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in inter-as mpls vpn option B,vpn label question

why asbr-pe rewrite vpn label because next-hop changed ,why.

if vpn label is not rewritten, is there any problem? what problem it is.

thank you!

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: in inter-as mpls vpn option B,vpn label question

Hi,

The problem arises when forwarding the packet, because of the construction of the label stack. In general the VPN label MUST be assigned by the BGP next hop of a VPNv4 route. Assume an ASBR-PE with next hop IP 1.1.1.1 will not rewrite a label learned f.e. VPN label 12345 and the ASBR-PE has no label 12345 assigned himself.

When the label stack is created by another PE in this AS in general it will look like this:

Top label: LDP label for 1.1.1.1/32 assigned by a P router

VPN label 12345

This will be sent towards the ASBR-PE. as the top label points to 1.1.1.1/32 typically the ASBR-PE will assign an implicit null label, i.e. penultimate hop popping occurs. This means that the penultimate hop will remove the top label and send the rest, i.e. VPN label 12345. This arrives at the ASBR-PE and no matching LFIB entry in my example would lead to packet drops.

So again, VPN labels MUST be assigned by the BGP next hop of a VPNv4 prefix or the packets will be dropped or forwarded to the wrong LSP.

This any modification of the BGP next hop needs to go along with a rewrite of the VPN label.

Hope this helps! Please rate all posts.

Regards, Martin

3 REPLIES
Cisco Employee

Re: in inter-as mpls vpn option B,vpn label question

Hi,

The problem arises when forwarding the packet, because of the construction of the label stack. In general the VPN label MUST be assigned by the BGP next hop of a VPNv4 route. Assume an ASBR-PE with next hop IP 1.1.1.1 will not rewrite a label learned f.e. VPN label 12345 and the ASBR-PE has no label 12345 assigned himself.

When the label stack is created by another PE in this AS in general it will look like this:

Top label: LDP label for 1.1.1.1/32 assigned by a P router

VPN label 12345

This will be sent towards the ASBR-PE. as the top label points to 1.1.1.1/32 typically the ASBR-PE will assign an implicit null label, i.e. penultimate hop popping occurs. This means that the penultimate hop will remove the top label and send the rest, i.e. VPN label 12345. This arrives at the ASBR-PE and no matching LFIB entry in my example would lead to packet drops.

So again, VPN labels MUST be assigned by the BGP next hop of a VPNv4 prefix or the packets will be dropped or forwarded to the wrong LSP.

This any modification of the BGP next hop needs to go along with a rewrite of the VPN label.

Hope this helps! Please rate all posts.

Regards, Martin

New Member

Re: in inter-as mpls vpn option B,vpn label question

Hi Martin:

I understand.

why doesn't ABSR-pe check VPNv4 route table to see traffic belong which traffic.

for example , two lsp. take same vpn lable arrived at ABSR-PE. but it is have different RD in vpnv4 route.

when two lsp traffic come back, i know the vpn lable is same. but ABSR-PE can further check VPNV4 destination address in mpls packet and check VPNV4 LFIB to determine the right LSP.

when config ABSR-PE,i can see vpnv4 routes in show ip bgp vpnv4 all.

oh, i know , ABSR-PE don't put this VPNV4 route into route table and LFIB in ABSR-PE,the LFIB don't have this information.

why ABSR-PE don't put vpnv4 bgp routes into route table and LFIB,if ABSR-PE put vpnv4 route LFIB , ABSR can check VPNV4 route to detemine right LSP,like summary in ospf .

oh, i know, sorry,because the return traffic has one vpn label, ABSR first check vpn label, but two lsp may have same vpn lable.

traffic will send to wrong lsp,it is defferent in ospf summary,last label is poped.

(if design option B like this, if ABSR PE receive mpls vpn traffic, ABSR-PE pop the last vpn lable and check vpnv4 route table/LFIB to determine correct next hop and vpn lable(first ABSR-PE must put vpnv4 bgp route into routing table and lfib), but absr-pe will do cef check,not lable switching,it affect performance of ABSR-PE.)

so asbr-pe rewrite vpnv4 lable is a best choice, when vpnv4 lable rewrite ,absr-PE will creat a LFIB, so LFIB entry has one outgoing to other AS vpnv4 lable ,one orginal vpnv4 lable, one orignal next hop label. this is pure lable switching.don't need check vpnv4 routes or cef.

does i understand correctly

thank you,you give me great knowledge.it has been confusing me many days.

oh another question, ASBR-PE will send all VPNV4 routes to other AS, can i limit just only some VPNV4 routes can send to other AS on ASBR-PE. may be i need check some document.

Jun

New Member

Re: in inter-as mpls vpn option B,vpn label question

Martin,

will you please clear it again in little bit detail? as VPN label is exchange between the two PE only right so here next hop you are talking is the IP address of remote PE or what?

and will you please again explain little bit in detail why we need to change the vpn label as next hop will change?

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