[XR as RR with bgp additional-paths]

[Carlos A. Silva]

Hi,

Does XR support RR operation when you want to send multiple equal paths (multiple best routes) to route-reflector-clients (bgp additional-paths)?

I've worked with this on a lab where RRs are XE and support the "neighbor advertise additional-paths best" command, which is not available in XR. The XE setup works, but when I try it with an XR as RR, it doesn't. The RR doesn't even send more than one prefix (the best one obviously).

XR RR works fine, except for that bit. All I have configured (for bgp additional paths) on the XR RR is:

address-family ipv4 unicast

additional-paths send

additional-paths receive

I've tried deleting each command, just to try something, to no avail.

Any suggestions?

Thanks!

c.

[xthuijs]

hi carlos, could it be that the additional paths are non equal?

I mean for bgp mpath to work in general, the as-path needs to be the same length for instance and through the same AS. it may be the case that the RR did best path and didnt find the additional paths equal enough for them to be advertised.

can you share a show bgp <prefix> output from one of the paths that we assumed to be supportive for addpath?

all the attributes need to be the same, and I think that is where there may be a crux in your case.

(XR is a bit more strict by default than IOS).

cheers

xander

[Carlos A. Silva]

Yup, I was playing with a couple default-routes with basically same BGP attributes.

Thanks,

c.

[Sandy Breeze]

Hi Xander,

I came across this post of yours whilst looking for clarification on how IOX treats 'group-best' vs IOS.

You mention IOX is stricter than IOS here.  I have both doing RR for IPv4 Unicast, but, they seem to have different ideas as to which 'group-best' prefixes they should advertise to addpath negotiated clients.

For example, on my IOX RR I see paths p1, p2, p3 for a prefix.  p1 comes from next-hop n1.  p2 and p3 both come from next-hop n2.  All paths have identical local-pref, med, as-path length and IGP distance to n1 and n2 are the same.  All paths have different 1st AS.  They all get marked as 'group-best'.  IOX only decides to send p1 and p2 to clients and leaves out p3.  IOS sends all 3.  Why would that be?

Adding some more paths with different nh's and different 1st AS get marked as group-best and get advertised.  Can I assume IOX is actually doing group-best per-next-hop/per-neighbour ?

Cheers

Sandy

[Sandy Breeze]

For completeness of anyone stumbling across this post with the same questions;

- It was a conscious design decision in XR to implement group-best per-next-hop.

- A documentation bug has been filed under CSCvh68039 to align XR's description of group-best with its actual behaviour

Sandy

[Adam Vitkovsky · Accepted solution]

Hi,

I think there’s a need to also use the policy as well”

route-policy RP-PIC

  set path-selection backup 1 advertise

end-policy

router bgp 100

  address-family vpnv4 unicast

    additional-paths selection route-policy RP-PIC

Also check whether the capabilities between RRs and Clients have been exchanged and negotiated successfully:

show bgp neighbor <ip>

…look for:

AF-dependent capabilities:

   Additional-paths Send: advertised and received

   Additional-paths Receive: advertised and received

Oh and don’t forget to configure clients with:

router bgp 100

  address-family ipv4 unicast

     advertise best-external

-in order to allow them to advertise alternate paths to RRs

set path-selection { backup number | group-best | all | best-path } [install [multipath-protect]] [advertise]

backup             Specifies the BGP backup path.

<number>           Specifies the BGP backup path number, which must be a value of 1.

group-best         Specifies all the BGP group best paths

                   A group best path is the best path received from an AS.

                   For example, if paths are received from 3 ASes,

then there are 3 group best paths.

all                Specifies all BGP paths.

best-path          Specifies the BGP best path.

install            Installs the path.

advertise          Advertises the path.

multipath-protect  Selects a backup path that is not also a multipath

                   All selected paths should have unique next hops.

                   For example, if two paths have the same next hop,

then one of them is not selected.

                   The backup path is selected such that it does not have

the same next hop as any other path that is a multipath.

                   Note: Multipaths are configured with the maximum-paths command.

adam

netconsultings.com

::carrier-class solutions for the telecommunications industry::

[Carlos A. Silva]

That was it, Adam. Thank you very much.

[ZAhmad04890]

Hi Adam,

how can we differentiate  between multipath advertise vs  multipath-protect advertise in IOS XR as shown in below,

also when we use  backup 1 install does it has a similar behaviour like  maximum-paths ibgp 2  command in terms of multipath installation in FIB. Thanks ?

route-policy AP1
set path-selection multipath advertise

vs

route-policy AP2
set path-selection  multipath-protect advertise

vs

route-policy AP3
set path-selection backup 1 install  multipath-protect advertise

Thanks

[aghufran]

Hi All,

I tried all the combinations given in the following Cisco document for ADD-PATH Selection and am sharing the results for those in similar situation. Couple of the options were not available due to the code I had. It was tried on eveNG community version 2.0.3-112 and with Cisco XRV9k running code 7.1.2.

Cisco Document Title: Configure and Verify BGP Path Selection in IOS-XR

Configuration shared is for the RR (route-reflector) and RRC (client) between which add-path, multi-path and backup-path options were tried. RR (AS64512) was receiving the same BGP prefix 111.111.111.1/32 from various Intra-AS(AS64512) and Inter-AS BGP(AS64513, AS64514) peers. There were two BGP peers from the AS64513 and only one from AS64514.

Attached is relevant configuration from RR and RRC and output of 'show bgp ipv4 u 111.111.11.1/32' and 'show route 111.111.111.1/32' commands from the RR and RRC. This shows the prefixes used locally on the RR, advertised to RRC and used locally on the RRC. I apologise for using a public prefix for testing.

I hope it helps those interested in learning more about the add-path, multi-path and backup-path selection and advertisement mechanisms.

Regards,

Abid Ghufran