Signaling And Rpf On Gre Tunnels; Pim Registration; Auto-Rp - Cisco ASR 9000 Series Configuration Manual

MVPN over GRE
• Transport header support is limited to IPv4.
• Path MTU discovery will not be supported over GRE tunnel interfaces. When size of the packet going
over GRE tunnel interface exceeds the tunnel MTU, the microcode will punt the packet to the slow path
for best effort fragmentation. Since punted packets are policed, this doesn't provide real fragmentation
support. This combined with no support for path MTU discovery means that user is responsible for
making sure the MTUs configured along the tunnel path are large enough to guarantee the GRE packet
will not be fragmented between tunnel source and destination routers.
• No support for optional checksum, key, and sequence number fields.
• No support for nested and concatenated GRE tunnels. If packets with nested GRE header are received
they will be dropped.
• No L3 features (like QoS, ACL and netflow) support for GRE tunnel interfaces. Features configured on
the underlying physical interface will be applied.
• ASR9000 SIP-700 linecard unicast GRE is NOT supported on VRFs.
• Support for up to 500 GRE tunnels per system for multicast.

Signaling and RPF on GRE Tunnels

Signaling will use the same mechanism when a unicast GRE tunnel terminated at an ingress linecard regardless
of whether the GRE tunnel interface belongs to a VRF or not. In the case of mVPN-GRE the Master Linecard
/ Master NP mechanism must still be used for egress punts of decapsulated VRF packets.
RPF selection can be static configured via a route policy configuration. Static RPF is more preferred and
expected if the RPF should be the GRE tunnel. RPF may be selected dynamically via RIB updates for the
upstream router's unicast reach-ability, although this is not preferred.

PIM Registration

PIM registration packets can be forwarded on a unicast GRE tunnel as long as the IPv4 unicast GRE interface
is selected by FIB for unicast forwarding of the encapsulated PIM registration packets toward the PIM RP.
In this case, the packet is essentially double encapsulated with unicast, ie, the original multicast packet is
encapsulated by PIM in a unicast PIM register packet. This is then encapsulated with the unicast GRE tunnel
header.
At the PIM RP, outermost unicast header will be removed and the PIM registration packets will be delivered
to PIM via LPTS as in the current PIM registration packet processing. It is advisable to avoid any MTU/TTL
or ACL/QoS configuration issues that result in the registration packets getting dropped.

Auto-RP

Auto-RP lite on PEs, Auto-RP/BSR/static-RP/ Anycast-RP with MSDP peering etc can be supported over
GRE tunnels with MFIB netio chain support. It is advisable to avoid any MTU/TTL or ACL/QoS configuration
issues that result in the registration packets getting dropped. Auto-RP routes will flood autp-rp packets to
every multicast egress interface including IPv4 unicast GRE tunnels.
Cisco ASR 9000 Series Aggregation Services Router Multicast Configuration Guide, Release 6.0.x
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Implementing Layer-3 Multicast Routing on Cisco IOS XR Software