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New Member

BGP Multihoming to single ISP

Drawing1.jpg We have a hosting center and a corporate office that will be connecting to the same ISP.  The ISP gave us a /24 netblock and the hosting center and corp office are connected via metro ethernet.  I would like to use a /25 at each location and have failover capabilites through the metro in the event of one of the Internet connections failing.  My rough-draft config is attached. I desperately need configuration assistance.

Thank you in advance,


Hall of Fame Super Silver

Re: BGP Multihoming to single ISP

Hello Jason,

to advertise the non prepended IP subnet you need


route-map HosttoISP permit 20


route-map corpToISP permit 20

the first block provides the backup path but you still need the additional block in order to advertise the other /25 IP prefix.

As I had explained in your previous thread

Without route-map block 20 you are advertising only one prepended IP prefix to ISP on each of your routers.


to have this working in real world you need an agreement with ISP in order for having them to accept your /25 advertisements.

It would be wise to also generate an aggregate-address

Hope to help


New Member

Re: BGP Multihoming to single ISP


I updated the configuration for the 2nd netblock, but not sure how to use the aggregate command. Can you assist?

router bgp 65000
neighbor remote-as 1234
neighbor route-map CorpToISP out
next-hop self

ip prefix-list test seq 5 permit

route-map CorpToISP permit 10
match ip address prefix test
set as-path prepend 65000 65000

route-map CorpToISP permit 20
match ip address prefix netblock2
set as-path prepend 65000

Hall of Fame Super Silver

Re: BGP Multihoming to single ISP

Hello Jason,

it should be simply

router bgp 65000

aggregate-address mask

but again it should be permitted by one of the two prefix-lists you use.

I would send a prepended aggregate out one router and a non prepended one out the other router.

Note also that  you don't need to do AS path prepend on second block, a single occurrence of AS 65000 is automatically added before sending the updates to ISP provider router.

You cannot see this from your router but it happens.

Hope to help