BGP advertisement

[jay_7301]

Hi,

I was wondering if someone can help me answer the following question.

If I have a public ASN and I have multiple border routers peering with 2 of more tier 1 providers and also connect to another ISP over a private peering session, and I advertise say a public network 82.10.31.0/22 address space with was purchased from ripe, should I expect the company I privately peered will advertise my address space

I presume this will cause issues as all inbound traffic could route via the private peering session apposed to via the tier 1's.

In this situation would you shutdown the session to them and get them to stop advertising your prefix's?

Any help would be appreciated.

Thanks

[paul driver]

Hello

Is this private peering using the same ASN as your stie?

If so you can untilise the bgp community attribute of No-Export , which will adverise the prefix within its own asn but not to any EBGP peers.

res
Paul

[jay_7301]

sorry when I say private peering session we have an EBGP neighbour to them to our public AS on a /30 link we just peer direct.

so between the direct link we just want to advertise our local networks and not our transit ( tier 1 links ) however this company seem to be advertising our prefix to the rest of the internet, which I presume they should not be doing this. As we don't want our /22 address space seen via them to the rest of the internet, they should be seen via our transit link instead?

Sorry Paul hope this makes sense as I understand the no export community wont help in this situation I we are peering over an EBGP session.

Maybe shut down the session to them and tell them to check the filters?

Thanks

[paul driver]

Hello

If i understand you wish not to become a transit between ISPs

So the best way would be to apply a filter

ip as-path access-list 1 ^$

router bgp xx
neigbour (isp1) filter-list 1 out
neigbour (isp2) filter-list 1 out

This will just advertise your local routes ONLY and not routes from either ISP thus preventing you becoming a transit site for either ISP


res
Paul

 

[jay_7301]

Hi Paul,

Yes I don't want to become a transit AS but also this provider should not be advertising my local prefix's to the rest of the internet correct?

My example subnet 82.10.31.0/22 within my AS should only be advertised them to but they should then not be advertising my prefix's out to the rest of the internet? If they are advertising my prefix's this could cause my inbound traffic to come in via them and not my tier1 transit.

Sorry hope I'm clear on what is happening

Thanks.

[paul driver]

Hello

So you can perform AS prepending to advertse your local routes to a specifc ISP for least preffered incoming path selection and  maybe also incorporate negating transit routing as the same time.

You can match on a sepcifi prefix if you wish or prepend all local routes

route-map AS-Prepend
set as path-prepend (your asn  your asn  your asn)

router bgp xx
neigbour (isp1) filter-list 1 out
neigbour (isp2) filter-list 1 out
neigbour (isp1) route-map AS-Prepend out



res
Paul

 

[jay_7301]

Also would it be a good idea to speak to the ISP and tell them to update there filters to tell them not to advertise our prefix to the rest of the world?

Not sure how it works but i thought another AS was not aloud to advertise your prefix's unless you give them permission am i correct?

Thanks

[paul driver]

Hello

This way you would then have no resilency if the primary ISP link failed so you might awell not advertied it to them in the first place

res

Paul

[jay_7301]

we already have multiple tier 1 providers like NTT Cogent etc so i assume we would prefer to route via them. 

I think the whole point in getting this private session setup is to provide less hops within the AS path thus lower latency ( so is doesn't travel via the tier 1 and goes direct over the private session) if any traffic is destined to there prefix.

Thanks

[paul driver]

Hello

Then pre pending will do the job