MPLS OSPF L3 VPN and SHAM links
Building this lab on our previous lab
we will run OSPF between PE and CE and we will see how the OSPF process number on the PE affects the type of routes advertised to the CE
We will also see how to setup sham links in mpls environment and why they are used.
Configuration:
R1
| interface Loopback0 ip address 1.1.1.1 255.255.255.0 ! interface FastEthernet0/0 ip address 10.1.12.1 255.255.255.0 interface FastEthernet0/1 no ip address shutdown duplex auto speed auto ! router ospf 1 router-id 1.1.1.1 net 0.0.0.0 0.0.0.0 area 0 |
R2
| ip vrf CUST_A rd 2.2.2.2:1 route-target export 1:1 route-target import 1:1 interface Loopback0 ip address 2.2.2.2 255.255.255.255 interface FastEthernet0/0 ip vrf forwarding CUST_A ip address 10.1.12.2 255.255.255.0 interface FastEthernet0/1 ip address 10.1.23.2 255.255.255.0 mpls ip router ospf 1 log-adjacency-changes network 10.1.12.2 0.0.0.0 area 0 router bgp 100 no bgp default ipv4-unicast bgp log-neighbor-changes neighbor 4.4.4.4 remote-as 100 neighbor 4.4.4.4 update-source Loopback0 address-family vpnv4 neighbor 4.4.4.4 activate neighbor 4.4.4.4 send-community extended exit-address-family |
R3
| interface FastEthernet0/0 ip address 10.1.23.3 255.255.255.0 mpls ip ! interface FastEthernet0/1 ip address 10.1.34.3 255.255.255.0 mpls ip ! router ospf 1 router-id 3.3.3.3 log-adjacency-changes network 0.0.0.0 255.255.255.255 area 0 |
R4
ip vrf CUST_A rd 4.4.4.4:1 route-target export 1:1 route-target import 1:1 interface Loopback0 ip address 4.4.4.4 255.255.255.255 interface FastEthernet0/0 ip vrf forwarding CUST_A ip address 10.1.45.4 255.255.255.0 interface FastEthernet0/1 ip address 10.1.34.4 255.255.255.0 mpls ip router ospf 1 router-id 4.4.4.4 log-adjacency-changes network 10.1.45.4 0.0.0.0 area 0 router bgp 100 no bgp default ipv4-unicast bgp log-neighbor-changes neighbor 2.2.2.2 remote-as 100 neighbor 2.2.2.2 update-source Loopback0 address-family vpnv4 neighbor 2.2.2.2 activate neighbor 2.2.2.2 send-community extended exit-address-family |
R5
| interface Loopback0 ip address 5.5.5.5 255.255.255.0 interface FastEthernet0/1 ip address 10.1.15.5 255.255.255.0 shutdown router ospf 1 log-adjacency-changes network 0.0.0.0 255.255.255.255 area 0 |
so we have our basic setting ready, we need to enable OSPF for the VRF and redistribute between OSPF and MP-BGP
on both R2 and R4
| router ospf 3 vrf CUST_A redistribute bgp 100 subnets router bgp 100 address-family ipv4 vrf CUST_A redistribute ospf 3 vrf CUST_A exit-address-family |
Note how the ospf vrf is configured, even the vrf is referenced the same process id can’t be re-used, you need to create a new opsf process for each vrf process
Now let’s check the OSPF table on R1
| R1(config-if)#do sh ip route ospf 5.0.0.0/32 is subnetted, 1 subnets O IA 5.5.5.5 [110/21] via 10.1.12.2, 00:39:34, FastEthernet0/0 10.0.0.0/24 is subnetted, 2 subnets O IA 10.1.45.0 [110/11] via 10.1.12.2, 00:39:34, FastEthernet0/0 |
so we see that routes are received as inter-area routes
let’s check the BGP for that specific VRF on R2 and R4
| R4(config-router-af)#do sh bgp vpn unica vrf CUST_A 1.1.1.1 | in ID BGP routing table entry for 4.4.4.4:1:1.1.1.1/32, version 224 Paths: (1 available, best #1, table CUST_A) Not advertised to any peer Local, imported path from 2.2.2.2:1:1.1.1.1/32 2.2.2.2 (metric 21) from 2.2.2.2 (2.2.2.2) Origin incomplete, metric 11, localpref 100, valid, internal, best Extended Community: RT:1:1 OSPF DOMAIN ID:0x0005:0x000000030200 OSPF RT:0.0.0.0:2:0 OSPF ROUTER ID:10.1.12.2:0 mpls labels in/out nolabel/18 ID:44.4.4.4:0 mpls labels in/out 22/nolabel R4(config-router-af)#do sh bgp vpn unica vrf CUST_A 5.5.5.5 BGP routing table entry for 4.4.4.4:1:5.5.5.5/32, version 236 Paths: (1 available, best #1, table CUST_A) Advertised to update-groups: 1 Local 10.1.45.5 from 0.0.0.0 (4.4.4.4) Origin incomplete, metric 11, localpref 100, weight 32768, valid, sourced, best Extended Community: RT:1:1 OSPF DOMAIN ID:0x0005:0x000000030200 OSPF RT:0.0.0.0:2:0 OSPF ROUTER ID:44.4.4.4:0 mpls labels in/out 22/nolabel |
As we can see the OSPF Domain-ID matches on both PE that’s why routes are received as Inter-area routes, if the domain ID doiesn’t match the routes should appear as External routes on the CE .The Domain-ID is the ospf process ID on the PE in Hexa this can be verified by changing the OSPF domain-id on any of the CE.
So let’s change the Domain-ID under R4 to 40
| router ospf 3 vrf CUST_A domain-id 0.0.0.40 clear ip os process |
now let’s check the routes on R4
| ! R4(config-router-af)#do sh bgp vpn unica vrf CUST_A 1.1.1.1 BGP routing table entry for 4.4.4.4:1:1.1.1.1/32, version 224 Paths: (1 available, best #1, table CUST_A) Not advertised to any peer Local, imported path from 2.2.2.2:1:1.1.1.1/32 2.2.2.2 (metric 21) from 2.2.2.2 (2.2.2.2) Origin incomplete, metric 11, localpref 100, valid, internal, best Extended Community: RT:1:1 OSPF DOMAIN ID:0x0005:0x000000030200 OSPF RT:0.0.0.0:2:0 OSPF ROUTER ID:10.1.12.2:0 mpls labels in/out nolabel/18 R4(config-router-af)#do sh bgp vpn unica vrf CUST_A 5.5.5.5 BGP routing table entry for 4.4.4.4:1:5.5.5.5/32, version 236 Paths: (1 available, best #1, table CUST_A) Advertised to update-groups: 1 Local 10.1.45.5 from 0.0.0.0 (4.4.4.4) Origin incomplete, metric 11, localpref 100, weight 32768, valid, sourced, best Extended Community: RT:1:1 OSPF DOMAIN ID:0x0005:0x000000280200 OSPF RT:0.0.0.0:2:0 OSPF ROUTER ID:44.4.4.4:0 mpls labels in/out 22/nolabel |
we see the domain ID for 1.1.1.1 is 3 in HEX while the domain ID for 5.5.5.5 is 28 HEX
Now let’s check the routes on R5
| R5(config-if)#do sh ip route ospf 1.0.0.0/32 is subnetted, 1 subnets O E2 1.1.1.1 [110/11] via 10.1.45.4, 00:32:41, FastEthernet0/0 10.0.0.0/24 is subnetted, 2 subnets O E2 10.1.12.0 [110/1] via 10.1.45.4, 00:32:41, FastEthernet0/0 |
As expected the routes are now showing as External OSPF routes
Now imagine that this customer has a secondary link that runs directly between R1 and R5, the backup link should be used only when primary MPLS links fail
so let’s un-shut f0/1 interface on both R1 and R5 and check the OSPF
| R1(config-if)#do sh ip route ospf 5.0.0.0/32 is subnetted, 1 subnets O 5.5.5.5 [110/11] via 10.1.15.5, 00:12:32, FastEthernet0/1 10.0.0.0/24 is subnetted, 3 subnets O 10.1.45.0 [110/20] via 10.1.15.5, 00:12:32, FastEthernet0/1 R5(config-if)#do sh ip route ospf 1.0.0.0/32 is subnetted, 1 subnets O 1.1.1.1 [110/11] via 10.1.15.1, 00:12:14, FastEthernet0/1 10.0.0.0/24 is subnetted, 3 subnets O 10.1.12.0 [110/20] via 10.1.15.1, 00:12:14, FastEthernet0/1 |
As we can see, both R1 and R5 are using the direct link to reach each other, the reason that inter-area routes will always be preferred over any other routes so cost wouldn’t help much in this scenario sham links was introduced for this type of scenarios
to configure sham links over MPLS PE routers we need the following
1- Two loopback interfaces with /32 prefix
2- advertise the new prefixes under the BGP-MP
3- configure sham link under the PE VRF OSPF process using this command
area X sham-link x.x.x.x y.y.y.y
where x.x.x.x is the source and y.y.y.y is the source of the sham link on each router
so let’s try this on R2 and R4
R2
| interf lo1 ip vrf forwarding CUST_A ip address 22.2.2.2 255.255.255.255 router bgp 100 address-family ipv4 vrf CUST_A net 22.2.2.2 mask 255.255.255.255 router ospf 3 area 0 sham-link 22.2.2.2 44.4.4.4 |
R4
| interface Loopback1 router bgp 100 router ospf 3 |
now let’s check the routes on R5
| R5(config-if)#do sh ip route os 1.0.0.0/32 is subnetted, 1 subnets O 1.1.1.1 [110/11] via 10.1.15.1, 00:13:51, FastEthernet0/1 22.0.0.0/32 is subnetted, 1 subnets O E2 22.2.2.2 [110/1] via 10.1.45.4, 00:05:46, FastEthernet0/0 10.0.0.0/24 is subnetted, 3 subnets O 10.1.12.0 [110/20] via 10.1.15.1, 00:13:51, FastEthernet0/1 44.0.0.0/32 is subnetted, 1 subnets O E2 44.4.4.4 [110/1] via 10.1.45.4, 00:07:40, FastEthernet0/0 |
We see that R5 is still using R1 to reach 1.1.1.1 prefix and the cost is 11
now let’s check the OSPF routes on R4 for that particular VRF
| R4(config-router)#do sh ip route vrf CUST_A ospf Routing Table: CUST_A 1.0.0.0/32 is subnetted, 1 subnets O 1.1.1.1 [110/12] via 2.2.2.2, 00:05:41 5.0.0.0/8 is variably subnetted, 2 subnets, 2 masks O 5.5.5.5/32 [110/11] via 10.1.45.5, 00:11:06, FastEthernet0/0 10.0.0.0/24 is subnetted, 3 subnets O 10.1.15.0 [110/20] via 10.1.45.5, 00:11:06, FastEthernet0/0 O 10.1.12.0 [110/11] via 2.2.2.2, 00:05:41 |
We see that we do receive the route from R2 as Intra area routes and the cost is 12
if we change the cost on R5 to R1 we should be able to use the mpls link as primary
| R5(config)# interf f0/1 R5(config-if)# ip ops cost 100 R5(config-if)#do sh ip route os 1.0.0.0/32 is subnetted, 1 subnets O 1.1.1.1 [110/22] via 10.1.45.4, 00:00:14, FastEthernet0/0 22.0.0.0/32 is subnetted, 1 subnets O E2 22.2.2.2 [110/1] via 10.1.45.4, 00:10:11, FastEthernet0/0 10.0.0.0/24 is subnetted, 3 subnets O 10.1.12.0 [110/21] via 10.1.45.4, 00:00:14, FastEthernet0/0 44.0.0.0/32 is subnetted, 1 subnets O E2 44.4.4.4 [110/1] via 10.1.45.4, 00:12:05, FastEthernet0/0 |
As expected we see that after changing the cost on R5 forced it to use the R4 link to R1 loopback interface
Also check R4 and R2 OSPF neighborship
| R4(config-router)#do sh ip os ne Neighbor ID Pri State Dead Time Address Interface 10.1.12.2 0 FULL/ - - 22.2.2.2 OSPF_SL0 |
R2
R2(config-router)#do sh ip os ne Neighbor ID Pri State Dead Time Address Interface 10.1.45.4 0 FULL/ - - 44.4.4.4 OSPF_SL0 |
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