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New Member

what is the second level label of a mpls packet?

i thought it was the RT or RD,but i can not associate the MBGP atrribute with the mpls label,confused,

please help me!

recommending a related book would be helpful

thanks

1 ACCEPTED SOLUTION

Accepted Solutions

Re: what is the second level label of a mpls packet?

Hi,

Yes you are totally right.

HTH,

Mohammed Mahmoud.

22 REPLIES

Re: what is the second level label of a mpls packet?

Hi,

With MPLS VPN, the second label is the MPLS VPN label which is exchanged between egress PEs via MBGP, and it will be responsible to deliver the VPN packets between the customers' sites, while on the other hand the RT will be used to control the distribution of the customers' routes between the PEs, the following book is one of the nice books that explains MPLS concepts:

MPLS Fundamentals

Luc De Ghein, CCIE

No. 1897

HTH,

Mohammed Mahmoud.

Re: what is the second level label of a mpls packet?

Hi,

Actually both RT and RD will be there in Second Label... Here your IP address and unique RD value make the VPNv4 address which will make the IP address unique in the MPLS network if you have multiple customer using the same IP addressing scheam... so RD must be there to creat the Unique VPNv4 addresss... now packet with the VPNv4 address will travel from one PE to other PE... now let me ask you one more question how remote PE will forward that VPNv4 packet to appropriate customer??? now RT will come in to the picture... RT with VPNv4 address will tell the remote end PE which customer PE has to forward the packet... so as a second label you will have both RD and RT...

first of all IPv4 address+RD=VPNv4... now To identify and proper forwarding to the remote CE RT will be added to VPNv4 address so now...

(IPv4+RD)+RT=VPNv4+RT=Second level label of MPLS

please let us know if you need any further explanation...

regards

Devang

Re: what is the second level label of a mpls packet?

WL & Devang,

When a PE router distributes a VPN-IPv4 route via BGP, it uses its own address as the "BGP next hop". It also assigns and distributes

an MPLS label. (Essentially, PE routers distribute not VPN-IPv4 routes, but Labeled VPN-IPv4 routes. When the PE processes a

received packet that has this label at the top of the stack, the PE will pop the stack, and send the packet directly to the site from to which the route leads. This will usually mean that it just sends the packet to the CE router from which it learned the route.

So RD is used to make the IPV4 route unique and the RT is to assign membership of the route to a given VRF. So they have no direct role in creation of the second label. And the second label does not contain the RD or RT values, it just another MPLS label with the same label format as the IGP label.

Reference : Lookup the RFC 2547 section "4.2.2. Route Distribution Among PEs by BGP"

Also what you are trying to do, and what did you manipulate to get what output would be good to start with.

HTH-Cheers,

Swaroop

New Member

Re: what is the second level label of a mpls packet?

Swaroop,

I am again confused now in one of my past post you gave explanation is:http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Service%20Providers&topic=MPLS&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.1ddf971b

so what is the exact 2nd label is?

what will be there in 2nd label?

see if it telll remote PE router which customer he has to send packet using 2nd label then it should be the RT label right??????

Re: what is the second level label of a mpls packet?

Hi Dan,

As Swaroop has stated, VPN labels has no relation with RT and RD, VPN labels are exchanged via MBGP (between PEs only, P router are not aware of the VPN labels), and it is imposed as the second label on a MPLS VPN packet, the first label is the LDP label and it is responsible for delivering the packet to the appropriate egress PE, and the second label (VPN label) is responsible to forward the packet to the desired CE router.

On the other hand RD is used to create the VPNv4 unique address of the customer (VPNv4 = IPv4 + RD) and RT is used to exchange the routes itself between VRFs on PEs, and as it is obvious there is no slight relation between VPN label and RT or RD.

I hope that i've been informative.

HTH,

Mohammed Mahmoud.

Re: what is the second level label of a mpls packet?

Dan, both replies are very much consitent. The second label is not the RT label but the label assigned to the prefix in a VPN/VRF routing table.

So again to conclude, RD is used to make the customer/VPN routes unique, as many customers may be using similar private addresses (192.168/172.16/10.), and the RT is used to exchange routes between VPN's.

And to these VPN routes the owner(the egress PE) of the routes assigns VPN label (second label). And advertises these routes to other PE's, with the label using MPBGP (VPNv4 Add Fly).

I would be glad to clarify any other questions to the best of my knowledge.

HTH-Cheers,

Swaroop

New Member

Re: what is the second level label of a mpls packet?

the RT and RD only used int exchanging route(control plane),and no relation with the actual traffic(forwarding plane),right?

Re: what is the second level label of a mpls packet?

Hi,

Yes you are totally right.

HTH,

Mohammed Mahmoud.

Re: what is the second level label of a mpls packet?

Hi,

As i've recommended earlier please check out this book:

MPLS Fundamentals

Luc De Ghein, CCIE

No. 1897

HTH,

Mohammed Mahmoud.

New Member

Re: what is the second level label of a mpls packet?

Mohammed

another question arise,when does the PE router insert the vpn label?and how to map this labeled packet to the mpls backbone label?

thank you

wlsj

Re: what is the second level label of a mpls packet?

Hi wlsj,

When the PE (Egress PE) is advertising the vpnv4 prefix via MP-BGP to other PEs, it also signals the VPN label that is associated with the vpnv4 prefix. And if by the mpls backbone label you mean the LDP label (Top label), then its simply discovered as the label corresponding to the next-hop of the BGP routes (or in other words the router advertising the route, which is the Egress router).

I hope that i've been informative.

HTH,

Mohammed Mahmoud.

Re: what is the second level label of a mpls packet?

Wlsj, if you go through this thread, when and how does the LDP label is mapped to the VPN label would be evident.

Without making it repititive, if would be nice if you can let us know what you are trying to do, and why do you want to map the VPN label with a specific LDP label, as this happends automatically as per the label stack formation.

HTH-Cheers,

Swaroop

Re: what is the second level label of a mpls packet?

Swaroop,

will you please take a look at this link:

http://www.cisco.com/application/pdf/en/us/guest/netsol/ns465/c654/cdccont_0900aecd80375d78.pdf page# 8 and step#2... so as per this document one label is used to identify the customer VPN prefix and other is MPLS label for forwarding...

regards

Devang

New Member

Re: what is the second level label of a mpls packet?

Second (top in the label stack) label is used by P's to forward packets between PE's. RT is propagated as a BGP ext community and never used for forwarding but to control inter-VRF export/import of routes.

Re: what is the second level label of a mpls packet?

Devang, the VPN label is used to identify the customer VPN prefix, in the very same way the IGP/LDP label is used to identify the global prefix. But none of the labels carry the RD or RT or the prefix into the label. Its just a mapping of the label values to the prefixes.

HTH-Cheers,

Swaroop

Re: what is the second level label of a mpls packet?

I got it Swaroop... so you mean to say 2nd label is generated by Egress PE and that will be used by only PE routes right!!! now one more question is as PE to PE forwarding is done by the MPLS Label which is our 1st label now why PE router need 2nd label... as PE will decide which VRF it has to install that route is based on the RT value so why we need 2nd label?

regards

Devang

Re: what is the second level label of a mpls packet?

Devang, IGP label(1st label) is required to label switch the path till the IBGP nexthop (which is other PE's advertising the VPN routes).Once it packet reaches the egress PE it has to find the right VRF to which the route belongs and then find the right outgoing interface. So from MPLS perspective it assigns a second label which is the mapping of VPN routes to label value. And as per this label (second label it identifies the right VPN). Without the second label there is no mapping to identify which VPN the packet belongs to.

Its more of the control plane and forwarding plane thing. When BGP sends the update it sends it with RT RD and VPN label to be used.

But when the packet from the customer side(data traffic) arrives that packet in its L3header only has a destination IP. No RD or RT so how would the PE's take the forwarding decision for that packet, so for that purpose the second label which was assigned as the part of control plane for a destination route (where in the routes's RD and RT values were known) is used.

So to conclude when forwarding packets there is no RD or RT information, its just labels which have been assigned when the packet entered the MPLS cloud.(1st label the IGP label for the BGP nexhop, and the second label (VPN label for the VPN prefix) to identify the right VPN for that packet.

HTH-Cheers,

Swaroop

Re: what is the second level label of a mpls packet?

Swaroop,

So it means there are two MPLS label one is for the IPv4 route which is the first label and other is for the VPNv4 route which is the second label right... and second label just only to identify the VPN only and that will be exchanged only between PEs... right!!!

so when packet arrived at the egress PE it will remove the second label and depends on the RT vaule it will put that route in appropriate VRF... so again VPN label will used only for the data transfer but installing route in VRF will only be handl by RT... and in BGP update PE will exchange the VPNv4 route,RT and VPN label...right!!!

regards

Devang

Re: what is the second level label of a mpls packet?

Yes Devang you got it right!

HTH-Cheers,

Swaroop

New Member

Re: what is the second level label of a mpls packet?

Swaroop and Devang,

I got benifit from your conversation thanks to both of you and originator of the post too.

Some are the things are very little but creating lots of confusion but you guys are there to make it clear. coollllllll

Re: what is the second level label of a mpls packet?

Thanks Swaroop for your help...

If i will have any further question then i will ask it again:)... thanks again

regards

Devang

Re: what is the second level label of a mpls packet?

Dan, Devang, Thank you :-) and you are welcome.

Cheers,

Swaroop

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