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[–]_chrisjhartCCNA R&S[S] 7 points8 points  (0 children)

Static Neighborships

If changing the OSPF network type from NBMA to broadcast doesn’t suit your fancy, we’ll need to configure static OSPF neighbors for each of the PVCs. Keep in mind that OSPF DR/BDR elections occur on NBMA networks as well, so we will need to retain our OSPF priority configuration from our previous adventure with dynamic neighborships.

B1(config)#router ospf 1
B1(config-router)#neighbor 1.1.1.10

#########################

B2(config)#router ospf 1
B2(config-router)#neighbor 1.1.1.10

#########################

B3(config)#router ospf 1
B3(config-router)#neighbor 1.1.1.10

#########################

HQ(config)#router ospf 1
HQ(config-router)#neighbor 1.1.1.1
HQ(config-router)#neighbor 1.1.1.2
HQ(config-router)#neighbor 1.1.1.3

Shortly afterwards, we will see neighborships form on the HQ router.

*Mar 1 14:39:56.524: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.3.1 on Serial1/0 from LOADING to FULL, Loading Done
*Mar 1 14:39:56.528: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.1.1 on Serial1/0 from LOADING to FULL, Loading Done
*Mar 1 14:39:56.532: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.2.1 on Serial1/0 from LOADING to FULL, Loading Done

We can also confirm on our branch routers that routes have been installed, and that the HQ router is the DR.

B1#show ip route
Codes: 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
 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
 o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

1.0.0.0/24 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Serial1/0
 192.168.10.0/32 is subnetted, 1 subnets
O 192.168.10.1 [110/65] via 1.1.1.10, 00:01:39, Serial1/0
C 192.168.1.0/24 is directly connected, Loopback0
 192.168.2.0/32 is subnetted, 1 subnets
O 192.168.2.1 [110/65] via 1.1.1.2, 00:01:39, Serial1/0
 192.168.3.0/32 is subnetted, 1 subnets
O 192.168.3.1 [110/65] via 1.1.1.3, 00:01:39, Serial1/0

B1#show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
192.168.10.1 255 FULL/DR 00:01:56 1.1.1.10 Serial1/0

Subinterfaces

Subinterfaces are simply virtual interfaces attached to a physical interface that primarily allow us to overcome split horizon issues that can be encountered when Frame Relay is configured on the physical interface. Frame Relay subinterfaces must be configured in one of two modes: point-to-point, and multipoint. Interestingly enough, the configuration for each mode matches configuration that we’ve already performed on physical interfaces in this lab! Both point-to-point and multipoint subinterfaces utilize the frame-relay interface-dlci [xxx] (where xxx is the DLCI of the link) command and the frame-relay map ip [x.x.x.x] [yyyy] (where x.x.x.x is the Layer 3 IP address, and yyyy is the Layer 2 DLCI) command, depending on whether Layer 3 addresses can be discovered via Inverse ARP or whether Layer 3 addresses must be mapped to specific Layer 2 addresses.

To demonstrate the configuration of each, we are going to set up point-to-point subinterfaces on all three branch routers, then set up three point-to-point subinterfaces on HQ for all three branch routers. In order to do this, we’ll need to change our IP addressing scheme slightly, as a single subnet can only be configured on one interface at a time (within the same VRF, of course.) To make it simple, B1 and HQ will share 1.1.1.0/30, B2 and HQ will share 2.2.2.0/30, and B3 and HQ will share 3.3.3.0/30. We will also need to rip out OSPF and convert back to EIGRP so that split horizon is proven to not be an issue with subinterfaces. The below configuration assumes that IP addresses and Frame Relay configuration (aside from encapsulation frame-relay, of course) were removed from the physical interfaces of all routers.

B1(config)#interface Serial1/0.10 point-to-point
B1(config-subif)#ip address 1.1.1.1 255.255.255.252
B1(config-subif)#frame-relay interface-dlci 110
B1(config)#no router ospf 1
B1(config)#router eigrp 1
B1(config-router)#network 1.1.1.0 0.0.0.3
B1(config-router)#network 192.168.1.0 0.0.0.255

#########################

B2(config)#interface Serial1/0.10 point-to-point
B2(config-subif)#ip address 2.2.2.1 255.255.255.252
B2(config-subif)#frame-relay interface-dlci 210
B2(config)#no router ospf 1
B2(config)#router eigrp 1
B2(config-router)#network 2.2.2.0 0.0.0.3
B2(config-router)#network 192.168.2.0 0.0.0.255

#########################

B3(config)#interface Serial1/0.10 point-to-point
B3(config-subif)#ip address 3.3.3.1 255.255.255.252
B3(config-subif)#frame-relay interface-dlci 310
B3(config)#no router ospf 1
B3(config)#router eigrp 1
B3(config-router)#network 3.3.3.0 0.0.0.3
B3(config-router)#network 192.168.3.0 0.0.0.255

#########################

HQ(config)#interface Serial1/0.10 point-to-point
HQ(config-subif)#ip address 1.1.1.2 255.255.255.252
HQ(config-subif)#frame-relay interface-dlci 101
HQ(config)#interface Serial1/0.20 point-to-point
HQ(config-subif)#ip address 2.2.2.2 255.255.255.252
HQ(config-subif)#frame-relay interface-dlci 102
HQ(config)#interface Serial1/0.30 point-to-point
HQ(config-subif)#ip address 3.3.3.2 255.255.255.252
HQ(config-subif)#frame-relay interface-dlci 103
HQ(config)#no router ospf 1
HQ(config)#router eigrp 1
HQ(config-router)#network 1.1.1.0 0.0.0.3
HQ(config-router)#network 2.2.2.0 0.0.0.3
HQ(config-router)#network 3.3.3.0 0.0.0.3
HQ(config-router)#network 192.168.10.0 0.0.0.255

After some time, all subinterfaces should be up/up and EIGRP should converge. We should also see EIGRP routes in our routing table, despite the fact we have not configured no ip split-horizon eigrp 1 on any of HQ’s interfaces.

HQ#show ip eigrp neighbors
IP-EIGRP neighbors for process 1
H Address Interface Hold Uptime SRTT RTO Q Seq
 (sec) (ms) Cnt Num
2 3.3.3.1 Se1/0.30 14 00:00:37 988 5000 0 3
1 2.2.2.1 Se1/0.20 11 00:00:40 529 3174 0 3
0 1.1.1.1 Se1/0.10 13 00:00:43 432 2592 0 3
HQ#show ip route eigrp
 1.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D 1.0.0.0/8 is a summary, 00:00:46, Null0
 2.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D 2.0.0.0/8 is a summary, 00:00:46, Null0
 3.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D 3.0.0.0/8 is a summary, 00:00:43, Null0
D 192.168.1.0/24 [90/2297856] via 1.1.1.1, 00:00:47, Serial1/0.10
D 192.168.2.0/24 [90/2297856] via 2.2.2.1, 00:00:44, Serial1/0.20
D 192.168.3.0/24 [90/2297856] via 3.3.3.1, 00:00:41, Serial1/0.30

Conclusion

We’ve explored what Frame Relay is, why it was created, how it works, and some potential issues one could encounter when configuring a routing protocol over a Frame Relay WAN. It is my hope that now that you are armed with this information, you are able to configure and troubleshoot Frame Relay enough to resolve common issues with it in the real world, as well as have the knowledge to logically answer Frame Relay questions that appear on your CCNP ROUTE exam!

I hope this has been helpful!

[–]karjune01 2 points3 points  (6 children)

Don't want to ask for too much, but can you PDF this and send it to me ? Greatly appreciated.

[–]_chrisjhartCCNA R&S[S] 2 points3 points  (3 children)

I can! Give me a few hours and I'll throw something together for you.

[–]karjune01 0 points1 point  (0 children)

Thanks man! Really appreciate that!

[–]erh_PracticalNetworking.net 0 points1 point  (1 child)

Would also love the PDF. Great write up!

Edit: Saw you have it up on your blog as well... nice!

https://chrisjhart.com/2017/12/06/bridging-the-gap-frame-relay/

[–]Hu5k3rA+ CCENT 1 point2 points  (1 child)

Dang dude. Nice write up.

[–]_chrisjhartCCNA R&S[S] 1 point2 points  (0 children)

Thank you!