area0                     area0  


     N1                       N2

CE1,CE2 are same customer edge routers. PE1,PE2 are provider Edge routers of same ISP. and they connected as show in image.

we are running OSPF between PE,CE for BGP/MPLS IP VPNs. PE1-CE1 (Network N1) & PE2-CE2 (Network N2) links are in area0.

OSPF on PE1,PE2 have same domain ID configured to make OSPF routes from one CE to other treated as IA routes.


Now the N1 is connected network to PE1 and Administrative distance for connected is higher than OSPF. so,PE1 will have N1 as connected route.

Then we have only choice to redistribute the N1 to CE2 is by enabling 'redistribute connected' in BGP of PE1. As the domain ID will not be inserted when redistributing connected routes, CE2 will treat it as External Type 2 Network.

Doesn't it wrong to treat a network of same area/domain as external..?

  • just curious, ...I have heard of such CPE configurations but always thought they were overly complicated. Do you own the CPE (CE routers)? Was the provider unable to deliver a layer 2 WAN, or a VPLS? Jun 18, 2015 at 18:21
  • I'm a network test engineer. I'm just testing some scenarios in our lab. Dec 7, 2015 at 14:15
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 11, 2017 at 16:55

1 Answer 1


Since you have enabled OSPF on the connected interface, it will be an internal route for the networks behind the CE1.

But all OSPF routes on the CE1 side will be external routes on the CE2 side, because they all are redistributed to/from mBGP. To make them internal, you need a sham link.

Administrative distance doesn't really come into play here. AD affects how the router forwards packets, but not necessarily what the routing protocol does.

  • Thanks Ron,I forgot to mention this point. Same domain ID configured in OSPF of PE1,PE2. now BGP will distribute them with LSA3. May 19, 2015 at 10:17
  • does it correct to treat CE-PE links as external..? even after setting same domain ID! May 22, 2015 at 10:47
  • If they're the same domain ID, they're internal routes,
    – Ron Trunk
    May 22, 2015 at 12:26
  • But this is what happening in my case: the BGP configured in PE1 is taking N1 as connected. so, it not advertising N1 towards CE2 since we said only redistribute ospf. if we do redistribute connected, it advertising N1 as external. doesn't it volation accroding to OSPF..? same area routes are showing as E2 May 27, 2015 at 18:45
  • Maybe I'm misunderstanding your diagram. Is N1 the link between CE1 and PE1 running OSPF? If so, it should be an internal route because you have ospf on that interface.
    – Ron Trunk
    May 27, 2015 at 19:02

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