[c-nsp] tag-switching advertise-tags for?

[Robert Kiessling]

Jon Lewis wrote:
> If I wanted to use MPLS/tag-switching only for the transport of MPLS VPN 
> traffic, leaving all other IP traffic routed rather that tag-switched, 
> could I use some invocation of tag-switching advertise-tags for do this?

You will need separate BGP sessions for IPv4 and VPNv4 prefixes,
with different session endpoints (usually Loopback interfaces) [1].

interface Loopback0
  ip address 10.0.1.1 255.255.255.255
!
interface Loopback10
  ip address 10.0.2.1 255.255.255.255
!
no tag-switching advertise-tags
tag-switching advertise-tags for ldp_filter
!
ip access-list standard ldp_filter
  permit 10.0.2.0 0.0.0.255
  deny   any

Or the "mpls ldp advertise-label" variety if you use current
IOSes.

Then use Loopback0 as source-interface for the ipv4 sessions
and Loopback10 for the vpnv4 sesions.

> Does tag-switching advertise-tags for <acl number> apply only to main 
> table IP traffic, or would it also affect tag advertisement for connected 
> vpnv4 vrf prefixes?

You run LDP only in the global routing domain, so it can only
affect global prefixes. I don't believe Cisco supports LDP inside
a VRF (which you could use for label exchange in a
carrier-of-carrier-scenario).

> Will interfaces connecting 2 routers with "tag-switching ip" configured 
> tag switch tagged packets and "ip route / cef switch" non-tagged packets?

"mpls ip" enables MPLS but does not force it (how could that work?).
If your routing tables doesn't have a label for the destination,
it will use plain old IP forwarding.

Robert

[1] Not strictly true since you could mess with the next hops
in one session, but that's just messy.