UMH (Upstream Multicast Hop) Challenge
Last updated
Last updated
Load umh.challenge.init.cfg
Profile 13 is pre-configured, and all customer routers are using PIM-SSM for the default range. PE2 and PE3 are originating a dummy 100.100.100.100/32 route in the CUSTOMER VRF. C1 has joined (100.100.100.100, 232.1.1.1).
The operator has noticed that PE1 is selecting PE2 as the upstream router due to the lower IGP cost. However, the operator has other routers in the network that are older and only support using the highest IP address as the tiebreaker. Find a way to fix PE1 so that it selects PE3 as the upstream router without changing IGP costs or influencing the BGP path selection for the unicast 100.100.100.100/32 route. Configure all PEs with this setting.
When a PE has multiple nexthops to a unicast VPNv4 address, it must make a decision as to which PE to “direct” the (S, G) type 7 route towards. The egress PE decides which import RT to place in the type 7 route, which controls which ingress PE will import the route and install the (S, G) state in the VRF MRIB.
It is important that all routers in the network use the same logic when selecting the upstream multicast hop (UMH). If some ingress routers choose different ingress PEs, there could be duplicate traffic in the network. By default, IOS-XE and IOS-XR choose the IGP closest PE as the UMH. This can produce duplicate streams in itself depending on the placement of the egress routers in the topology. It appears that IOS-XE and IOS-XR simply use the bestpath VPNv4 route to decide which PE to use.
Examine the current state of PE1. It’s bestpath to 100.100.100.100 is via PE2 because of the lower IGP cost to the nexthop.
PE1 uses PE2’s MVPN VRF route target for the type 7 route that it generates based on the received (S, G) PIM Join from CE1.
To make this more predictable, and solve cases where you don’t want multicast ingress PEs sending duplicate traffic, you can set the UMH decision to always use the highest IP address, regardless of the best unicast VPNv4 route. This is configured under the BGP VRF ipv4/unicast AFI on both IOS-XE and IOS-XR.
Notice that the VPNv4 route is still best via PE2:
However, now the type 7 route is generated using PE3 as the ingress PE, not PE2. This is simply because PE3’s loopback is higher.
When all routers are configured for this setting, then only the PE with the highest loopback address will send multicast traffic onto the MDT.
