[j-nsp] EBGP multihop, prevent next-hop self on advertisement to an IBGP peer

[Anton Smith]

Hi all,

I am having a problem with a J series router. I have a network as below:

R1---mp-ibgp----R2----multihop-ebgp----R3


The ebgp session between R2 and R3 is a normal EBGP multihop session. 
Let us say that it is between a PE and another box located somewhere on 
the internet.

The mp-ibgp session between R1 and R2 is used for advertising vpnv4 
prefixes.

The BGP session between R2 and R3 terminates inside a VRF on R2.

My problem is that R2 is rewriting the next-hop for the routes from R3 
when it advertises them on to R1.

I am definitely not setting next-hop self anywhere on R2.


When I show received routes for the routes from R3, I get the following 
output:

admin at j-peering-gw1> show route receive-protocol bgp z.z.z.z
<snip>
vrf-internet.inet.0: 216203 destinations, 363085 routes (216203 active, 
0 holddown, 2 hidden)
   Prefix                  Nexthop              MED     Lclpref    AS path
* x.x.x.x/19         z.z.z.z       0                  12345 I

At this stage, the next-hop, z.z.z.z is intact and is the desired 
next-hop that I want advertised on to R1.

When I show route extensive, things also *seem* (to me) to be ok 
(apologies if the following output is not very clean, I have also 
sanitised it a bit to replace IP addresses):

admin at j-peering-gw1> show route table vrf-internet x.x.x.x/19 extensive
------------------------------------------------------------------------
vrf-internet.inet.0: 216037 destinations, 362760 routes (216037 active, 
0 holddown, 2 hidden)
x.x.x.x/19 (1 entry, 1 announced)
TSI:
Page 0 idx 0 Type 1 val c4b5f30
         *BGP    Preference: 170/-101
                 Next-hop reference count: 4
                 Source: z.z.z.z
                 Next hop: 172.17.100.26 via fe-0/0/0.0, selected
                 Label operation: Push 493, Push 109(top)
                 Protocol next hop: z.z.z.z
                 Indirect next hop: 8884af8 264765
                 State: <Active Ext>
                 Peer AS: 12345
                 Age: 23:48:05   Metric: 0       Metric2: 1
                 Task: BGP_39532.z.z.z.z+37854
                 Announcement bits (3): 0-BGP RT Background 1-KRT 
2-Resolve tree 2
                 AS path: 12345I
                 Communities: 1234:1234
                 Localpref: 100
                 Router ID: 5.6.7.8
                 Indirect next hops: 1
                         Protocol next hop: z.z.z.z Metric: 1
                         Indirect next hop: 8884af8 264765
                         Indirect path forwarding next hops: 1
                                 Next hop: 172.17.100.26 via fe-0/0/0.0
                         10.0.0.0/30 Originating RIB: vrf-internet.inet.0
                           Metric: 1                       Node path 
count: 1
                           Indirect nexthops: 1
                                 Protocol Nexthop: 3.3.3.3 Metric: 1 
Push 493
                                 Indirect nexthop: 89fae04 264578
                                 Indirect path forwarding nexthops: 1
                                         Nexthop: 172.17.100.26 via 
fe-0/0/0.0
                                 3.3.3.3/32 Originating RIB: inet.3
                                   Metric: 1 
    Node path count: 1
                                   Forwarding nexthops: 1
                                         Nexthop: 172.17.100.26 via 
fe-0/0/0.0
-----------------------------------------------------------------------

The protocol next-hop looks correct, z.z.z.z.

However, when I show advertised routes towards R1, I see that the 
next-hop has been changed:

admin at j-peering-gw1> show route advertising-protocol bgp 4.4.4.4 
x.x.x.x/19 extensive

vrf-internet.inet.0: 216241 destinations, 363161 routes (216241 active, 
0 holddown, 2 hidden)
* x.x.x.x/19 (1 entry, 1 announced)
  BGP group route-reflectors type Internal
      Route Distinguisher: 1111:1111
      VPN Label: 107440
      Nexthop: Self
      Flags: Nexthop Change
      MED: 0
      Localpref: 100
      AS path: [4321] 12345 I
      Communities: 1234:1234 target:1111:1111

Notice the lines that read "Nexthop: Self" and "Flags: Nexthop Change". 
The flag seems strange to me, does anybody know what it means?

Nowhere in my policies do I set next-hop self:

admin at j-peering-gw1> show configuration | match next-hop

admin at j-peering-gw1>


I have tried configuring, under the multihop bgp section, the parameter 
'no-nexthop-change' but this does not have any effect ( I have set this 
both for the ibgp group and the ebgp group to no avail ).

This behaviour seems unexpected to me - as though the box is 
automatically setting next-hop self for some reason. Normally in all of 
my experience (albeit with plain ibgp/ebgp setups) you need to 
explicitly turn on next-hop self.

Does anybody have any idea why this is happening? My suspicion is it is 
definitely something to do with multihop bgp.

Any help much appreciated, since at present I am completely stuck. And I 
am aware that under normal circumstances this would be desirable 
behaviour, but in this particular instance I have a specific requirement 
that the next-hop not be changed.

Best regards,
Anton