The difference between BGP RD and RT

Friday, 10 Aug 2018

BGP Route Descriptor and Route Target

Let me caveat this post by saying I'm not a Service Provider (SP) kid by trade; I spend my life doing Enterprise, Data Centre and Wireless - so all this MPLSery is new territory for me, and my imaginary sidekick-dog friend ("Hi Jake!") - which means this might be technically incorrect, but this is how the concepts of RD and RT finally "clicked" for me.

How it was explained to me

Adventure Time with BGP RT and RD

When I first starting Googling for Dear Life (TM) about this (because I needed to spin up a new VRF/IPVPN/L3VPN on our MPLS Network), and looked at a few existing config excerpts, I thought they were both the same thing, which seems valid:

vrf definition ADVENTURE-TIME-VRF
 route-target export 65432:999
 route-target import 65432:999

I didn't really question the fact that the Export/Import Route Target (RT) was the same (and didn't know about "Full Mesh VRF" vs "Hub-and-Spoke VRF"), but it did strike me as odd that the RD wasn't the same as the RT, given all the explanation I'd read said things like:

The RD is used to keep all prefixes in the BGP table unique between Customers or VRFs...

Which I read thinking:

"Hmm, that makes sense; BGP will just append the RD in-front of the Prefix, to identify the VRF it belongs to. But wouldn't that mean the RD should be the same for each PE Router, the same for each instantiation of that VRF/Customer across the network?"

So then why the differing RD from the RTs?

Why bother with the extra admin work of creating a different value each time, between the RD and RT?

How I now understand it

Adventure Time with BGP RT and RD continued

When I started exploring Full Mesh VRF vs Hub-and-Spoke VRF, it started to click into place - the RT and RD aren't really related, and I think there's some missing text from the common definition of how RD's are enacted: - RD = Route(r) Descriptor - RT = Rout(ing Table) Target

When I looked around the configs we had elsewhere, the pattern become clear; it decomposed like this:

vrf definition <VRF Human-friendly Name>
 rd <Router Loopback0>:<VRF RT No>
 route-target export <Router ASN>:<VRF RT No>
 route-target import <Router ASN>:<VRF RT No>

It's starting to click

Then you step back a bit more, and realise the VRF Name and RT/RD have pretty much no association (and then it suddenly clicks what they mean when they say "Locally Significant"...), and we - as humans - use the same VRF Name everywhere because it's easier for us, like a sort of "Poor Man's DNS for VRF RTs". So there's no reason this config wouldn't just stitch VRF "Bob" to VRF "Jane" between two Routers in the same MPLS Domain - but it'd be a pain in the arse to troubleshoot when it scaled to more than a few Routers:

Router_PE1#vrf definition Bob
 route-target export 65432:999
 route-target import 65432:999

Router_PE2#vrf definition Jane
 route-target export 65432:999
 route-target import 65432:999

Great Scott! He's got it!

Which is when it clicks - when you look at two Router's configurations and realise the RT is the same, but the RD changed; within what we've established is the same VRF "Container" (even though we renamed it across Routers, to cause pain to that guy in Ops that looked at our wife wrong during that Christmas Do, yeah - "Bob"...). So roughly then:

  • An RD can be thought of as the "Router Descriptor"
    • i.e. "Who injected that Prefix into my VRF?"
    • Probably makes sense to use a Loopback, or unique attribute of a Router; then you can jump on your Route Reflector (RR) and have a quick "Whodunnit?"
      • Router_RR1#sh ip bgp vpnv4 all | sec <Router Loopback0>:<VRF RT No>
  • An RT can be thought of as the "Routing Table Target"
    • i.e. "So that's just a VLAN-equivalent Tag for a VRF Container on the MPLS Domain then..."
    • If it's the same RT you're import/exporting everywhere, we're rocking Full Mesh; if it's not (or I'm suddenly doing loads of import statements/one export statement, or vice versa), we're looking at a pesky Hub-and-Spoke
      • Got multiple RT Import statements and one Export? You're probably on a Hub Router (for that VRF)
      • Got one RT Import statement and multiple Exports? You're probably on a Spoke Router (for that VRF)

Am I right here?

That's how I understand all this MPLS VRFery anyway; if I'm wrong, why not:

  • Tweet me @notworkd and tell me "U iz well wrong, Bruv..."
  • Write a comment below and tell me "Dude, do you even MPLS, Bro?"