Subject: Re: [MPLS-OPS]: Nos of Labels in CE

From: John Smith <[email-masked]>
Date: Fri, 20 Jun 2003 09:28:38 +0100 (BST)

Aamer,
 
> > WHat is that implementations generally do when they have multiple prefixes
> > associated
> > with a single interface.
> 
> I think you want to say same next-hop rather than interface.

No, I mean the same interface. What difference does it make to me if i have a prefix
which has different next-hops and both the next-hops are reachable to me thru the same
interface/sub-interface (e.g. ethernet)

> 
> >DO they advertise a single label for all these routes
> > OR does it
> > advertise multiple unique labels per route?
> > 
> > I think it should be the former.
> > 
> > Is there any case when one might want to do the latter?
> 
> In general, cases where it might be useful to be able to distinguish between
> prefixes.
> 
> One specific case is in mpls-vpn if the prefix is reachable via two
> different  nexthop's on the same PE. This allows the PE to loadshare between
> the two next-hops.

PE can only load share if both the next-hops are accessable through different interfaces.
Thus in this case too, the PE will be advertisiing differnet labels per interface. IMHO
the label should be associated with an interface. So that when this PE receives a MPLS
packet with this label, it does nothing but pushes the native IPv4 packet out on the
associated interface!

Am i missing something?

Regards,
John Smith 

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---

From: Aamer Akhter <[email-masked]>
Date: Fri, 20 Jun 2003 08:38:22 -0400

> interface/sub-interface (e.g. ethernet)

Which next-hop would you forward the packet to, in the case of multiple
next-hops reachable via a multi-access interface (eg ethernet)? Keep in mind
that unlike point-point interfaces you have to do a L2 rewrite that includes
a MAC address on multi-access interfaces.

> packet with this label, it does nothing but pushes the native IPv4 packet out

I am guessing you will advertise the prefix twice in BGP? BGP's best path
will make sure that only one of the labels will ever get used. (there are
exceptions to the rule in this case, of course).

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-- 
Aamer Akhter / [email-masked]
NSITE - cisco Systems

---

From: "Vinay Bannai" <[email-masked]>
Date: Fri, 20 Jun 2003 06:40:51 -0700

John,

If both the next hops are available on the same interface (ethernet as you
mentioned), then you need to have a distinct mac address for each of the
next hop.

Either you have one label per route and associate the next hop L2
information (usually the destination mac address) with the label and save it
in the lookup table OR you pop the label and do a lookup on the IP DA to
figure out the next hop that you can ARP (assuming it is still ethernet) to
get the next hop L2 information.

However, if you have point to point link (say PPP) as the outgoing
interface, then it might make some sort of sense to associate label binding
for IP routes based on outgoing links. But if you do that you lost your
ability to load balance unless you can figure out a way to associate
multiple label bindings for the same IP routes emanating from different
interfaces.

Vinay Bannai
Luminous Networks

-----Original Message-----
From: John Smith [mailto:[email-masked]]
Sent: Friday, June 20, 2003 1:29 AM
To: [email-masked]
Cc: [email-masked]
Subject: Re: [MPLS-OPS]: Nos of Labels in CE

Aamer,

> I think you want to say same next-hop rather than interface.

No, I mean the same interface. What difference does it make to me if i have
a prefix
which has different next-hops and both the next-hops are reachable to me
thru the same
interface/sub-interface (e.g. ethernet)

> different  nexthop's on the same PE. This allows the PE to loadshare
between

PE can only load share if both the next-hops are accessable through
different interfaces.
Thus in this case too, the PE will be advertisiing differnet labels per
interface. IMHO
the label should be associated with an interface. So that when this PE
receives a MPLS
packet with this label, it does nothing but pushes the native IPv4 packet
out on the
associated interface!

Am i missing something?

Regards,
John Smith

-------

---

From: "Alok Dube" <[email-masked]>
Date: Wed, 25 Jun 2003 14:51:04 +0530

Aamer,

I would like to ask the following:

> > No, I mean the same interface. What difference does it make to me if i
have a
> > prefix
thru
> > the same

agreed,

but wherever the "label switching starts" too,you have to do an L3 lookup..

take the case of a packet entring a VRF with multiple interfaces in the
VRF.(L3VPN)

how does one determine where to Label switch the packet to without doing an
L3 lookup?

atleast in the case of binding labels to P2P interfaces we have an advantage
of avoiding an L3 lookup at egress point.

---

From: Aamer Akhter <[email-masked]>
Date: Wed, 25 Jun 2003 09:53:06 -0400

> but wherever the "label switching starts" too,you have to do an L3 lookup..
Not really. You only really need to look at the ip address (lets ignore
internal loadsharing implementations for now) if the label type is
'aggregate'. In the aggregate case the label does not convey enough
information about what the forwarding behavior should be.

> how does one determine where to Label switch the packet to without doing an

When you allocate the label, you should know what the forwarding behavior is
going to be for the label. If you need to do a arp later on, you do it and
cache it.