The logic is that an RPF check is executed, which is checking to make sure the source traffic is coming in on the interface the router would used to forward back to the source, reverse path forwarding. If the interface is a multi-access interface, more than one destination reachable out that interface, the local router may not be the correct RPF neighbor on that interface. If R15 was dually connected to R1 and R4, and both routers were receiving multicast traffic from R15, R15 could send twice the multicast flows, one to R1 and one to R4.
The remote PE, like R7 for instance could receive a multicast flow from R1 and R4. If PIM is used as the overlay signaling protocol, this is not likely but not impossible to happen, as asserts would be triggered and one of the flows would be stopped. Since we are using BGP as the C-Mcast signaling, this is guaranteed not happen, asserts that is, so IOS ingress PEs need to have a way to prevent duplicate multicast flows. This capability doesn't apply to our design since each CE is singly attached to a PE and not dually attached.
Feel free to review this Cisco guide on this, I have offered my interpretation of the issue.
https://www.cisco.com/c/en/us/support/docs/ip/multicast/118677-technote-mvpn-00.html
R1, R3, R7
vrf definition MVPN
address-family ipv4
mdt preference mldp
mdt auto-discovery mldp
mdt strict-rpf interface
mdt partitioned mldp p2mp
mdt overlay use-bgp
!
mpls mldp logging notifications
XR1, XR5 and XR6
router pim
vrf CORE
address-family ipv4
rpf topology route-policy RPL_MLDP_PART_P2MP
mdt c-multicast-routing bgp
!
mpls ldp
mldp
logging notifications
Since we are dealing with on demand flows from SSM joined groups, ASM groups are also supported but not tested. We will join a couple new SSM groups, we have enabled MLDP logging on all the routers as well. Not much has changed in the BGP or MPLS verification's so I will skip those and focus on new group signaling with the associated log messages and then verify the reachability.
R18 will join a new SSM group, 232.18.18.18 with a source of R16s PE facing interface.
R18
int l00
ip igmp join-group 232.18.18.18 source 100.64.204.16
We configure R18 to join a new SSM group.
R7
%MLDP-5-ADD_BRANCH: [gid 7 (0x00000007)] Root: 192.0.2.21, Add P2MP branch MDT remote label
R7 is the last hop router, it needs to add a new label mapping message and new P2MP MDT with a root of XR1, which is the PE that connects to the customer source.
XR1
mpls_ldp[1181]: %ROUTING-MLDP-5-BRANCH_ADD : 0x0002A [global-id 7] P2MP 192.0.2.21, Add PIM MDT branch remote label no_label
mpls_ldp[1181]: %ROUTING-MLDP-5-BRANCH_ADD : 0x0002A [global-id 7] P2MP 192.0.2.21, Add LDP 192.0.2.22:0 branch remote label 92026
XR1 also generated a log message, with 2 different labels for the new MDT, one "no_label" for the multicast label and label 92026 to reach the next hop of XR2.
R16#sh ip mroute ssm
(100.64.204.16, 232.18.18.18), 00:10:06/00:03:28, flags: sPT
Incoming interface: GigabitEthernet1.204, RPF nbr 0.0.0.0
Outgoing interface list: Null
We check R16 and see an (S, G) entry proving that the signaling worked.
R7#sh ip multicast vrf MVPN mpls vif
Interface Next-hop Application Ref-Count Table / VRF name Flags
Lspvif0 0.0.0.0 MDT N/A 1 (vrf MVPN) 0x1
Lspvif2 192.0.2.1 MDT N/A 1 (vrf MVPN) 0x1
Lspvif1 192.0.2.21 MDT N/A 1 (vrf MVPN) 0x1
We check the local VIF database and see that LSPVIF1 points to XR1.
R7#sh ip rpf vrf MVPN 100.64.204.16
RPF information for ? (100.64.204.16)
RPF interface: Lspvif0
Strict-RPF interface: Lspvif1
RPF neighbor: ? (192.0.2.21)
RPF route/mask: 100.64.204.0/24
RPF type: unicast (bgp 50693)
Doing distance-preferred lookups across tables
BGP originator: 192.0.2.21
RPF topology: ipv4 multicast base, originated from ipv4 unicast base
We check R7's output for the RPF connection, this is where we verify the "strict RPF interface" command which points to LSPVIF1 which is the overlay tunnel we'll use to reach the source of the traffic.
R7#sh ip pim vrf MVPN neighbor
PIM Neighbor Table
Mode: B - Bidir Capable, DR - Designated Router, N - Default DR Priority,
P - Proxy Capable, S - State Refresh Capable, G - GenID Capable,
L - DR Load-balancing Capable
Neighbor Interface Uptime/Expires Ver DR
Address Prio/Mode
100.64.214.18 GigabitEthernet1.214 2d10h/00:01:37 v2 1 / DR S P G
192.0.2.21 Lspvif1 04:37:57/00:01:42 v2 1 / DR G
Technically there shouldn't be any PIM adjacencies on R7 except to the CE device, but we have one towards XR1, which ties to LSPVIF1 which is our RPF interface. I can't find a way to disable PIM on XR to resolve this so I just ignore it knowing it's not necessary.
R7#sh mpls forwarding-table labels 7026 - 7029
Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or Tunnel Id Switched interface
7026 [T] No Label [gid 7 (0x00000007)][V] \
1220 aggregate/MVPN
7027 [T] No Label [gid 3 (0x00000003)][V] \
0 aggregate/MVPN
7028 [T] No Label [gid 1 (0x00000001)][V] \
0 aggregate/MVPN
7029 [T] No Label [gid 4 (0x00000004)][V] \
0 aggregate/MVPN
We check the MPLS forwarding table for "MVPN" and see 4 labels, which we then filter down to just those labels.
R16#ping 232.18.18.18 repeat 10
Type escape sequence to abort.
Sending 10, 100-byte ICMP Echos to 232.18.18.18, timeout is 2 seconds:
Reply to request 0 from 192.0.2.18, 30 ms
Reply to request 1 from 192.0.2.18, 8 ms
Reply to request 2 from 192.0.2.18, 8 ms
Reply to request 3 from 192.0.2.18, 9 ms
Reply to request 4 from 192.0.2.18, 8 ms
Reply to request 5 from 192.0.2.18, 8 ms
Reply to request 6 from 192.0.2.18, 13 ms
Reply to request 7 from 192.0.2.18, 8 ms
Reply to request 8 from 192.0.2.18, 10 ms
Reply to request 9 from 192.0.2.18, 7 ms
On the source, R16, we ping to the SSM group 232.18.18.18 and get replies back.
R7#sh mpls forwarding-table labels 7026 - 7029
Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or Tunnel Id Switched interface
7026 [T] No Label [gid 7 (0x00000007)][V] \
1830 aggregate/MVPN
7027 [T] No Label [gid 3 (0x00000003)][V] \
0 aggregate/MVPN
7028 [T] No Label [gid 1 (0x00000001)][V] \
0 aggregate/MVPN
7029 [T] No Label [gid 4 (0x00000004)][V] \
0 aggregate/MVPN
Checking the LFIB on R7 we see label 7026 has incremented bytes label switched.
Thanks for stopping by!
Rob Riker, CCIE #50693
