When OSPF is used as the routing protocol on a provider edge to customer edge (PE-CE) link in a multiprotocol label switching (MPLS) VPN.
PE routers mark OSPF routes with the domain attribute derived from the OSPF process number to indicate whether the route originated within the same OSPF domain or from outside it.
If the OSPF process numbering is inconsistent on PE routers in the MPLS VPN, the domain-id OSPF mode command should be used to mark that the OSPF processes with different
numbers belong to the same OSPF domain.
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In the example built as shown in the figure, two customer routers (CE1, CE2) are connected to a provider MPLS / VPN network. OSPF is used to redistribute customer routes between the CE and PE routers. By default the OSPF domain-id will be the same as the configured OSPF process number.
In this configuration example different process numbers are used for the OSPF processes between the PE en CE routers. As shown in de outputs bellow in this case routes in the routing table of PE1 routes coming from PE2 will shown as external routes. This can be overcome by configuring the same domain-id on both PE routers.
CE1#sh ip route
-- output omitted --
10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C 10.1.13.0/24 is directly connected, FastEthernet0/0
O E2 10.1.4.4/32 [110/2] via 10.1.13.1, 00:33:53, FastEthernet0/0
C 10.1.3.3/32 is directly connected, Loopback0
O E2 10.1.24.0/24 [110/1] via 10.1.13.1, 00:34:08, FastEthernet0/0
PE1#sh ip ospf 2
Routing Process "ospf 2" with ID 10.1.13.1
Domain ID type 0x0005, value 0.0.0.2
-- output omitted --
PE2#sh ip ospf 3
Routing Process "ospf 3" with ID 10.1.24.2
Domain ID type 0x0005, value 0.0.0.3
-- output omitted --
After configuring the same OSPF domain-id on both PE routers, the routes coming from the other PE will be indicated as OSPF inter area (O IA) routes.
PE2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
PE2(config)#router ospf 3 vrf VPN_A
PE2(config-router)#domain-id 0.0.0.2
PE2(config-router)#^Z
PE2#
PE2#sh ip ospf 3
Routing Process "ospf 3" with ID 10.1.24.2
Domain ID type 0x0005, value 0.0.0.2
CE1#sh ip route
-- output omitted --
10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C 10.1.13.0/24 is directly connected, FastEthernet0/0
O IA 10.1.4.4/32 [110/3] via 10.1.13.1, 00:00:17, FastEthernet0/0
C 10.1.3.3/32 is directly connected, Loopback0
O IA 10.1.24.0/24 [110/2] via 10.1.13.1, 00:00:17, FastEthernet0/0
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Configurations:
Router PE1
hostname PE1
!
ip vrf VPN_A
rd 1:100
route-target export 1:100
route-target import 1:100
!
interface Loopback0
ip address 150.1.1.1 255.255.255.255
!
interface FastEthernet0/0
ip address 150.1.12.1 255.255.255.0
tag-switching ip
!
interface FastEthernet1/0
ip vrf forwarding VPN_A
ip address 10.1.13.1 255.255.255.0
!
router ospf 1
network 150.1.1.1 0.0.0.0 area 0
network 150.1.12.1 0.0.0.0 area 0
!
router ospf 2 vrf VPN_A
log-adjacency-changes
redistribute bgp 1 subnets
network 10.1.13.1 0.0.0.0 area 0
!
router bgp 1
no synchronization
neighbor 150.1.2.2 remote-as 1
neighbor 150.1.2.2 update-source Loopback0
neighbor 150.1.2.2 next-hop-self
!
address-family vpnv4
neighbor 150.1.2.2 activate
neighbor 150.1.2.2 send-community both
exit-address-family
!
address-family ipv4 vrf VPN_A
redistribute ospf 2
no auto-summary
no synchronization
exit-address-family
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Router PE2
hostname PE2
!
ip vrf VPN_A
rd 1:100
route-target export 1:100
route-target import 1:100
!
interface Loopback0
ip address 150.1.2.2 255.255.255.255
!
interface FastEthernet0/0
ip address 150.1.12.2 255.255.255.0
tag-switching ip
!
interface FastEthernet1/0
ip vrf forwarding VPN_A
ip address 10.1.24.2 255.255.255.0
!
router ospf 1
network 150.1.2.2 0.0.0.0 area 0
network 150.1.12.2 0.0.0.0 area 0
!
router ospf 3 vrf VPN_A
log-adjacency-changes
redistribute bgp 1 subnets
network 10.1.24.2 0.0.0.0 area 0
!
router bgp 1
no synchronization
neighbor 150.1.1.1 remote-as 1
neighbor 150.1.1.1 update-source Loopback0
neighbor 150.1.1.1 next-hop-self
!
address-family vpnv4
neighbor 150.1.1.1 activate
neighbor 150.1.1.1 send-community both
exit-address-family
!
address-family ipv4 vrf VPN_A
redistribute ospf 3
no auto-summary
no synchronization
exit-address-family
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Router CE1
hostname CE1
!
interface Loopback0
ip address 10.1.3.3 255.255.255.255
!
interface FastEthernet0/0
ip address 10.1.13.3 255.255.255.0
!
router ospf 1
network 10.1.3.3 0.0.0.0 area 0
network 10.1.13.3 0.0.0.0 area 0
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Router CE2
hostname CE2
!
interface Loopback0
ip address 10.1.4.4 255.255.255.255
!
interface FastEthernet0/0
ip address 10.1.24.4 255.255.255.0
!
router ospf 1
network 10.1.4.4 0.0.0.0 area 0
network 10.1.24.4 0.0.0.0 area 0
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