Auto-created Tunnel interfaces on Sup2T

[michacox]

I recently migrated from a Cat6k Sup720 running s72033-advipservicesk9_wan-mz.122-33.SXI3.bin to a Sup2T running s2t54-adventerprisek9-mz.SPA.150-1.SY1.bin. The config is basically the same, but on the new Sup2T I'm getting 7 Tunnel interfaces which are being created automatically, so to speak I guess. They aren't in the config. See below snippet from the show ip int brief output.

Tunnel0                10.30.2.98      YES unset  up                    up     

Tunnel1                10.30.2.98      YES unset  up                    up     

Tunnel2                10.30.2.98      YES unset  up                    up     

Tunnel3                10.30.2.98      YES unset  up                    up     

Tunnel4                10.30.2.98      YES unset  up                    up     

Tunnel5                10.30.2.98      YES unset  up                    up     

Tunnel6                10.30.2.98      YES unset  up                    up     

The address they all use is the Gig port that is the link to the WAN router (routed OSPF), below.

interface GigabitEthernet2/1

description WAN edge

ip address 10.30.2.98 255.255.255.248

no ip redirects

no ip proxy-arp

ip flow monitor Netflow-Monitor-1 input

ip pim query-interval 1

ip pim sparse-mode

ip ospf network point-to-point

ip ospf hello-interval 1

flowcontrol receive desired

Is anyone aware of what process or feature might be creating these interfaces? They don't appear to affect performance or functionality, but I am curious why they are there and where they came from since this behavior isn't seen on the old Sup720. Thanks.

Michael

[Peter Paluch · Accepted solution]

Hi Michael,

I believe these tunnels are related to the PIM Sparse Mode operation and they probably represent a PIM-Register tunnel used during the multicast source registration process. I have no firm proof of that but I have also seen these tunnels being created on the fly on software ISR router platforms on recent IOSes when PIM-SM was run.

Would it actually be possible to elicit any output from show run int tun0 up to show run int tun6 (yes, I know they are not in the config but sometimes this command works for dynamically configured interfaces, too)? Or at least show interface tun0 and show ip interface tun0?

Best regards,

Peter

[Steve Fuller · Accepted solution]

Hi Michael,

Peter is absolutely correct. These tunnels are created automatically by the Multicast Forwarding Information Base (MFIB) as part of the PIM sparse-mode registration process.

From the IPv4 Multicast Layer 3 Features section of the Catalyst 6500 Software Configuration Guide:

The PIM-SM protocol requires the first-hop router for a multicast source to send a register packet to the Rendezvous Point (RP) when the source starts sending packets. On the Supervisor Engine 2T, the PIM control-plane uses the MFIB infrastructure to represent the PIM register tunnel as an interfaceand adds it to the outgoing interface list when PIM register packets are sent. When a new PIM RP is configured or learned, a PIM register tunnel interface is created and used for PIM register packets. The interface is deleted when a register-stop packet is received. On the RP, an additional interface is created and used to decapsulate the PIM register packet when it is received.

You can see the tunnels created and the groups associated with those tunnels using the show ip pim tunnel command.

Regards

[michacox]

Thanks to both of you.

Yes, the show ip pim tunnel output does show the PIM-related tunnels:

#sh ip pim tunnel
Tunnel0 
  Type  : PIM Encap
  RP    : 10.28.254.253
  Source: 10.30.2.98
Tunnel1 
  Type  : PIM Encap
  RP    : 10.28.254.254
  Source: 10.30.2.98

The show run int tun0 command only produces:

#sh run int tun0
Building configuration...

Current configuration : 5 bytes
end

However, the show int does indeed show some valid output though:

#sh int tun0
Tunnel0 is up, line protocol is up 
  Hardware is Tunnel
  Interface is unnumbered. Using address of GigabitEthernet2/1 (10.30.2.98)
  MTU 17864 bytes, BW 100 Kbit, DLY 50000 usec, 
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation TUNNEL, loopback not set
  Keepalive not set
  Tunnel source 10.30.2.98 (GigabitEthernet2/1), destination 10.28.254.253
   Tunnel Subblocks:
      src-track:
         Tunnel0 source tracking subblock associated with GigabitEthernet2/1
          Set of tunnels with source GigabitEthernet2/1, 7 members (includes iterators), on interface 
  Tunnel protocol/transport PIM/IPv4
  Tunnel TOS/Traffic Class 0xC0,  Tunnel TTL 255
  Tunnel transport MTU 1472 bytes
  Tunnel is transmit only
  Tunnel transmit bandwidth 8000 (kbps)
  Tunnel receive bandwidth 8000 (kbps)
  Last input never, output never, output hang never
  Last clearing of "show interface" counters never

Good to know!

Michael

[aian792003]

You can use the "show derived-config interface tunnel #" to get the interface derived configuration:

Router#show derived-config interface tunnel 1
Building configuration...

Derived configuration : 199 bytes
!
interface Tunnel1
description Pim Register Tunnel (Encap) for RP x.x.x.x
ip unnumbered Vlanxxx
tunnel source Vlanxxx
tunnel destination x.x.x.x
tunnel tos 192
no routing dynamic
end

Best regards.