Why BGP LU route prefer over BGP ipv4?

[inderjeet452]

Hi Folks,

Thanks all for tour support every time.

Today I required your support to answer one question.

In MP-BGP we configured ipv4 and LU address family.

Just wanted to know why BGP LU routes are preferred over ipv4 routes.

[David Ruess]

Hello,

I wouldn't say its preferred, just more optimized. I believe when the router forms an IPv4 and a LU neighborship it just uses the LU routes. No need to send both IPv4 AND LU routes if they are the same. Take a look at this article, maybe it will help explain it better. See Summary at the end if you don't want to read the whole thing.

BGP Labeled Unicast on Cisco IOS « ipSpace.net blog

-David 

[inderjeet452]

Still answer is not there why it is being preferred over ipv4.

[Giuseppe Larosa]

Hello @inderjeet452 ,

the document provided by @David Ruess contains the following note:

>> Even though the routers negotiated IPv4 and IPv4-LU address families on the BGP session, the IPv4 BGP updates are sent only within the IPv4-LU address family. IPv4 address family is not used at all.

This is the optimization noted by David.

However, it makes sense to have BGP LU preferred over BGP IPv4 unicast because the first one provides directly an MPLS FEC , in other words it is more specific as it provides an MPLS label to be used when sending packets to the destination prefix via the BGP next-hop.

Hope to help

Giuseppe

[MHM Cisco World]

Dont confuse 

BGP vpnv4 use for intra AS of SP

BGP LU use for inter AS of SP

That it

MHM

[inderjeet452]

Not cleared

vpnv4 and ipv4 are different.

Need logic why BGP LU address family is prefer over BGP ipv4 address family.

[MHM Cisco World]

first there is BGP address family  ipv4 and vpnv4 there is no LU address family 

the BGP LU is called to BGP IPv4 when we config it to send label with prefix it not address family 

the BGP in MPLS 
1- for intra AS it use VPNv4 (which send label with prefix)
2- for Inter AS it use BGP LU (which send label with prefix)

MHM

[inderjeet452]

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/216498-labeled-and-unlabeled-together-on-one-bg.html

[MHM Cisco World]

even so new S-AFI for label unicast the principle is same 
from link you share
the AFI IPv4 have two S-AFI one prefix without label and other prefix with label, the 
s-AFI labeled is use to inter-AS mpls SP or extended label into DC. 

Unlabeled unicast for IPv4 is AFI 1 and SAFI 1.

Labeled unicast for IPv4 is AFI 1 and SAFI 4.

Unlabeled unicast for IPv6 is AFI 2 and SAFI 1.

Labeled unicast for IPv6 is AFI 2 and SAFI 4.

the book make it complex and un clear but it simple 

MHM

[MHM Cisco World]

https://iosonounrouter.wordpress.com/2022/01/11/advertising-routes-with-labels-bgp-labeled-unicast/

check this link also

MHM