request for documents to read about the PIM GRE tunnel

[Ab26]

Hi, PIM creates automatically mGRE tunnel. I've been searching for more info about this kind of automatically created tunnel of PIM but with no luck. All I can find is either about PIM or GRE. Anyone can help me and share few links to read?

Is there anything to look at on this GRE tunnel? for example troubleshoot? 

Router#show derived-config interface tunnel0
Building configuration...
Derived configuration : 133 bytes
!
interface Tunnel0
ip unnumbered Loopback0
no ip redirects
ip mtu 1500
tunnel source Loopback0
tunnel mode gre multipoint
end

[Giuseppe Larosa]

Hello @Ab26 ,

this is Draft Rosen Multicast L3 VPN , the multipoint GRE reprents a logical distribution tree that is the default MDT Multicast Delivery Tree associated to Global VRF multicast address that is the basis for this data plane solution.

Customer multicast in VRF is encapsulated into multicast GRE packets with source = local loopback desitnation = MDT address

This tunnel is created automatically on each PE node. There is nothing to tune on it.

You can see it as the logical interface used for encapsulating customer multicast in VRF into multicast GRE. And for decapsulation of received packets from other PE nodes as well.

All PE nodes must agree on the MDT address used for a specific L3 VPN.

the multipoint GRE provides a virtual LAN segment where all PE nodes are connected to and they exchange PIM messages over it.

The modern service to support multicast traffic is called NG MVPN and it can use mLDP on Cisco devices and appropriate MP BGP AF.

in your case you should be Draft Rosen native

>> Rosen GRE (native): MVPN uses GRE with unique multicast distribution tree (MDT) forwarding to enable scalability of native IP Multicast in the core network. MVPN introduces multicast routing information to the VPN routing and forwarding table (VRF), creating a Multicast VRF. In Rosen GRE, the MCAST customer packets (c-packets) are encapsulated into the provider MCAST packets (p-packets), so that the PIM protocol is enabled in the provider core, and mrib/mfib is used for forwarding p-packets in the core.

Draft Rosen does not use MPLS for forwarding and it cannot take advantage of MPLS FRR, this has been the main driver for introducing NG MVPN.

see

https://www.cisco.com/c/en/us/td/docs/iosxr/ncs5xx/multicast/710x/b-mcast-cg-710x-ncs540/implementing-multicast.html?dtid=osscdc000283#con_2631058

Hope to help

Giuseppe

[Ab26]

Thanks a million @Giuseppe Larosa for the detailed explanation! I've unfortunately never heard of NG MVPN. The link you shared is for XR IOS, does NG MVPN works on regular IOS, i.e XE ? I found for Nexus IOS but not XE. 

[Giuseppe Larosa]

Hello @Ab26 ,

NG MVPN is supported on selected platforms  for sure ASR 9000 and ASR 1000.

see the following link from ASR 1000 configuration guide

https://www.cisco.com/c/en/us/td/docs/routers/ios/config/17-x/ip-multicast/b-ip-multicast/m_imc_mldp-based_mvpn.html

As I have explained NG VPN can be see as a specialized feature that can be used by ISP that need sub second multicast convergence or they need to support multicast in a high number of L3 VPN ( scalability reasons).

I will look to find a document that explains Draft Rosen MVPN.

Hope to help

Giuseppe

[paul driver]

Hello
When multicast is enable on a rtr and a rendezvous point is defined (pim-sparse mode - static/auto/bsr pim) multicast (MC) tunnel interface(s) are automatically created.

So lets say for instance a static RP mapping is created on a mc enabled rtr ( ip pim rp-address x.x.x.x) the rtr will believe its directly connected to the MC source, as such a pim tunnel will be auto created, These tunnels are used to encapsulate registration messages from the FHR/DR towards the RP

Similar tunnels are create on the RP rtr itself but are used to decpasualte the received encapsulated messages.

Note: these dynamic MC tunnel interfaces begin with tunnel 0 and increment when additional rp-mappings are created, so it best practice to keep this in mind when you create manual gre/ipsec tunnel interface(s) and begin with a high value, so not to duplicate the tunnel numbers.

example:

conf t
ip multicast-routing
ip pim rp-address 1.1.1.1 10
ip pim rp-address 2.2.2.2 11

sh derived-config | in Tun
interface Tunnel0
description Pim Register Tunnel (Encap) for RP 1.1.1.1
interface Tunnel1
description Pim Register Tunnel (Encap) for RP 2.2.2.2

(config)int tun 0
%Tunnel0 used by PIM for Registering, configuration not allowed

(config)#int tun 1
%Tunnel1 used by PIM for Registering, configuration not allowed