topic Re: Route Reflector in Routing and SD-WAN

Route Reflector

saishreevj — Wed, 08 Dec 2021 08:58:28 GMT

Do we need to consider both Route Reflectors and Next Hop self while Planning for IBGP Peerings?

Re: Route Reflector

balaji.bandi — Wed, 08 Dec 2021 08:23:55 GMT

Not sure what is the use case here.

Both do different jobs. next-hop not going to change on route reflectors in iBGP routes, it does only for eBGP learned routes ( as per i know)

Re: Route Reflector

saishreevj — Wed, 08 Dec 2021 08:59:59 GMT

@balaji.bandi I mean configuring Next Hop Self inside IBGP network.

Re: Route Reflector

paul driver — Wed, 08 Dec 2021 10:04:47 GMT

Hello

Yes it is applicable to apply it on the RR only not on any RRC

Re: Route Reflector

saishreevj — Wed, 08 Dec 2021 12:07:56 GMT

@paul driver thanks, If route learned from RR client will be propagated to EBGP via RR and need to configure Next Hop Self on non RR I believe

Re: Route Reflector

MHM Cisco World — Wed, 08 Dec 2021 12:08:39 GMT

Yes you can, next-hop-self don't make loop since the RR add originate router to prefix learn from client and this prevent loop inside iBGP with RR.

Re: Route Reflector

paul driver — Wed, 08 Dec 2021 12:43:50 GMT

Hello

@MHM Cisco World @saishreevj
No you do not require to add next-hop-self on any RRC, This feature ONLY needs to appended on the RR towards the RRC.

RRC are internal ibgp rtrs and have no direct connection to ebgp peers. any ebgp prefix being advertised into the ASN will have its next-hop change to be the RR so any ibgp rtrs will see the next-hop NLRI for ebgp prefixes as the RR and not the advertising ebgp rtr.

The same applies to any internal prefix being advertised externally by the RR the next-hop advertised will be the RR itself.

Re: Route Reflector

paul driver — Wed, 08 Dec 2021 16:02:58 GMT

Hello

@MHM Cisco World wrote:

Yes you can, next-hop-self don't make loop since the RR add originate router to prefix learn from client and this prevent loop inside iBGP with RR.

The loop prevention in RR design as far as I understand is performed by the Cluster-list/originator id's, Originator id prevent loops within its as single cluster and a cluster id it works when you have multiple RR, just like the path sequence verification in the AS Path, So if a router see its own cluster id in any bgp update then its discarded.

Re: Route Reflector

Harold Ritter — Wed, 08 Dec 2021 13:05:17 GMT

Hi @paul driver ,

> No you do not require to add next-hop-self on any RRC, This feature ONLY needs to appended on the RR towards the RRC.

I disagree with this statement. Generally, "next-hop-self" is applied to the route reflector client, which normally peers with the other ASNs. It is also generally not recommended to change the next hop on the route reflector itself. In fact IOS and IOS-XR by default only allow the modification of the next hop for paths learnt via eBGP (if any eBGP peer are directly connected to the RR). This default behavior can be changed, but is only required if the RR needs to be inline, such as in an MPLS BGP LU scenario for example.

Here's with RFC4456 says about changing the next hop on the RR:

10.  Implementation Considerations

   Care should be taken to make sure that none of the BGP path
   attributes defined above can be modified through configuration when
   exchanging internal routing information between RRs and Clients and
   Non-Clients.  Their modification could potentially result in routing
   loops.

   In addition, when a RR reflects a route, it SHOULD NOT modify the
   following path attributes: NEXT_HOP, AS_PATH, LOCAL_PREF, and MED.
   Their modification could potentially result in routing loops.

https://datatracker.ietf.org/doc/html/rfc4456

Regards,

Re: Route Reflector

paul driver — Wed, 08 Dec 2021 14:34:05 GMT

Hello Harold

Thank for your feedback it’s very much appreciated.

Generally, "next-hop-self" is applied to the route reflector client,

Just to confirm when you state “to” you mean towards the rrc not on the rrc towards the rr correct?

also

In fact IOS and IOS-XR by default only allow the modification of the next hop for paths learnt via eBGP (if any eBGP peer are directly connected

isnt this what i am saying which is that if the RR have ebgp peers or infact come to think of it if the rrc also, that the next-hop-self is valid .

However then you provide this little nugget

In addition, when a RR reflects a route, it SHOULD NOT modify the
   following path attributes: NEXT_HOP, AS_PATH, LOCAL_PREF, and MED

which is now stating the opposite from above unless i have missed something…which is very possible

Re: Route Reflector

Harold Ritter — Wed, 08 Dec 2021 14:33:30 GMT

Hi @paul driver ,

> Just to confirm when you state “to” you mean towards the rrc not on the rrc towards the rr correct?

No, I mean on the RRC towards the RR.

> isnt this what i am saying which is that if the RR have ebgp peers or infact come to think of it if the rrc also, that the

> next-hop-self is valid .

The case where RR has ebgp sessions is more of a corner case. The RR is normally dedicated to being a RR and doesn't handle the eBGP sessions.

> which is now stating the opposite from above unless i have missed something…which is very possible

I meant next-hop-self should be applied on the RRC towards the RR.

Regards,

Re: Route Reflector

saishreevj — Wed, 08 Dec 2021 18:08:54 GMT

@Harold Ritter Thanks for the explanation, much appreciated.

Re: Route Reflector

paul driver — Wed, 08 Dec 2021 18:36:49 GMT

Hello Harold

@Harold Ritter wrote:

The case where RR has ebgp sessions is more of a corner case. The RR is normally dedicated to being a RR and doesn't handle the eBGP sessions.

Okay if the RR or RRC did have ebgp sessions then the NHS would be applicable however as you state normally it would be either a RR client or non-client within the same ASN that would have the ebgp peering as such they would implement the NHS feature, And as both RR are basically non clients to each other they will reflect each other IGBP routes with the NHS already set from the RRC/non client rtr.

Re: Route Reflector

Harold Ritter — Wed, 08 Dec 2021 18:51:09 GMT

Hi @paul driver ,

> Okay if the RR or RRC did have ebgp sessions then the NHS would be applicable however as you state normally it would be

> either a RR client or non-client within the same ASN that would have the ebgp peering as such they would implement the

> NHS feature, And as both RR are basically non clients to each other they will reflect each other IGBP routes with the NHS

> already set from the RRC/non client rtr.

That is correct Paul.

Regards,

Re: Route Reflector

Harold Ritter — Wed, 08 Dec 2021 18:54:42 GMT

You are very welcome @saishreevj

Re: Route Reflector

MHM Cisco World — Wed, 08 Dec 2021 19:32:00 GMT

As I know non-client client and RR can make eBGP with outer AS.

https://blog.ipspace.net/2014/04/changes-in-ibgp-next-hop-processing.html

this article talk a little about neighbor next-hop-self all <- which use specifically for RR.

Re: Route Reflector

paul driver — Wed, 08 Dec 2021 22:08:13 GMT

Hello Harold

Cheers mate for that clarification,
By the way barcleona 2019 cisco live, I coulndt get into the ipv6 adv lab I believe now you were presenting, I wasnt a happy bunny at the time but all good in the end due to the on demend sessions!

Was you booked for 2022 Amsterdam?

Re: Route Reflector

Harold Ritter — Wed, 08 Dec 2021 22:27:25 GMT

Hi Paul,

> By the way barcleona 2019 cisco live, I coulndt get into the ipv6 adv lab I believe now you were presenting, I wasnt a happy

> bunny at the time but all good in the end due to the on demend sessions!

Barcelona was my last in-person CiscoLive. Sorry to hear you couldn't get in the IPv6 session. It would have been nice to meet you in person. We always try to get at least two slots, but even that isn't enough. I always fight to get a 3rd slot, but it is not easy due to the number of overall sessions. We did have 3 sessions this year in the virtual event.

> Was you booked for 2022 Amsterdam?

Yes, I was. It was very sad to hear that it got cancelled last week. I was so looking forward to going back to an in-person CiscoLive. Hope to get a chance to meet you at CiscoLive Amsterdam 2023.

Regards,

Re: Route Reflector

Sam666 — Sun, 08 May 2022 09:35:17 GMT

Hi @Harold Ritter thanks for your information. I am also tring to modify the refelected route's local-preference on RR outgoing direction, now I know that RR outgoing NEXT_HOP, AS_PATH, LOCAL_PREF and MED modification has been ignored due to the loop prevention. Here I have a question, what if I modify the attribute on the RR incoming direction, will it also potentially cause a loop (refer to the attached diagram, it seems that modify on the incoming or outgoing has the same effect to the whole network)? Should we avoid any route attribute modification on RR for loop prevention consideration?

Re: Route Reflector

MHM Cisco World — Sun, 08 May 2022 11:51:07 GMT

what want to achieve here? why looking for modify ? is RRC is EBGP also?