Routing Protocols

Tutorial: OSPF Network Types and Frame Relay Part 3

In our previous tutorials on OSPF Network types (you can read Part 1 here, and Part 2 here), we looked at the OSPF broadcast, non-broadcast, point-to-multipoint, and point-to-multipoint non-broadcast network types. In this article we will be taking a look at the last of the ospf network types, namely point-to-point and loopback network types.

Lets start with a configuration. We will be using the following topology in this tutorial:

Dynagen .net file Configuration

ghostios = True
sparsemem = True
model = 3640

[localhost]

    [[3640]]
        image = \Program Files\Dynamips\images\c3640-jk9o3s-mz.124-12.bin
        # On Linux / Unix use forward slashes:
        # image = /opt/7200-images/c7200-jk9o3s-mz.124-7a.image
        ram = 96

    [[ROUTER R1]]
        f0/0 = LAN 1
        s1/0 = FRAME 1
        console = 2000
        model = 3640

    [[ROUTER R2]]
        f0/0 = LAN 2
        s1/0 = FRAME 2
        console = 2001
        model = 3640

    [[FRSW FRAME]]
        1:102 = 2:201

You can download this dynagen configuration file here.

Basic Configuration
Lets set up our basic configuration. We will disable inverse arp and manually map the frame-relay dlci’s to the ends of the point-to-point link.

hostname R1
!
interface fastEthernet0/0
 ip address 1.1.1.1 255.255.255.0
!
interface Serial1/0
 ip address 192.168.1.1 255.255.255.0
 encapsulation frame-relay
 frame-relay map ip 192.168.1.2 102 broadcast
 no frame-relay inverse-arp

hostname R2
!
interface fastEthernet0/0
 ip address 2.2.2.2 255.255.255.0
!
interface Serial1/0
 ip address 192.168.1.2 255.255.255.0
 encapsulation frame-relay
 serial restart-delay 0
 frame-relay map ip 192.168.1.1 201 broadcast
 no frame-relay inverse-arp

R1#sh frame map
Serial1/0 (up): ip 192.168.1.2 dlci 102(0x66,0x1860), static,
              broadcast,
              CISCO, status defined, active

Looks good so far. We have a static mapping between our dlci and ip address with broadcast capability configured. Lets check communication between the two routers:

R1#ping 192.168.1.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 72/104/164 ms

So far so good, we have connectivity between the two routers.

OSPF Over Frame-Relay – Point-to-Point Network Type

As the name suggests, the OSPF Point-to-Point network type is designed to be used on networks with a single pair of routers (for example T1, DS-3 or SONET links). In our case R1 and R2 are the only pair of routers configured over a frame relay cloud. Let’s take a look at the configuration:

hostname R1
!
interface fastEthernet0/0
 ip address 1.1.1.1 255.255.255.0
!
interface Serial1/0
 ip address 192.168.1.1 255.255.255.0
 encapsulation frame-relay
 frame-relay map ip 192.168.1.2 102 broadcast
 no frame-relay inverse-arp
 ip ospf network point-to-point
!
router ospf 1
 router-id 1.1.1.1
 network 0.0.0.0 255.255.255.255 area 0

hostname R2
!
interface fastEthernet0/0
 ip address 2.2.2.2 255.255.255.0
!
interface Serial1/0
 ip address 192.168.1.2 255.255.255.0
 encapsulation frame-relay
 serial restart-delay 0
 frame-relay map ip 192.168.1.1 201 broadcast
 no frame-relay inverse-arp
 ip ospf network point-to-point
!
router ospf 1
 router-id 2.2.2.2
 network 0.0.0.0 255.255.255.255 area 0

We’ve configured OSPF such that all interfaces will be placed in area 0. The default non-broadcast network type has been changed to the point-to-point network type using the ip ospf network point-to-point command.

R1#sh ip ospf nei

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           0   FULL/  -        00:00:30    192.168.1.2     Serial1/0

You can see that R1 has become fully adjacent with R2. You will also notice with the above show ip ospf neighbor command that there is no DR on this segment. The Point-to-Point network type does not require a DR to operate. OSPF packets on this networks will always be addressed to the well known 224.0.0.5 (except retransmitted LSA’s which are always unicast).

Lets verify the OSPF network type and take a look at the default timers:

R1#sh ip ospf int s1/0
Serial1/0 is up, line protocol is up
  Internet Address 192.168.1.1/24, Area 0
  Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 64
  Transmit Delay is 1 sec, State POINT_TO_POINT,
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:07
  Supports Link-local Signaling (LLS)
  Index 2/2, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 1, Adjacent neighbor count is 1
    Adjacent with neighbor 2.2.2.2
  Suppress hello for 0 neighbor(s)

The default timers on a Point-to-Point link is 10 and 40 seconds for hello and dead intervals respectively . We can change this using the ip ospf hello-interval and ip ospf dead-interval commands. We will be taking a look at changing the default timers in our next article when we mix and match ospf network types.

R1#sh 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, FastEthernet0/0
     2.0.0.0/24 is subnetted, 1 subnets
O       2.2.2.0 [110/65] via 192.168.1.2, 00:02:25, Serial1/0
C    192.168.1.0/24 is directly connected, Serial1/0

R1#ping 2.2.2.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 44/100/176 ms

Looks like we have full connectivity between the routers!

OSPF Over Frame-Relay – Loopback Network Type

The last of the OSPF network types that we will be looking at is the Loopback network type. This is a special network type that is only seen on loopback interfaces. We cannot manually change a network to be a Loopback network type, IOS automatically assigns the network type to loopback interfaces.

Lets configure a loopback interface. We don’t have to worry about activating OSPF on this interface as our ospf network statement covers all interfaces.

R2#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#int lo1
*Mar  1 01:13:10.671: %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback1, changed state to up
R2(config-if)#ip add 22.22.22.22 255.255.255.0

R2#sh ip ospf int
Loopback1 is up, line protocol is up
  Internet Address 22.22.22.22/24, Area 0
  Process ID 1, Router ID 2.2.2.2, Network Type LOOPBACK, Cost: 1
  Loopback interface is treated as a stub Host
Serial1/0.12 is up, line protocol is up
  Internet Address 192.168.1.2/24, Area 0
  Process ID 1, Router ID 2.2.2.2, Network Type POINT_TO_POINT, Cost: 64
  Transmit Delay is 1 sec, State POINT_TO_POINT,
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:08
  Supports Link-local Signaling (LLS)
  Index 2/2, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 1, Adjacent neighbor count is 1
    Adjacent with neighbor 1.1.1.1
  Suppress hello for 0 neighbor(s)
FastEthernet0/0 is up, line protocol is up
  Internet Address 2.2.2.2/24, Area 0
  Process ID 1, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 2.2.2.2, Interface address 2.2.2.2
  No backup designated router on this network
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:00
  Supports Link-local Signaling (LLS)
  Index 1/1, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 0, maximum is 0
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 0, Adjacent neighbor count is 0
  Suppress hello for 0 neighbor(s)

You can see the output of the show ip ospf interfaces command, the loopback interface is treated as a stub host. No advertisements are sent out that interface, but it is advertised in the OSPF routing domain. Lets check the routing table on R1 to verify this.

R1#sh 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, FastEthernet0/0
     2.0.0.0/24 is subnetted, 1 subnets
O       2.2.2.0 [110/65] via 192.168.1.2, 00:02:24, Serial1/0.12
     22.0.0.0/32 is subnetted, 1 subnets
O       22.22.22.22 [110/65] via 192.168.1.2, 00:02:24, Serial1/0.12
C    192.168.1.0/24 is directly connected, Serial1/0.12

You can see by the routing table on R1 that the loopback network type advertises a /32 route for the loopback interface into the routing domain. This is because the loopback network type represents a sing stub host, a loopback! We can change the default network type on a loopback interface just like a normal interface using the ip ospf network command.

Let take a look at what happens when we change the network type to a point-to-point network type:

hostname R2
!
interface Loopback1
 ip address 22.22.22.22 255.255.255.0
 ip ospf network point-to-point
!
interface FastEthernet0/0
 ip address 2.2.2.2 255.255.255.0
 duplex auto
 speed auto
!
interface Serial1/0
 no ip address
 encapsulation frame-relay
 no frame-relay inverse-arp
!
interface Serial1/0.12 point-to-point
 ip address 192.168.1.2 255.255.255.0
 frame-relay interface-dlci 201
!
router ospf 1
 router-id 2.2.2.2
 log-adjacency-changes
 network 0.0.0.0 255.255.255.255 area 0

R1#sh 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, FastEthernet0/0
     2.0.0.0/24 is subnetted, 1 subnets
O       2.2.2.0 [110/65] via 192.168.1.2, 00:03:01, Serial1/0.12
     22.0.0.0/24 is subnetted, 1 subnets
O       22.22.22.0 [110/65] via 192.168.1.2, 00:03:01, Serial1/0.12
C    192.168.1.0/24 is directly connected, Serial1/0.12

You can see that by changing the ospf network type to point-to-point we can get our loopback advertised as a /24 instead of a /32 into the ospf routing domain.

In our next tutorial we will be looking at mixing and matching ospf network types and looking at some of the problems this can potentially cause. Now back to Labs. HTH!

Summary:

• OSPF Point-to-Point network are designed to be used when there is a single pair of routers on a network segment.
• Point-to-Point networks don’t have DR/BDR relationships.
• Point-to-Point networks have Hello and Dead intervals of 10 seconds and 40 seconds respectively.
• OSPF Loopback Network Type is a special network type used on loopback interfaces only.
• Loopback Network types are treated as a stub host in the OSPF routing domain, and are advertised as a /32 route by default.
• We can change the network type of a loopback using the ip ospf network command. This allows us to advertise the route as something other than a /32 route.

Resources:

• Dynamips/Dynagen .net configuration file

© 2008 Arden Packeer. All rights reserved. The opinions expressed herein are my own personal opinions and do not represent my employer's view in anyway. This material is not sponsored or endorsed by Cisco Systems, Inc. Cisco, Cisco Systems, CCIE and the CCIE Logo are trademarks of Cisco Systems, Inc. and its affiliates