filter is 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 use BGP 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
add chain=isp1-out prefix=10.1.2.0/24 action=accept
#discard the rest
add chain=isp1-out action=discard

Outgoing filters to ISP2:

/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
add chain=isp2-out prefix=10.1.2.0/24 action=accept set-bgp-prepend=3
#discard the rest
add chain=isp2-out action=discard

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.

Outgoing filters to ISP1:

/routing filter
#accept our networks and prepend second network
add chain=isp1-out prefix=10.1.1.0/24 action=accept
add chain=isp1-out prefix=10.1.2.0/24 action=accept  set-bgp-prepend=3
#discard the rest
add chain=isp1-out action=discard

Outgoing filters to ISP2:

/routing filter
#accept our networks and prepend first network
add chain=isp2-out prefix=10.1.1.0/24 action=accept set-bgp-prepend=3
add chain=isp2-out prefix=10.1.2.0/24 action=accept
#discard the rest
add chain=isp2-out action=discard

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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