PIM-SSM RPF traffic flow not via PIM forwarding interfaces

Hi all,

I created a PIM-SSM lab on GNS3, as per the attached
diagram.  The aim was to see how SSM handles send traffic to a LAN
segment with multiple multicast routers (R2 and R4).   

The
default configuration ( basic ospf cfg, and ip pim ssm enable) worked as
expected, R4 was elected the DR (highest IP) for the 2.4.7.0/24 
network, and multicast traffic flowed from source
R6->R5->R3->R4->R7 host, trace and mtrace match.


Then
I  changed the ospf metrics  (R4 cost for 2.4.7.0 to 1000 )   Now the
path via R2is the best but the DR is R4.  Here I see a mismatch between
PIM forwarding interfaces (show ip mroute) and the trace/mtrace path to the host but traffic
is still forwarded, but im not sure why?

 


Default cfg

R4#sho ip mroute 232.1.1.1
(56.1.1.6, 232.1.1.1), 00:00:46/00:02:58, flags: sTI
  Incoming interface: FastEthernet0/0, RPF nbr 34.1.1.3
  Outgoing interface list:
    FastEthernet1/1, Forward/Sparse, 00:00:14/00:02:45
R4#
*Apr
15 21:22:40.075: IP(0): s=56.1.1.6 (FastEthernet0/0) d=232.1.1.1
(FastEthernet1/1) id=1437, ttl=252, prot=1, len=100(100), mforward

R4#sho ip pim interface
Address          Interface                Ver/   Nbr    Query  DR     DR
                                          Mode   Count  Intvl  Prior
4.4.4.4          Loopback0                v2/S   0      30     1      4.4.4.4
34.1.1.4         FastEthernet0/0          v2/S   1      30     1      34.1.1.4
2.4.7.4          FastEthernet1/1          v2/S   1      30     1      2.4.7.4
R4#

R5#mtrace 2.4.7.7
Type escape sequence to abort.
Mtrace from 2.4.7.7 to 35.1.1.5 via RPF
From source (?) to destination (?)
Querying full reverse path...
 0  35.1.1.5
-1  35.1.1.5 PIM  [2.4.7.0/24]
-2  35.1.1.3 PIM  [2.4.7.0/24]
-3  34.1.1.4 PIM  [2.4.7.0/24]
-4  2.4.7.7

R5#traceroute 2.4.7.7
  1 35.1.1.3 44 msec 52 msec 24 msec
  2 34.1.1.4 52 msec 24 msec 32 msec
  3 2.4.7.7 64 msec 24 msec 32 msec
R5#

R3#sho ip route 2.4.7.0
Routing entry for 2.4.7.0/24
  Known via "ospf 1", distance 110, metric 2, type intra area
  Last update from 23.1.1.2 on FastEthernet0/0, 00:00:06 ago
  Routing Descriptor Blocks:
  * 34.1.1.4, from 2.4.7.7, 01:02:53 ago, via FastEthernet1/0
      Route metric is 2, traffic share count is 1
    23.1.1.2, from 2.4.7.7, 00:00:06 ago, via FastEthernet0/0
      Route metric is 2, traffic share count is 1
R3#
R3#sho ip rpf 2.4.7.7
RPF information for ? (2.4.7.7)
  RPF interface: FastEthernet1/0
  RPF neighbor: ? (34.1.1.4)
  RPF route/mask: 2.4.7.0/24
  RPF type: unicast (ospf 1)
  RPF recursion count: 0
  Doing distance-preferred lookups across tables
R3#

 
After OSPF cost change on R4

R4#sho ip mroute 232.1.1.1
(56.1.1.6, 232.1.1.1), 00:46:14/00:02:59, flags: sTI
  Incoming interface: FastEthernet0/0, RPF nbr 34.1.1.3
  Outgoing interface list:
    FastEthernet1/1, Forward/Sparse, 00:08:44/00:02:15, A

R4#
*Apr
15 22:08:07.635: IP(0): s=56.1.1.6 (FastEthernet0/0) d=232.1.1.1
(FastEthernet1/1) id=2499, ttl=252, prot=1, len=100(100), mforward
R4#

R3 only forwards the traffic to R4

R3#SHO IP MROute 232.1.1.1
(56.1.1.6, 232.1.1.1), 00:05:20/00:03:25, flags: sT
  Incoming interface: FastEthernet1/1, RPF nbr 35.1.1.5
  Outgoing interface list:
    FastEthernet1/0, Forward/Sparse, 00:05:00/00:03:25


R2#sho ip mroute 232.1.1.1
(56.1.1.6, 232.1.1.1), 00:08:36/00:02:00, flags: sPTI
  Incoming interface: FastEthernet1/0, RPF nbr 23.1.1.3
  Outgoing interface list: Null

R2#
*Apr 15 22:14:52.211: IP(0): s=56.1.1.6 (FastEthernet1/1) d=232.1.1.1 id=2588, ttl=251, prot=1, len=114(100), not RPF interface

The below shows both unicast and multicast following the same path, but not via R4?

R5#traceroute 2.4.7.7

Type escape sequence to abort.
Tracing the route to 2.4.7.7

  1 35.1.1.3 76 msec 40 msec 20 msec
  2 23.1.1.2 8 msec 32 msec 24 msec
  3 2.4.7.7 56 msec 32 msec 76 msec

R5#MTRace 2.4.7.7

 0  35.1.1.5
-1  35.1.1.5 PIM  [2.4.7.0/24]
-2  35.1.1.3 PIM  [2.4.7.0/24]
-3  23.1.1.2 PIM  [2.4.7.0/24]
-4  2.4.7.7
R5#

R3#sho ip route 2.4.7.0
Routing entry for 2.4.7.0/24
  Known via "ospf 1", distance 110, metric 2, type intra area
  Last update from 23.1.1.2 on FastEthernet0/0, 00:30:21 ago
  Routing Descriptor Blocks:
  * 23.1.1.2, from 2.4.7.7, 00:30:21 ago, via FastEthernet0/0
      Route metric is 2, traffic share count is 1


R3#sho ip rpf 2.4.7.0
RPF information for ? (2.4.7.0)
  RPF interface: FastEthernet0/0
  RPF neighbor: ? (23.1.1.2)
  RPF route/mask: 2.4.7.0/24
  RPF type: unicast (ospf 1)
  RPF recursion count: 0
  Doing distance-preferred lookups across tables
R3#

 

 

imageimage

Sign In or Register to comment.