Internal vs external BGP VPNv4 neighborship between PE routers

[riad1990new]

Hi,

Consider a typical MPLS L3 VPN network (CE1 <-> PE1 <-> P <-> PE2 <-> CE2).

To my understanding, internal BGP VPNv4 neighborship is configured between PE1 and PE2, but what's the difference if we configure an external BGP VPNv4 neighborship between the two PE routers? Are there any advantages or disadvantages for doing so?

Thanks,
Riad. 

[MHM Cisco World]

you run one SP then you have one BGP AS number so you need iBGP VPNv4
you run one SP but it have two BGP AS number so you need eBGP VPNv4
there is tow diff SP then inter-AS need eBGP VPNv4

[Flavio Miranda]

Hi

 I think it should be considered based on the difference between protocols and the topology. I believe the Type of autonomous system may be the most important parameter for decision.

The table below summarizes the differences between iBGP and eBGP.

Description	iBGP	eGBP
Full name	Internal Border Gateway Protocol	External Border Gateway Protocol
Type of autonomous system (AS)	Runs between two BGP routers in the same AS	Runs between two BGP routers in different ASs
Default administrative distance	200	20
Route advertisement	Routes received from a peer cannot be advertised to another peer but can be advertised to an eBGP peer	Routes received from a peer can be broadcast to both eBGP and iBGP peers
Full mesh topology	Required	Not required
Usage	Within the same organization	Distinct networks (i.e., different organizations or internet service providers)
Loop prevention	Uses BGP split horizon	Uses path for loop prevention
Default peer setup	TTL = 255	TTL = 1
Local preference attributes	Sent	Not sent
Behavior on route advertisement to a peer	The next hop remains unchanged	The next hop is changed to the local router

[riad1990new]

Thanks Flavio, I wonder if (all) internal and external BGP neighborship differences apply to BGP VPNv4 neighborships as well? But even from pure design perspective, , is going with an iBGP design better? maybe can be scalable as you can utilize route reflectors with your iBGP VPNv4 neighborships? maybe going with an eBGP VPNv4 neighborship design would give you more control over routes advertisements?

[Flavio Miranda]

I believe it does  as the protocol behavior can not be changed according to platform or IOS version, etc. 

I would not say iBGP is better or worst but I believe it fits better in some situation and eBGP not and vice versa.

"maybe going with an eBGP VPNv4 neighborship design would give you more control over routes advertisements?"

But also would create you some problem with "Behavior on route advertisement to a peer".

[M02@rt37 · Accepted solution]

Hello @riad1990new,

In general, using iBGP VPNv4 neighborships is a common and scalable design choice for MPLS L3 VPN networks. Route reflectors can be used to simplify the iBGP configuration and reduce the number of BGP sessions required between PE routers. This can improve scalability and reduce complexity.

On the other hand, using eBGP VPNv4 neighborships between PE routers can provide more control over routing and route advertisements, but may also introduce additional complexity and configuration overhead. This approach may be more suitable for networks with specific requirements or constraints, such as the need for advanced policy routing or the use of multiple service providers.

[MHM Cisco World]

iBGP VPNv4 different than iBGP IPv4 in one point, 
this point is Next-hop is by default change to self in iBGP VPNv4 
this way the VPN LO is used by PE for forward traffic to CE. 
that only different other is same. 
again the design force you iBGP or eBGP, we talk here about public AS number not prviate AS number that you can config any one.

[MHM Cisco World]

I think you new learn SP, 
SP have one AS how you can config eBGP ?  You can not, you must use iBGP (if there is I want to see how you can config it)
the case you can use eBGP VPNv4 what I list above 
 SP have two AS or two SP each have it unique AS number.
again you force to use iBGP or eBGP it is not your option

FYI:- if I have 100 PE in SP core and I run eBGP then I need 100 AS public Number!!