BGP routing from same AS - behavior?

[jayh]

Equipment is Gen. 2 TA908e running R10.5.3 .

We have a situation where a customer has multiple sites connected via MPLS.  There is a service provider BGP with a different AS, simplified scenario as follows:

TA900                                                  TA900

Site A<-----BGP------>Service provider<------BGP------>Site B

AS100                       AS200                      AS100

With Cisco equipment, Site A will discard routes advertised from Site B and vice-versa because the EBGP routes have the same origin AS as the destination with a different transit AS.  This can be overridden with the "allowas-in" command facing the service provider neighbor. 

I don't see an "allowas-in" option in AOS.

What is Adtran's default behavior here?  Will the routes from Site A be learned by Site B?

If not, is there a command to override the default similar to allowas-in to allow the routes to propagate?  p

[jayh]

Thanks for posting the AOS guide for configuring BGP.  I have Read That Fine Manual and it doesn't address the scenario in my question.

[Anonymous]

:

Thank you for asking this question in the support forum.  I believe this question can be answered by example 2 (page 37) in the Configuring BGP in AOS guide.

Some multihoming network designs require a customer to appear as a different AS number to individual service providers. Also, service providers sometimes assign the same AS to multiple sites, which can cause problems due to BGP’s loop avoidance check mechanism. The local-as command rectifies both situations by substituting an AS number that is different from the one specified in the command router bgp <AS number>.

Use the local-as command to specify an AS number for the unit to use when communicating with this BGP neighbor on the default VRF.

(config-bgp-neighbor)#local-as <value>

<value> Specifies the AS number to use when communicating with this neighbor. The value must be different than the AS number for this router and the peer router. It is only valid for eBGP connections. Range is 0 to 4294967295. When 0 is used, it indicates that the BGP process local AS is used, because 0 is not a valid AS number.

I hope that answers you question, but please do not hesitate to reply with any additional questions or information.  I will be happy to help in any way I can.

Levi

[jayh]

I saw that but it didn't seem to quite fit.

levi wrote:

Also, service providers sometimes assign the same AS to multiple sites, which can cause problems due to BGP’s loop avoidance check mechanism. The local-as command rectifies both situations by substituting an AS number that is different from the one specified in the command router bgp <AS number>.

Use the local-as command to specify an AS number for the unit to use when communicating with this BGP neighbor on the default VRF.

(config-bgp-neighbor)#local-as <value>

If, as in the scenario above, the service provider assigns the same AS to multiple sites, communicating with the service provider with a different AS will prevent the session from coming up due to the AS mismatch.

Or, does the local-AS command keep the same AS on the actual EBGP connection but somehow trick AOS into interpreting the conneactio as if the local router has a different AS?

Would this work? 

TA900                                                  TA900

Site A<-----BGP------>Service provider<------BGP------>Site B

                                                       AS101

AS100                       AS200                      nei w.x.y.z local-as AS100

Here the service provider is peering with AS100 on both (all) legs but the defined primary AS on the AOS devices is something different.

Or, is it required that the actual EBGP connections themselves not re-use the AS?

(I take it that the "allowas-in" functionality isn't supported?)

[Anonymous · Accepted solution]

:

Here is a sample configuration for your application, based on the information you've provided:

AOS Router @ site 1:

router bgp 1

  neighbor 1.1.1.1

    local-as 100

    remote-as 200

    no shutdown

    exit

  address-family ipv4

    neighbor 1.1.1.1

      no shutdown

      exit

    exit

AOS Router @ site 2:

router bgp 2

  neighbor 2.1.1.1

    local-as 100

    remote-as 200

    no shutdown

    exit

  address-family ipv4

    neighbor 2.1.1.1

      no shutdown

      exit

    exit

AOS does not support the "allowas-in" functionality, currently.  Please, let me know what additional questions you have based on this sample configuration provided, or the example 2 (page 37) in the Configuring BGP in AOS guide.

Levi

[jayh]

Thanks, that works.  Somewhat counter-intuitive to someone like me with a Cisco background.  The allowas-in command would be useful.