Thursday, November 10, 2011

Dual Hub Equal cost Load-Sharing Point-to-Multipoint Broadcast OSPF Frame Relay Network Topology


Recently I got into Frame Relay networks, so I decided to dig into details.
  • Above is the Dual Hub Frame Relay Network with Dual Hubs representing the Head Offices for the multiple Spoke locations. 
  • Spoke-Spoke communication is only possible through Hub locations. 
  • This topology is useful for implementing Access lists or Route filters at Hub locations.
  • Each spoke has Equal Cost routes to other spokes through both the Hub routers, OSPF equal cost load-sharing is performed here.
  • Packets from Spoke to Spoke locations will use different hubs according to the algorithm & ACL/Filter results at Hub locations. 
  • This topology adds overheads on to network as direct Spoke-Spoke communication is not mapped here [Requirement says so :-( ].
  • Frame Relay IP addresses are mapped to DLCI with "broadcast" capability.
  • Point-to-Multipoint Broadcast OSPF Network type is required for proper operation of the Network. 
  • Point-to-Multipoint Broadcast OSPF Networks does not require DR/BDR routers.
  • Neighbor configuration is not mandatory for P-to-M Broadcast networks, however I configured if with equal costs in OSPF process.
  • Each routers local DLCI needs to be mapped to his own IP address to make Interface IP addresses reachable in Frame Relay. 10.1.1.1 needs to be mapped to 103 with frame relay map to ensure Router R1 can ping 10.1.1.1.
  • OSPF Hello & Dead Timers are adjusted accordingly.
  • Everything is Statically Mapped, so there is no need for Inverse ARP LMI messages.
  • OSPF Router Id's are configured with routers Loopback interface IP addresses.
  • If Hub 2 loses connectivity to any spoke or any particular DLCI but maintains communication with Hub 1, all Spokes still will be able send packets to Hub 2 through Hub 1.OSPF protocol is intentionally used here for complete Network Transparency.
Hub 1 Router configuration -
(Lines omitted for brevity)

interface Serial0/0
 no ip address
 no ip directed-broadcast
 encapsulation frame-relay
 serial restart-delay 0
!
interface Serial0/0.1 multipoint
 ip address 10.1.1.1 255.255.255.0
 no ip directed-broadcast
 ip ospf network point-to-multipoint
 ip ospf hello-interval 20
 ip ospf dead-interval 40
 frame-relay map ip 10.1.1.1 103 broadcast
 frame-relay map ip 10.1.1.2 701 broadcast
 frame-relay map ip 10.1.1.3 103 broadcast
 frame-relay map ip 10.1.1.4 104 broadcast
 frame-relay map ip 10.1.1.5 105 broadcast
 frame-relay interface-dlci 103
 frame-relay interface-dlci 104
 frame-relay interface-dlci 105
 frame-relay interface-dlci 701
 no frame-relay inverse-arp
!
router ospf 1
 router-id 11.1.1.1
 network 10.1.1.0 0.0.0.255 area 0
 network 11.1.1.0 0.0.0.255 area 0
 network 103.1.1.0 0.0.0.255 area 0

Spoke 1 configuration -
(Lines omitted for brevity)

interface Serial0/0
 no ip address
 no ip directed-broadcast
 encapsulation frame-relay
 serial restart-delay 0
!
interface Serial0/0.3 multipoint
 ip address 10.1.1.3 255.255.255.0
 no ip directed-broadcast
 ip ospf network point-to-multipoint
 ip ospf hello-interval 20
 ip ospf dead-interval 40
 frame-relay map ip 10.1.1.1 301 broadcast
 frame-relay map ip 10.1.1.2 302 broadcast
 frame-relay map ip 10.1.1.3 301 broadcast
 frame-relay interface-dlci 301
 frame-relay interface-dlci 302
 no frame-relay inverse-arp
!
router ospf 1
 router-id 13.1.1.1
 network 10.1.1.0 0.0.0.255 area 0
 network 13.1.1.0 0.0.0.255 area 0
 network 100.1.1.0 0.0.0.255 area 0
 neighbor 10.1.1.2 cost 48
 neighbor 10.1.1.1 cost 48

Hub 1 Router OSPF neighbor adjacency -

Neighbor ID     Pri   State           Dead Time   Address         Interface
14.1.1.1          1   FULL/  -        00:00:29    10.1.1.4        Serial0/0.1
15.1.1.1          1   FULL/  -        00:00:25    10.1.1.5        Serial0/0.1
12.1.1.1          1   FULL/  -        00:00:24    10.1.1.2        Serial0/0.1
13.1.1.1          1   FULL/  -        00:00:25    10.1.1.3        Serial0/0.1
103.1.1.2         1   FULL/DR         00:00:30    103.1.1.2       FastEthernet1/0

Spoke 1 Router OSPF neighbor adjacency -

Neighbor ID     Pri   State           Dead Time   Address         Interface
12.1.1.1          1   FULL/  -        00:00:25    10.1.1.2        Serial0/0.3
11.1.1.1          1   FULL/  -        00:00:24    10.1.1.1        Serial0/0.3
100.1.1.2         1   FULL/DR         00:00:34    100.1.1.2       FastEthernet1/0

Equal Cost Load Sharing for Spoke to Spoke communication -

R3#sh ip route 101.1.1.0
Routing entry for 101.1.1.0/24
  Known via "ospf 1", distance 110, metric 97, type intra area
  Redistributing via ospf 1
  Last update from 10.1.1.2 on Serial0/0.3, 00:07:05 ago
  Routing Descriptor Blocks:
  * 10.1.1.1, from 101.1.1.2, 00:07:05 ago, via Serial0/0.3
      Route metric is 97, traffic share count is 1
    10.1.1.2, from 101.1.1.2, 00:07:05 ago, via Serial0/0.3
      Route metric is 97, traffic share count is 1

Happy Networking.