Advertise network LE /25 into eBGP

[Timohamoto]

Hello community.

At nowadays by having own AS, and PI /24 network

Does it possible to devide that network let's say /25 and do advertise each part by peering with two different ISP into global eBGP?

I asking cause currious if such networks /25, /26, ... are legal in eBGP and won't be filtered by some AS

[MHM Cisco World]

why not if the SP will advertise it to other Branchs

[Giuseppe Larosa]

Hello @Timohamoto ,

it is possible with cooperation and agreement between the two upstream providers.

Over the public internet the aggregate /24 will be published by your upstream providers. They can agree on accepting the two /25 sub blocks on their direct peering between each other and this make possible what you would like to achieve.

Hope to help

Giuseppe

[Timohamoto]

Hello @Giuseppe Larosa appreciate your answer.

Please take a look to attached diagram.

In case two dedicated offices with 2 upstream on each.

• ISP 1, and ISP2 will receive 100.100.1.0/25
• ISP 3, and ISP4 will receive 100.100.1.1/25

Will that works? cause as I'm aware in modern eBGP everything what's LE /25 will be filtered.

And no need ISP1, ISP2 to aggregate it to /24

as there no reason cause there no services running with another part of IP addresses. Cause as I assume if they will do it, and traffic will arrive to Office1 for ip from pool of Office2 it's will be just dropped by border routers.

[Giuseppe Larosa]

Hello @Timohamoto ,

the two subnets should be

100.100.1.0/25

and

100.100.1.128/25

However, the scenario you have presenteds  is quite complex as you have one office in Europe and one office in Asia.

The number of involved ISPs is four and they are two in Europe and two in Asia.

In addition to this, your public IP address space /24 i.e. 100.100.1.0/24 in our example has been received either in Europe from RIPE either in ASIA from APNIC the two RIR Regional Internet Registry for Europe and ASIA.

In a case like yours you can do one of  the followings:

a) apply for another public /24 address block on RIPE if your current public block has been assigned by APNIC (or viceversa )

b) look for an MPLS L3 VPN service spanning over multiple MPLS SPs that allow you to connect your two offices using private IP addresses. In this second option your public IP address block is used for internet handoff in the correct region . Internet access on the other office could be provided using a ISP provided address block (even a single IP address just for internet surfing)

Hope to help

Giuseppe

[Timohamoto]

Hello @Giuseppe Larosa!

Yeap you are right, that's typo for ...1.0/25, 1.128/25

So ok, actually if we run offices in same region, let's say Europe. and block /24 from RIPE.

So without option "b" - We will not be able to run our couple subnets of /25 from two different region in Europe?

Cause all providers will filter that /25 out? and the way it's obtain additional /24?

[Giuseppe Larosa]

Hello @Timohamoto ,

if the two offices are both in Europe and only two ISPs are involved you may be able to use /25 subnets in each office but it will require cooperation with both ISP stuff.

Hope to help

Giuseppe

[Timohamoto]

Hello @Giuseppe Larosa

Actually I leading to summary that there no /25 networks in modern eBGP

and in such way we will not be able to split /24 across sites and advertise them independently.

just wanna make sure that those statement is correct