Cisco ASR 9000 Series Software Manual page 42

MVPN over GRE
GRE Limitations
Listed below are the limitations for unicast GRE tunnels:
• GRE unicast tunnel supports IPv4 encapsulation only.
Note
• Native and mVPN traffic over underlying ECMP links are not supported.
Note
• IPv6 multicast for GRE unicast tunnels is not supported, in releases prior to IOS XR 5.3.2.
• 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.
Cisco ASR 9000 Series Aggregation Services Router Multicast Configuration Guide, Release 5.1.x
42
Starting from the IOS XR 5.3.2 release, GRE unicast tunnels support IPv6 encapsulation.
Starting with IOS XR 5.3.2 release, native and mVPN traffic over underlying ECMP
links, including bundles is supported.
Implementing Layer-3 Multicast Routing on Cisco IOS XR Software