Cluster id - ASBR ID Match stipulation when synchronisation is

Hi all

it is mentioned that when we have BGP synchronisation rule enabled , ASBR
must be the must also be route reflector. why is this so

to test out ...

R2----R1----R3

R1 is hub and R2 , R3 are spokes
172.16.101.1 , 172.16.102.1 , 172.16.103.1 are router id of R1,R2, and
R3.

*R2 as ASBR and RR*

router bgp 100
* synchronization*
 bgp log-neighbor-changes
 network 20.20.20.0 mask 255.255.255.0 ----> Advertises this network
 neighbor 172.16.20.10 remote-as 200
 neighbor 172.16.123.1 remote-as 100 ----> R1 as ibgp RR client
 neighbor 172.16.123.1 route-reflector-client
 neighbor 172.16.123.3 remote-as 100
 neighbor 172.16.123.3 route-reflector-client ---->R3 as ibgp RR
client

router ospf 1
 log-adjacency-changes
 redistribute bgp 100 subnets ----> obeying what synchronisation rule
says

*on R3*

R3#show ip bgp 20.20.20.0
BGP routing table entry for 20.20.20.0/24, version 3
Paths: (1 available, best #1, table Default-IP-Routing-Table,
RIB-failure(17))
  Advertised to update-groups:
     2
  Local
    172.16.123.2 from 172.16.123.2 (172.16.102.2)
      Origin IGP, metric 0, localpref 100, valid, internal, *synchronized,
best*

on R1 (ASBR ONLY , NOT RR)
R1#show ip bgp 20.20.20.0
BGP routing table entry for 20.20.20.0/24, version 9
Paths: (1 available, best #1, table Default-IP-Routing-Table,
RIB-failure(17))
  Advertised to update-groups:
     1
  Local
    172.16.123.2 from 172.16.123.2 (172.16.102.2)
      Origin IGP, metric 0, localpref 100, valid, internal, *synchronized,
best*

all ok here

now if i advertise on R1 network 10.10.10.0

router bgp 100
 synchronization
 bgp log-neighbor-changes
 network 10.10.10.0 mask 255.255.255.0

R3 receives it but says this

R3#show ip bgp 10.10.10.0
BGP routing table entry for 10.10.10.0/24, version 0
Paths: (1 available, no best path)
  Not advertised to any peer
  Local
    172.16.123.1 from 172.16.123.2 (172.16.102.2)
      Origin IGP, metric 0, localpref 100, valid, internal*, not
synchronized*
      Originator: 172.16.101.1, Cluster list: 172.16.102.2

if i look for a matching route in IGP
*
R3#SHOW IP ROUTE 10.10.10.0*
Routing entry for 10.10.10.0/24
  Known via "ospf 1", distance 110, metric 1, type extern 2, forward metric
64
  Last update from 172.16.123.1 on Serial1/0, 00:09:43 ago
  Routing Descriptor Blocks:
  * 172.16.123.1, from 172.16.101.1, 00:09:43 ago, via Serial1/0
      Route metric is 1, traffic share count is 1

Indeed the route is present ...

why does cluster id needs to be same as ASBR advertising ip when
synchronisation is enabled ????

is this as per RFC or just cisco implementation ???

i did debug ip bgp update ...to see actual message

i am advertising a new network now 100.100.100.0

*Mar 1 04:03:09.906: BGP(0): 172.16.123.2 rcvd UPDATE w/ attr: nexthop
172.16.123.1, origin i, localpref 100, metric 0,* originator 172.16.101.1,
clusterlist 172.16.102.2*
*Mar 1 04:03:09.914: BGP(0): 172.16.123.2 rcvd 100.100.100.0/24
*Mar 1 04:03:09.918: BGP(0): no valid path for 100.100.100.0/24
R3#
*Mar 1 04:03:23.842: BGP(0): *no valid path for 100.100.100.0/24*
R3#sr 100.100.100.0
Routing entry for 100.100.100.0/24
  Known via "ospf 1", distance 110, metric 1, type extern 2, forward metric
64
  Last update from 172.16.123.1 on Serial1/0, 00:00:51 ago
  Routing Descriptor Blocks:
  * 172.16.123.1, from 172.16.101.1, 00:00:51 ago, via Serial1/0
      Route metric is 1, traffic share count is 1

and when advertise on R2 (RR AND ASBR)

*Mar 1 04:09:51.070: BGP(0): 172.16.123.2 rcvd UPDATE w/ attr: nexthop
172.16.123.2, origin i, localpref 100, metric 0
*Mar 1 04:09:51.074: BGP(0): 172.16.123.2 rcvd 200.200.200.0/24
*Mar 1 04:09:51.078: BGP(0): *Revise route installing 1 of 1 routes for
200.200.200.0/24 -> 172.16.123.2(main) to main IP table*
*Mar 1 04:09:51.086: BGP(0): 172.16.35.5 send UPDATE (format)
200.200.200.0/24, next 172.16.35.3, metric 0, path Local
*Mar 1 04:09:51.190: BGP(0): Revise route installing 1 of 1 routes for
200.200.200.0/24 -> 172.16.123.2(main) to main IP table

*R3#SHOW IP BGP 200.200.200.0*
BGP routing table entry for 200.200.200.0/24, version 5
Paths: (1 available, best #1, table Default-IP-Routing-Table,
RIB-failure(17))
Flag: 0x820
  Advertised to update-groups:
     2
  Local
    172.16.123.2 from 172.16.123.2 (172.16.102.2)
      Origin IGP, metric 0, localpref 100, valid, internal, s*ynchronized,
best*

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Received on Mon Sep 23 2013 - 20:03:10 ART