How is provider doing this in BGP?

[John Blakley]

All,

How does a provider receive a /24 subnet and break that into multiple subnets using only BGP?

For example:

Our provider is receiving a full /24 from wherever they're getting it from. We have a /25 from that block. They are having us set a community name and then they're doing something with our /25 block.

Thanks!

John

[cadet alain]

Hi,

They don't  subnet using BGP. The community is a path attribute like a tag that your ISP is using for policy routing routes with this tag like modifying LOCAL_PREF or filtering.

Regards.

Alain.

[Marwan ALshawi]

Hi John

route subnet/Prefix and the BGP community are two different things

BGP community,  SP they use it as Alain described for BGP routing within the SP cloud and with other connected SPs to your directly connected SP

for the subnet/Prefix this is something that your ISP/SP allocated to you and they add/injuct a route point to your CE for that range or you do advertise it and they filter it from there end to let you advertise whatever allocated to you only and then get injected within your VRF in to their MPLS cloud

HTH

[Edison Ortiz]

John,

I'm assuming the /25 is owned by them, not yours? They allow you to use that /25 block along with another customer perhaps using the other /25 block and in turn the summarize both blocks before sending it out to the internet backbone routers.

I guess you were wondering how the internet allows your /25 block to traverse, right?

[John Blakley]

Edison,

That's what I think is happening. I believe that we have one /25 and another customer has another /25 and they're summarizing to their upstream ISP the /24 block. Isn't that the only way they'd be able to do it?

Here's some more backstory:

We ported our block from a different type of circuit with ATT. ATT said that in order for us to use the /25, we'd need to send a community with our BGP peering. We were on a legacy system at one point and moved over to a more current service. The block that was ported over didn't allow anything less than a /24. They told us that because of this we'd need to send a community to them. I'm assuming that, even if we're the only customer at the moment using the /25, that they are summarizing. I just wanted to know what the service provider was doing with the community once they received it. I haven't asked them yet

Thanks! (Good to talk to you again!)

[John Blakley]

I think I figured out how they're doing it. I think they're using inject-maps in BGP to split the /24 into 2 /25s. I can reproduce in GNS doing it this way....

[Edison Ortiz]

Inject-maps are one way, static routes to null0 along with network statements are another...

[Peter Paluch]

Edison,

Perhaps I have misunderstood something but let me ask...

The OP was asking about the ISP injecting more specific prefixes to BGP database. You have indicated that inject-maps are one way, the static routes to Null0 are yet another.

I see a problem with the static routes to Null0. Because they would be more specific than the original network, they would always match the respective traffic and discard it because of the Null0 adjacency. In this case, using such static routes would probably cause traffic blackholing.

The Null0 static routes are usually fine when doing an opposite scenario - advertising a stable summary/aggregate route. In that case, they merely work as discard routes automatically inserted by routing protocols when summarizing.

Best regards,

Peter

[Giuseppe Larosa]

Hello John,

>>

I think they're using inject-maps in BGP to split the /24 into 2 /25s. I can reproduce in GNS doing it this way....

no it is far easier: BGP protocol allows to advertise /25 prefixes and to receive them.

However, common best practice is that IPv4 prefixes longer then /24 SHOULD NOT  be advertised to other ASes.

But this does not mean that within a specific AS more specific prefixes do not exist.

The BGP community attribute is a way to signal "please accept this /25 IP subnet as we have an agreement on this"

The true advantage for a big provider is that it can build routing policies based on BGP communities instead of listing prefixes.

At other border routers of your ISP  your prefix is aggregated to /24 before being sent to other ASes and voilà.

It gives them scalability and manageability.

Alain had already pointed out this.

Edit:

you may be referring to a way to create multiple /24 starting from two conditions

a /25 exists in iBGP

a specific BGP community value is set

THEN

generate corresponding /24 prefix

now I see your points gentlemen.

ip prefix-list SPECIFIC-ROUTES permit 195.200.0.0/16 eq 25

a community list

ip community-list 12 permit AA:NN

a route-map invoking both of the above

and the route-map used as inject-map

route-map CREATE_AGGREGATE permit 10

match comunity

match ip address prefix SPECIFIC-ROUTES

!

However, inject-map can be used to create more specific routes without a match the opposite of what is needed here according to documentation

see

http://www.cisco.com/en/US/docs/ios/iproute_bgp/configuration/guide/irg_basic_net.html

So I keep my idea they use the community to signal A) accept the route B) aggregate it at AS border routers.

Hope to help

Giuseppe

[Marwan ALshawi]

i agree with Giuseppe because normally service providers they do not go with complex route-maps and polices when they building the MPLS cloud and if they use inject map with BGP i believe this will be complex when it being used with many customers

not to mention this is we assuming the PE is Cisco as i think some ISPs they use Juniper in for their PEs and i am not sure if Jinus dose support the BGP inject map or not