Subject: [mpls] RE: [MPLS-OPS]: basic doubt about LERs in MPLS

From: "Christopher Young" <[email-masked]>
Date: Thu, 18 Jan 2007 08:46:52 -0500

Shilpa,

Here is an example from an MPLS BGP VPN which shows the flow of
information you are looking for. In this example the destination address
would be 30.6.0.0. A route lookup in the virtual routing and forwarding
instance table indicates that the next hop is an MPLS next-hop with a
VPN label of 16 to be appended on to the packet, with a base label for
the LSP of 19 which goes out over the POS0/0 physical interface. You are
correct in that there are separate tables that maintain all of this
information. The detailed routing table display ties it all together and
shows the complete forwarding decision that is made for reaching this
destination of 30.6.0.0

test:pe1#sh ip route vrf vpn1 30.6.0.0 detail

Protocol/Route type codes:

  I1- ISIS level 1, I2- ISIS level2,

  I- route type intra, IA- route type inter, E- route type external,

  i- metric type internal, e- metric type external,

  O- OSPF, E1- external type 1, E2- external type2,

  N1- NSSA external type1, N2- NSSA external type2

  L- MPLS label, V- VRF, *- via indirect next-hop

30.6.0.0/30 Type: Bgp Distance: 200 Metric: 0 Tag: 0 Class: 0

  MPLS next-hop: 132, label 16, VPN traffic, resolved by MPLS next-hop
60

    MPLS next-hop: 60, resolved by MPLS next-hop 123, peer 31.0.0.1

      MPLS next-hop: 123, label 19 on POS0/0 (ip19000002.mpls.ip
[V:pe1]), nbr 11.13.0.1

This is an example of an MPLS forwarding table entry showing a label
binding to the FEC 31.0.0.1. If you look at the display above you see
that this label of 19 is displayed in the detailed "show ip route"
output as the base label to append to packets headed to the next hop PE
router 31.0.0.1 which originated the VPN route 30.6.0.0.

test:pe1#sh mpls ip binding 31.0.0.1

  31.0.0.1/32

    Out   19  neighbor 11.13.0.13

    Out   20  neighbor 11.14.0.14

Hope this helps in illustrating the forwarding decision.

Thanks,

Christopher Young
Resident Engineer
JNCIP-E ERX #9
(978) 973-0574
[email-masked]

________________________________

From: shilpa goel [mailto:[email-masked]] 
Sent: Thursday, January 18, 2007 5:03 AM
To: [email-masked]; [email-masked]; [email-masked]
Subject: [MPLS-OPS]: basic doubt about LERs in MPLS

---------- Forwarded message ----------
From: shilpa goel <[email-masked]>
Date: Jan 18, 2007 3:32 PM 
Subject: Re: [MPLS-OPS]: basic doubt about LERs in MPLS
To: Christopher Young <[email-masked]>

Christopher,

Thanks for the reply. Can you give me a little more insight on the
points given below?

You have mentioned that 'The destination of that packet will yield
whether or not the packet will be sent out over a regular IP interface
or encapsulated in MPLS and sent over an LSP'
As I understand routers maintain 2 routes - one in IP forwarding table
for some destination say 192.168.1.0 <http://192.168.1.0/> and one in
MPLS LFIB with the same destination address. I am considering the case
of LDP protocol setting up LSPs in response to route updates by IGPs.
Now when a LER interface receives an IP packet with destination
192.168.1.0 <http://192.168.1.0/> , will it consult some common table
which will indicate which of the 2 tables mentioned above (IP or MPLS)
will be used for routing of the packet? If so, how is this common table
configured? 

Also is it "possible" or "advisable" for all packets (control/protocol
or management) generated at the router to be IP routed by default? What
is the usually adopted implementation?

thanks,
Shilpa

On 1/17/07, Christopher Young < [email-masked]
<mailto:[email-masked]> > wrote: 

	Shilpa,

	For your first question an LER always does a lookup on the IP
destination address of a packet regardless of whether or not the packet
is meant to be encapsulated with MPLS or not. In other words, a router
still functions like a router even with MPLS turned on. The destination
of that packet will yield whether or not the packet will be sent out
over a regular IP interface or encapsulated in MPLS and sent over an
LSP.

	(NOTE: In L2 VPN's where you don't do a L3 lookup this is
different as usually a configured association is made between a CE-to-PE
interface and an LSP)

	For your second question, my statement above still applies. For
example if your IP routing table yields the LSP as the next-hop to the
loop back address of your IBGP peer then you send the BGP packets over
the LSP.  For ISIS,OSPF, RSVP and LDP control packets those will still
be sent out on the base POS/ATM/GE IP interface without MPLS
encapsulation, but that is an exercise left up to the individual vendor.
Note that if using hierarchical MPLS tunnels (LDP over RSVP) the LDP
packets would ride over the MPLS LSP. Also, of note is that in all
vendors you can control whether or not IP traffic and IGPs actually use
LSPs for forwarding and next-hop computation with configuration knobs. 

	Hope this helps.

	Thanks,

	Christopher Young
	Resident Engineer
	JNCIP-E ERX #9
	(978) 973-0574
	[email-masked]

________________________________

	From: shilpa goel [mailto:[email-masked]] 
	Sent: Wednesday, January 17, 2007 6:09 AM
	To: [email-masked]; [email-masked]; [email-masked]
	Subject: [MPLS-OPS]: basic doubt about LERs in MPLS

	Hi,

	 I have one basic doubt regarding LERs in IP/MPLS networks.
	
	How does a LER decide whether it should do IP routing (i.e. IP
Forwarding Table lookup) or labelling (i.e. LFIB lookup) of the
unlabeled IP packets that it receives at an interface? 
	
	Linked to this I have another doubt that how are
control/protocol packets (which are IP) routed in a router i.e. are they
label switched along the LSPs that exist for the FECs/destinations to
which they are addressed or by default IP routed? 

	regards,
	Shilpa

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