VRF Without Route Target – Will the Route Be Exported?

Yesterday I posted a tricky question to Twitter. If you have a working VPNv4 environment and create a VRF with only a Route Distinguisher (RD) but without Route Targets (RT), will the route be exported? The answer may surprise you! The configuration supplied in the question was similar to the one below:

vrf definition QUIZ
 rd 198.51.100.1:100 
 !
 address-family ipv4
 exit-address-family
!
interface GigabitEthernet2
 vrf forwarding QUIZ
 ip address 203.0.113.1 255.255.255.0
!
router bgp 65000
 !
 address-family ipv4 vrf QUIZ
  network 203.0.113.0

Notice how this VRF has a RD but no RT. Will this router, PE1, advertise the route into VPNv4? Most would say no, but the answer is yes. Let’s first check that we see the route locally on PE1 in VRF QUIZ:

PE1#show bgp vpnv4 uni vrf QUIZ 203.0.113.0
BGP routing table entry for 198.51.100.1:100:203.0.113.0/24, version 4
Paths: (1 available, best #1, table QUIZ)
  Advertised to update-groups:
     1         
  Refresh Epoch 1
  Local
    0.0.0.0 (via vrf QUIZ) from 0.0.0.0 (198.51.100.1)
      Origin IGP, metric 0, localpref 100, weight 32768, valid, sourced, local, best
      mpls labels in/out 18/nolabel(QUIZ)
      rx pathid: 0, tx pathid: 0x0
      Updated on Jan 2 2024 06:32:17 UTC

The route is there but has no RT. Is it being advertised?

PE1#show bgp vpnv4 uni all nei 198.51.100.2 adv 203.0.113.0/24
Route Distinguisher: 198.51.100.1:100 (default for vrf QUIZ)
BGP routing table entry for 198.51.100.1:100:203.0.113.0/24, version 4
  Paths: (1 available, best #1, table QUIZ)
  Advertised Attributes
    Local Preference: 100
    Metric: 0
    Origin: IGP
    Nexthop: 198.51.100.1

Yes, it is being advertised. Without RT, PE2 will not import it, though:

*Jan  2 06:37:17.920: BGP(4): 198.51.100.1 rcvd UPDATE w/ attr: nexthop 198.51.100.1, origin i, localpref 100, metric 0
*Jan  2 06:37:17.921: BGP(4): 198.51.100.1 rcvd 198.51.100.1:100:203.0.113.0/24, label 18 (0x12) -- DENIED due to:  extended community not supported;

A packet capture shows the VPNv4 update and that there is no RT:

Now let’s add the RT at PE1:

PE1(config)#vrf definition QUIZ
PE1(config-vrf)#route-target both 65000:100

This prefix now has an RT:

PE1#show bgp vpnv4 uni all 203.0.113.0         
BGP routing table entry for 198.51.100.1:100:203.0.113.0/24, version 7
Paths: (1 available, best #1, table QUIZ)
  Advertised to update-groups:
     1         
  Refresh Epoch 1
  Local
    0.0.0.0 (via vrf QUIZ) from 0.0.0.0 (198.51.100.1)
      Origin IGP, metric 0, localpref 100, weight 32768, valid, sourced, local, best
      Extended Community: RT:65000:100
      mpls labels in/out 18/nolabel(QUIZ)
      rx pathid: 0, tx pathid: 0x0
      Updated on Jan 2 2024 06:42:05 UTC

The debug shows that the prefix is now being imported on PE2:

*Jan  2 06:42:05.962: BGP(4): 198.51.100.1 rcvd UPDATE w/ attr: nexthop 198.51.100.1, origin i, localpref 100, metric 0, extended community RT:65000:100
*Jan  2 06:42:05.962: BGP(4): 198.51.100.1 rcvd 198.51.100.1:100:203.0.113.0/24, label 18 (0x12)
*Jan  2 06:42:05.963: BGP VPN-IMP: VPNv4 Unicast:base 198.51.100.1:100:203.0.113.0/24 Marking path (passed into bestpath).
*Jan  2 06:42:05.969: BGP: IMP Process work queue.
*Jan  2 06:42:05.969: BGP: tbl VPNv4 Unicast:base IMP Incremental export.
*Jan  2 06:42:05.969: BGP VPN-IMP: VPNv4 Unicast:base 198.51.100.1:100:203.0.113.0/24 Exporting doing PATHS.
*Jan  2 06:42:05.972: BGP VPN-IMP: VPNv4 Unicast:base Building ETL from VPN
*Jan  2 06:42:05.972: BGP VPN-IMP: VPNv4 Unicast:base GBL Building ETL.
*Jan  2 06:42:05.972: BGP VPN-IMP: VPNv4 Unicast:base -> global:IPv4 Unicast:base Creating Import Topo.
*Jan  2 06:42:05.972: BGP VPN-IMP: VPNv4 Unicast:base -> global:IPv4 Unicast:base GBL Adding topology IPv4 Unicast to ETL.
*Jan  2 06:42:05.972: BGP VPN-IMP: VPNv4 Unicast:base -> global:IPv4 Multicast:base Creating Import Topo.
*Jan  2 06:42:05.972: BGP VPN-IMP: VPNv4 Unicast:base -> global:IPv4 Multicast:base GBL Adding to ETL.
*Jan  2 06:42:05.972: BGP VPN-IMP: VPNv4 Unicast:base Building GBL ETL done.
*Jan  2 06:42:05.973: BGP VPN-IMP: VPNv4 Unicast:base Before adding to ETL bgp_topo_afi 0, is found
*Jan  2 06:42:05.973: BGP VPN-IMP: VPNv4 Unicast:base -> QUIZ:VPNv4 Unicast:base Adding to ETL.
*Jan  2 06:42:05.973: BGP VPN-IMP: VPNv4 Unicast:base 198.51.100.1:100:203.0.113.0/24 -> IPv4 Multicast:base Not importing in new table, not importing any paths.
*Jan  2 06:42:05.973: BGP VPN-IMP: VPNv4 Unicast:base 198.51.100.1:100:203.0.113.0/24 -> IPv4 Unicast:base Not importing in new table, not importing any paths.
*Jan  2 06:42:05.973: BGP VPN-IMP: global:VPNv4 Unicast:base 198.51.100.1:100:203.0.113.0/24 -> QUIZ:base Creating importing net.
*Jan  2 06:42:05.973: BGP VPN-IMP: QUIZ:VPNv4 Unicast:base 198.51.100.2:100:203.0.113.0/24 -> base Importing net created.
*Jan  2 06:42:05.974: BGP VPN-IMP: global:VPNv4 Unicast:base 198.51.100.1:100:203.0.113.0/24 198.51.100.1 -> QUIZ:base Called path modify.
*Jan  2 06:42:05.974: BGP VPN-IMP: global:VPNv4 Unicast:base 198.51.100.1:100:203.0.113.0/24 198.51.100.1 -> QUIZ:base Imported new path nexthop (198.51.100.1)
*Jan  2 06:42:05.974: BGP VPN-IMP: global:VPNv4 Unicast:base 198.51.100.1:100:203.0.113.0/24 198.51.100.1 -> QUIZ:base Creating imported path id 0.
*Jan  2 06:42:05.974: BGP VPN-IMP: VPNv4 Unicast:base Export time record 5 (0) msec for net 198.51.100.1:100:203.0.113.0/24 (1/3).
*Jan  2 06:42:05.974: BGP: IMP Work queue processed.
*Jan  2 06:42:05.975: BGP(4): Revise route installing 1 of 1 routes for 203.0.113.0/24 -> 198.51.100.1(QUIZ) to QUIZ IP table

The prefix is indeed in the BGP table now on PE2:

PE2#show bgp vpnv4 uni vrf QUIZ 203.0.113.0
BGP routing table entry for 198.51.100.2:100:203.0.113.0/24, version 3
Paths: (1 available, best #1, table QUIZ)
  Flag: 0x100
  Not advertised to any peer
  Refresh Epoch 2
  Local, imported path from 198.51.100.1:100:203.0.113.0/24 (global)
    198.51.100.1 (metric 3) (via default) from 198.51.100.1 (198.51.100.1)
      Origin IGP, metric 0, localpref 100, valid, internal, best
      Extended Community: RT:65000:100
      mpls labels in/out nolabel/18
      rx pathid: 0, tx pathid: 0x0
      Updated on Jan 2 2024 06:42:05 UTC

It has also been installed into the RIB:

PE2#show ip route vrf QUIZ

Routing Table: QUIZ
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, m - OMP
       n - NAT, Ni - NAT inside, No - NAT outside, Nd - NAT DIA
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       H - NHRP, G - NHRP registered, g - NHRP registration summary
       o - ODR, P - periodic downloaded static route, l - LISP
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR
       & - replicated local route overrides by connected

Gateway of last resort is not set

B     203.0.113.0/24 [200/0] via 198.51.100.1, 00:19:37

For completeness, here is BGP Update from packet capture now that RT has been set:

What did we learn? Be careful with assumptions. One would assume that RT is mandatory to get a route exported, but it’s not. Obviously another router won’t be able to import the route but it will be advertised. What we went through today will give you a deeper understanding of VPNv4 and being less focused on working configurations and more focused on how all the pieces interact.

In case you are interested in testing something similar, I have posted my initial configurations below. My test setup was a simple topology of three routers, PE1, P1, and PE2, running in CML:

PE1:

hostname PE1
!
vrf definition QUIZ
 rd 198.51.100.1:100
 !        
 address-family ipv4
 exit-address-family
!
interface Loopback0
 ip address 198.51.100.1 255.255.255.255
 ip ospf 1 area 0
!
interface GigabitEthernet1
 ip address 192.0.2.1 255.255.255.254
 ip ospf network point-to-point
 ip ospf 1 area 0
 mpls ip
!
interface GigabitEthernet2
 vrf forwarding QUIZ
 ip address 203.0.113.1 255.255.255.0
!
router ospf 1
!
router bgp 65000
 bgp log-neighbor-changes
 no bgp default ipv4-unicast
 neighbor 198.51.100.2 remote-as 65000
 neighbor 198.51.100.2 update-source Loopback0
 !        
 address-family ipv4
 exit-address-family
 !
 address-family vpnv4
  neighbor 198.51.100.2 activate
  neighbor 198.51.100.2 send-community extended
 exit-address-family
 !
 address-family ipv4 vrf QUIZ
  network 203.0.113.0
 exit-address-family

P1:

hostname P1
!
interface GigabitEthernet1
 ip address 192.0.2.0 255.255.255.254
 ip ospf network point-to-point
 ip ospf 1 area 0
 mpls ip
! 
interface GigabitEthernet2
 ip address 192.0.2.2 255.255.255.254
 ip ospf network point-to-point
 ip ospf 1 area 0
 mpls ip
!
router ospf 1

PE2:

hostname PE2
!
vrf definition QUIZ
 rd 198.51.100.2:100
 route-target export 65000:100
 route-target import 65000:100
 !        
 address-family ipv4
 exit-address-family
!
interface Loopback0
 ip address 198.51.100.2 255.255.255.255
 ip ospf 1 area 0
!
interface GigabitEthernet1
 ip address 192.0.2.3 255.255.255.254
 ip ospf network point-to-point
 ip ospf 1 area 0
 mpls ip
!
router ospf 1
!
router bgp 65000
 bgp log-neighbor-changes
 no bgp default ipv4-unicast
 neighbor 198.51.100.1 remote-as 65000
 neighbor 198.51.100.1 update-source Loopback0
 !
 address-family ipv4
 exit-address-family
 !
 address-family vpnv4
  neighbor 198.51.100.1 activate
  neighbor 198.51.100.1 send-community extended
 exit-address-family