How many legitimate cases when Origin AS in BGP announcement changed by another AS?

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1

Hi

I am looking for example routing policies when any AS receiving BGP advertisement changes Origin AS in BGP AS set attribute to remove the received AS number and puts its own AS number.[legitimate cases]

1. customer AS advertises the prefix however provider AS announce the aggregate(super prefix) shall put it's AS number in the BGP announcement.It may or may not suppress the customer BGP announcement based on multihoming.Well, we may say this is not a change in origin AS but new BGP announcement.

2.Can it happen in the case of private/public peering ?

3.ASes managed by same organization?

4.Are there cases it can be done for sub prefix or exact prefix announcement forwarding?

Thank you.

Akmal
PhD Student
MMLAB,SNU,Korea

2

Hi

Last week I faced a case like this in an african NREN.

This NREN provides Internet and Academic traffic to thier members, but there are some members leases a layer 3 VPN from local ISP in order to get to thier NREN POPs.
Engineers of this NREN desided for some reseans to use eBGP from thier POPs to this VPN, and to use OSPF from the VPN to thier members (member routers do not support BGP).

In this case the route will originate form the ISP which not actually posses those prefixes.

No upstream will accept this unless the NREN removes the local ISP ASN and announce new fresh routes, and this which actually happened with aggregate and suppress-map cisco commands.

I hope this a legitmate case.

Thanks

Tarig Yassin Ahmed

3

a message of 443 lines which said:

I am looking for example routing policies when any AS receiving BGP
advertisement changes Origin AS in BGP AS set attribute to remove
the received AS number and puts its own AS number.[legitimate cases]

When the old origin AS was a private one? Today, with 32bits AS,
everyone should have a public AS number but, in the real world, some
people do BGP with private AS that the upstream provider has to
change.

4

When the old origin AS was a private one?

NO.Even when old origin AS is not private one.

Don't you think private/pubic AS won't matter in case of provider aggregation?

Thanks

Akmal

5

a message of 44 lines which said:

>>When the old origin AS was a private one?
NO.Even when old origin AS is not private one.

You misunderstood me. I replied to your query "When is it legitimate
to change an origin AS?" by adding a possibility you did not mention:
"When the origin AS is a private one".