Frame Relay Point-to-Point with Sub Interfaces and OSPF Single Area

Frame Relay Point-to-Point Sub-interfaces with OSPF Single Area:

Introduction:

The purpose of this lab is to configure a frame relay network using sub interfaces and configure single area OSPF to run over that network.  One of the advantages of using point-to-point sub-interfaces over point to multipoint on the physical interface is the split-horizon issue is removed.

Recapping: the split-horizon rule states that if a routing update is learned on an interface it will not be advertised back out of the same interface, which on a point-to-multipoint network can stop routing updates.   Sub-interfaces are treated as separate physical interfaces so a routing update learned on one sub interface is allowed to be sent out other sub interfaces without breaking the split horizon rule.

Lab Environment:

GNS 3.0.8.6
3 x 3725
c3725-adventerprisek9-mz.124-15.T8.image

Download GNS3 pre config and solution + word doc 

Network Diagram:

Addressing Scheme:

Device	Interface	IP Address
CORE_1	SERIAL 0/0	NO IP ADDRESS
CORE_1	SERIAL 0/0.100	172.16.50.1 /24
CORE_1	SERIAL 0/0.400	172.16.51.4 /24
CORE_2	SERIAL 0/0	NO IP ADDRESS
CORE_2	SERIAL 0/0.200	172.16.50.2 /24
CORE_3	SERIAL 0/0	NO IP ADDRESS
CORE_3	SERIAL 0/0.300	172.16.51.3 /24

Stage 1 – Set up frame relay on each serial interface:

Activity 1 – Setup the serial interfaces for connectivity over frame relay:

Issue the following commands repeating for the serial 0/0 interfaces for CORE_1, CORE_2, CORE_3

CORE_X(conf ig)interface serial 0/0 (selects the interface)
CORE_X(conf ig-if)clock-rate 64000 (sets the clock rate on that interface)
CORE_X(conf ig-if)encapsulation frame-relay ietf
CORE_X(config-if)no shut

Stage 2 – Set up the sub interfaces for each router

Activity 1: Setuing up the sub-interfaces for connectivity over frame relay

Issue the following commands:

CORE_1(conf ig)interface serial 0/0.100 point-to-point
CORE_1(conf ig-subif) frame-relay  interface-dlci 100
CORE_1(conf ig-subif)ip address 172.16.50.1 255.255.255.0
CORE_1(conf ig)interface serial 0/0.400 point-to-point
CORE_1(conf ig-subif) frame-relay  interface-dlci 400
CORE_1(conf ig-subif)ip address 172.16.51.4 255.255.255.0

CORE_2(conf ig)interface serial 0/0.200 point-to-point
CORE_2(conf ig-subif) frame-relay  interface-dlci 200
CORE_2(conf ig-subif)ip address 172.16.50.2 255.255.255.0

CORE_3(conf ig)interface serial 0/0.300 point-to-point
CORE_3(conf ig-subif) frame-relay  interface-dlci 300
CORE_3(conf ig-subif)ip address 172.16.50.3 255.255.255.0

At this point you should have complete local connectivity across the frame relay PVC’s

Note: On this lab I had issues when I played around with the dlci's on the frame relay switch.the symptoms are that one or more sub-interfaces refuse to come up.  After investigating and much

playing around I noticed that if you closed the lab and re-opened it the frame relay switch had somehow got lots of wrongly configured DLCI's.  Removing them and recreating did not

work, when you rebooted they were just the same. The solution is if this happens is to delete the frame relay switch reload the project add a new switch and reconfigure it.

Stage 3 – Testing your frame relay

Test 1 – Pinging Local Interfaces

CORE_1# ping 172.16.50.2
CORE_1#ping 172.16.51.3 
CORE_3#ping 172.16.50.2

Question:  Did any of the pings fail, if so why?
Answer:  Yes, because there is no route between CORE_3 and CORE_2

Test 2 – Check frame relay pvc status

 On Core_1 issue the command “show frame-relay pvc” as below:

The output should show Active for both DLCI’s, take some time to examine the output, notice the BECN and FECN counters etc..

Repeat for CORE_2 and CORE_3

Question:  How many PVC’s should you see on CORE_2 and CORE_3
Answer: Only one PVC on each.

Stage 2 – Implement Single Area OSPF

Recall earlier that you could not ping from Core_2 to Core_3 and Vica-Versa by implementing a routing protocol such as OSPF we will be able to ping across the entire frame relay network. 

Add the following configurations in this order:

CORE_2(config)#router ospf 1
CORE_2(config-router)#network 172.16.50.0 0.0.0.255 area 0

CORE_1(config)#router ospf 1
CORE_1(config-router)#network 172.16.51.0 0.0.0.255 area 0

Question:  Can you ping from Core_2 to Core_3 yet, if not why?
Answer:  No, because there is not routing between them yet and they are not neighbors.

CORE_1(config)#router ospf 1
CORE_1(config-router)#network 172.16.50.0 0.0.0.255 area 0
CORE_1(config-router)#network 172.16.51.0 0.0.0.255 area 0

Notice the neighbor relationships come up on all three routers.

Stage 3 – Investigating the network

Test 1 – Show IP Protocols to ensure the correct networks are being advertised

Issue the command show ip protocols  on each router and ensure OSPF is advertising the correct networks as below:

 

Test 2 – Show IP Route to ensure each router has a route to all advertised networks

Issue the show ip route on all routers and fill in the blanks on the following table

Router	Route Description	Learned Via	Code
Core_1	172.16.0.0 /24 subnetted 3 subnets	Directly connected	C
Core_1	172.16.50.0 is directly connected	Directly connected	C
Core_1
Core_2
Core_2
Core_3
Core_3

Question: have all routes been learned?
Answer: Yes
Question: are all routing tables identical?
Answer: No
Question: what does [110/128] in the output mean?
Answer: this is the default administrative distance(110)  and the default cost (128).

Test 3: - Checking the OSPF Link State Database

Question:  Before you look at the ospf database on each router if all routers are configured correctly what would you expect them to have in common?

Answer:  The link state database within an area should be identical, remember the SPF algorithm is run against this database to produce the SPF tree and the shortest routes then populate the routing table.

Issue the command show ip ospf database and record your findings below:

Router	Link ID	Advertising Router	Link Count
Core_1	172.16.50.2	172.16.50.2
Core_1
Core_1
Core_2
Core_2
Core_2
Core_3
Core_3
Core_3

Question:  Look at the output you what is the RID of CORE_1, why is this the RID?
Answer:  Because it is the highest IP Address

Question:  What type of LSA’s are being generated and shown under Router Link States (area 0)?
Answer:  These are type 1 LSA’s and describe each routers links into an area

Question:  Why aren’t there any type 2 LSA’s being generated?
Answer: Our network is running over point-to-point links which do not elect a BDR/DR relationship without this there is no DR to build a type 2 LSA.

Question:  How many links are there in area 0, what can you do to check that they are all in the ospf database?

Answer:  There are four links in area 0, if you look at the output for area 0 it shows a total of 4 links which should be the same physically – Core_1 has two links or interfaces, Core_2 and Core_3 both have a single link in each.

Test 4: - A closer look at the OSPF Database:

Take a closer look at the LSA information being generated by the routers by issuing the command show ip ospf database router 172.16.50.2 on Core_1 results as below:

Things to note:

11.       Notice it gives you the RID of the router you have issued the command on: 172.16.51.4

22.       Notice it gives you the interface address of the actual link i.e. 172.16.50.2 (Link ID)

33.       Notice it gives you the advertising router ID i.e. 172.16.50.2 (the same in this case)

44.       Notice the link count for that router is 2

55.       Notice the type 1 has two entries:

a.       A link connected to another router (point-to-point) with the neighbors RID

b.      Link connecting to: a stub network, in these terms it is just a description of that subnet. (see note below)

Note: In OSPF terms a point to point link is described by both routers with a type 1 LSA that has two entries one describing the interface and remote router and the other describing the subnet of that link as a stub network

Note: In OSPF terms a transit network is a multi-access network that contains 2 or more routers whilst a stub network contains only one other, however it’s not quite as simple as that even if you have a point to point link that leads to another area (think Core_2 to Core_1 to Core_3) OSPF still describes the links as STUB! As above only a multi-access network with 2 or more routers is described as a transit network.   There’s a lot more rules involved in that but if you work on that basis you won’t go wrong!

Question: Why is the routing table different on each router but the ospf database is the same?

Answer:  Each router should have an identical link state database as above, however the SPF algorithm will populate the routing table with the best routes to destinations from the perspective of the individual router which means that they should have different routing tables.

Stage 4 – Examining Loopback interfaces

Issue a show ip protocols on R1 record the router id  __________________________

Create a loopback interface on R1 with an ip address of 172.16.45.1 /24

Core_1(config) loopback 0
Core_1(config-if)ip address 172.16.45.1 255.255.255.0

Advertise that loopback interface into OSPF in area 0

Core_1(config)#router ospf 1
Core_1(config-router)#network 172.16.45.0 0.0.0.255 area 0

Issue a show ip route on Core_3:

Question: what is different about the route for the loopback on CORE_1

Answer:  All the others are show as /24 subnets of the major 172.16.0.0 network while the 172.16.45.0 is show as a 32 bit host route, this is default ospf behaviour.

To solve the default behaviour of OSPF with Loopbacks got to core 1 and issue: 

Core_1(config)interface loopback 0
core_1(config-if)ospf network point-to-point

Now go back to  to CORE_3 issue the show ip route again how is the route being advertised now?  It should be listed with the others as a /24 subnet of the major subnet 172.16.0.0!

Exploring the RID

Remember that OSPF uses a loopback or the highest IP Address on the router as the RID let’s find out what happens now we have added a loopback to Core_1:

Issue the show ip protocols again core_1 has the RID changed?  You should find it hasn’t.

Does clearing the ip ospf process command force the new RID to be used? Lets find out:

Issue the clear ip ospf process command on core_1

Wait a few moments for neighborships and routing tables to converge and issue the show ip protocols on core_1 has the RID changed? You should find that it hasn’t!!

Now save your configs with a copy run start, if your in GNS3 don’t forget to save your topology as well. 

Shutdown core_1 and then restart it

Wait a few moments for neighborships and routing tables to converge and issue the show ip protocols on core_1 has the RID changed? You should find that it is now the loopback address.  Remember this, you won’t change a RID unless you reboot the router!!

Stage 5 – Creating a pvc between CORE_2 and CORE_3 with a new subnet

On your frame relay switch create a mapping for DLCI 500 port 2 to DLCI 600 to port 3

On Core_2 and Core_3 issue the following commands:

CORE_2(conf ig)interface serial 0/0.500 point-to-point

CORE_2(conf ig-subif) frame-relay  interface-dlci 500

CORE_2(conf ig-subif)ip address 172.16.52.2 255.255.255.0

CORE_2(config)router ospf 1

CORE_2(config-router)network 172.16.52.0 0.0.0.255 area 0

CORE_3(conf ig)interface serial 0/0.600 point-to-point

CORE_3(conf ig-subif) frame-relay  interface-dlci 600

CORE_3(conf ig-subif)ip address 172.16.52.3 255.255.255.0

CORE_2(config)router ospf

CORE_3(config-router)network 172.16.52.0 0.0.0.255 area 0

Question:  What are the advantages of having an extra PVC,
Answer: If one of the routers go down the others can maintain connectivity.

Question: What are the disadvantages of having an extra PVC
Answer: Having fully meshed PVC’s can be expensive and increases the Administrative burden as well as overhead.

Question: Where is the single point of failure for each site?

Answer:  The link to the frame relay network over the serial interface.

Question: What is the advantage of point to point sub interfaces over point to multipoint with the broadcast option?

Answer:  The split-horizon rule is not violated

Question: What is the disadvantage of point to point sub-interfaces over point-to-mulitpoint frame relay?

Answer:  you have to have a separate subnet for each logical PVC.

Extra Tasks:

Think about the new subnet, how do you think this will affect routes and the generating of LSA’s write down what you think and then go back to the ospf database and routing table see if your right, if not work out why.

Shut down the serial 0/0.300 on core 3 now try and ping other interfaces do they succeed? If so why?

Answer: because of the extra PVC you created giving an alternative route.

I hope you enjoyed this guide, please send any constructive comments or additions to Renyk@systems-engineer.info