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Re: loopback /32 mask

  • From: Eric Osborne <eosborne@cisco.com>
  • Date: Sun, 18 Mar 2001 18:38:54 -0500
  • Cc: MPLS Mailing List <mpls-ops@mplsrc.com>
  • Resent-Date: Sun, 18 Mar 2001 19:39:41 -0500
  • To: Raymond Cheung <rcheung@slb.com>
  • User-Agent: Mutt/1.2.5i
  • X-GPG-Fingerprint: 6412 0836 E440 B3EA 980C 4951 611E 1819 2E71 8562

On Sun, Mar 18, 2001 at 01:59:21PM -0600, Raymond Cheung wrote:
> I also changed the mask to /30, and the tunnel is still set up. I also
> enable "ip ospf network point-to-point" under this loopback since OSPF
> advertises only host route for loopbacks no matter what mask is. I also
> rebooted the router, and the tunnel is still up. From the other P routers,
> /30 prefix did show up instead of /32. Then I changed the TE path from
> explicit path to dynamic path, the tunnel is torn down and then set up
> without any problems.

Right; like I said, I believe the /32 is mandatory in the specs, but
we don't enforce that.  I could be wrong about the mandating about the 
/32, but obviously we don't enforce it either way.

> I just don't understand why the /32 mask should be mandatory from some Cisco
> presentation. It looks like other masks work too.
> 

They do, for TE.

The funny restriction about /32s is not really about the endpoint of a
tunnel, but about the next-hop for a VPN route.  If you have a
next-hop that's not a /32 for a VPN route, you can't guarantee that
the route you *do* have (a /30 or /24 or whatever) wasn't the result
of address summarization at some point.  

And since you can't guarantee that, then there's no way to assure
you'll be able to always avoid running into an aggregate label.

And since the inner label on a VPN packet has no meaning to anyone but 
the PEs, you don't want a P somewhere in the middle of your network
trying to do a label lookup on a VPN label.

Thus, the mandatory /32 for VPN next-hops.



eric



> -----Original Message-----
> From: Raymond Cheung [mailto:rcheung@slb.com]
> Sent: Saturday, March 17, 2001 4:56 PM
> To: MPLS Mailing List
> Subject: loopback /32 mask
> 
> 
> Any particular reason for loopback to be /32 mask during RSVP-TE config? I
> understand it's good to have loopback as router id or headend, or ..., but
> my question is, is it mandatory that the mask should be /32? why other masks
> won't work?
> regards,
> 
> ================================================
> Yinglam (Raymond) Cheung
> Schlumberger Network Solutions
> CCIE (R/S), CCNP+ATM, CCDP
> Tel: (713) 513-3285
> 5599 San Felipe St Suite 400
> Houston, TX 77056
> Email: rcheung@slb.com
> ================================================
> 
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