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

[Aamer Akhter (aakhter)]

Couple of additional points:

*) one may use the same session but use the bgp next-hop feature (under vrf) or a route-map to modify the bgp next hop based on addresss family (already mentioned)

*) yes, IOS supports LDP in a vrf (one of the CSC cases)


*) on when traffic gets mpls encapped:

 *.*) 'mpls ip' will enable mpls processing for that interface, it will also enable ldp/tdp. But this is no guarentee that an ip packet coming into the LER will get mpls encapped. There are a variety of reasons for the ip packet to get a mpls label:

  1) LDP/TDP has advertised a FEC, the FEC is in the routing table, and the LSR advertising that FEC is reachable via an MPLS enabled interface
  2) eBGP (rfc3107) with constrains from 1
  3) TE tunnel

The best way to check what what will happen to an ip packet is to check the cef entry for that prefix.

hth


-- 
Aamer Akhter / aa at cisco.com
NSITE - cisco Systems 

> -----Original Message-----
> From: cisco-nsp-bounces at puck.nether.net 
> [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of 
> Robert Kiessling
> Sent: Monday, October 24, 2005 1:36 PM
> To: Jon Lewis
> Cc: cisco-nsp at puck.nether.net
> Subject: Re: [c-nsp] tag-switching advertise-tags for?
> 
> 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.
> 
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