OSPF Area Types and LSAs

 

In this post I wanted to go over the various OSPF area types and LSAs associated with each type, and then a configuration example for each, working with the above topology as in previous posts.

There is simply TONs of quality information and lessons out there on OSPF area types, LSAs etc, so I won’t re-invent the wheel here, just create (another) hopefully useful reference. 😊

Use this illustration below as a guide

Normal Area

Let’s first take a normal area – Area 3 – R8 – and run an ip ospf database command

You can see we have LSA types 1 – 5 – we expect the LSA types 4 and 5 since we have external AS routes being redistributed into our topology at the other side of the network.

Let’s look at the routing table on R8 also

Looks like this area is a good candidate to be a stub area – all the routes go through R7, so we can definitely make this more efficient.

Stub Area

To configure our Area 3 as a Stub area, we need to run this command on all routers in the area. So that’s R7 (the ABR), R8, and R9.

router ospf 1

area 3 stub

Let’s look at the OSPF database and routing table now on R8 now

We’ve replaced the external routes (E2 and E1) with a default route (via Type 3 LSA) and eliminated type 4 and 5 LSAs

But we still have all these inter-area routes that could use optimizing

Totally Stubby Area

Next, we’ll take this a step further and make Area 3 a Totally Stubby Area

Issue this command on ABR of the area – in this case, R7

router ospf 1

area 3 stub no-summary

R8’s route table and ospf database

We cut down the Type 3 Summary LSA’s down to a single LSA, and a single default route that represents all networks beyond the ABR…external and inter-area. Quite cool!

You can also run a show ip ospf command – and confirm that Area 3 is indeed a total stub area as shown below

Next, we’ll go over to the other side of the network and optimize those external AS routing

Here’s what we have so far

Not So Stubby Area (NSSA)

To make Area 2 a NSSA, we need to run this command on R3 (the ABR), R4, and the 2 ASBRs (R10 and R5)

router ospf 1

area 2 nssa

As show on R4’s routing table, the routes themselves don’t change, their designations do -the E2s become N2s, and E1s become N1s.

What does also change is type 5 LSAs are gone, replaced with Type 7 – as shown on R4

There is NO default rote injected in a NSSA – you need to manually inject the route from the ABR if you want to- the reason for this is that your ASBR may be injecting default routes into the area, for reaching the external networks, and you would not want multiple default routes. And in our case here, we have 2 external networks in Area 2

Let’s inject a default route into the nssa with tis command – appending default-information-originate

area 2 nssa default-information-originate

On R4, nothing magical happens – only difference is there is now a default route added (as a N2 route) with R3 as the next hop. Every other route has a longer prefix so the default route in this case does nothing to minimize the routing table, so this route is useless. Since the area contains ASBRs, advertising externally redistributed routes from 2 ASs, those routes remain.

Total Not So Stubby Area

Now we can try to reduce the type 3 LSAs (responsible of inter-area summary routes) here, and get a default route towards R3 for the destinations on the opposite side of the network. Also, I will remove the default-information-originate option

area 2 nssa no-summary default-information-originate

Here’s R4’s routing table now. The external routes remain for the 2 advertising ASBR routers, and now we have a single inter-area default route towards R3 for destinations beyond, and a smaller routing table ! This is as optimized as you can get with this topology.

Again, the no-summary option reduces the type 3 LSAs (to 1), and a default route towards the ABR is injected automatically for inter-area routes, without the need to specify default-information-originate, which was not helpful here anyway as it only represents external routes.

Thanks for reading this post- hope it serves as a useful reference!

OSPF - Path Selection – Route Type – Cost – Redistribution

  

In this post, I wanted to go over the OSPF path selection and route types, as well as a couple redistribution points and examples.

First thing is, let’s take a look at the two External AS routers. Both routers 5 and 10 are ASBRs- Autonomous System Boundary Routers – and are redistributing routes from EIGRP and iBGP respectively into OSPF and vice-versa

You can see this by issuing the following command on each ASBR

show ip ospf

Router 5 redistribution config: eigrp\ospf

router eigrp 11

network 10.1.56.0 0.0.0.255

redistribute ospf 1 metric 1000000 10 255 255 1500

!

router ospf 1

router-id 5.5.5.5

auto-cost reference-bandwidth 10000

redistribute eigrp 11 subnets

network 10.1.35.0 0.0.0.255 area 2

network 10.1.50.0 0.0.0.255 area 2

Router 10 redistribution config: bgp\ospf

router ospf 1

router-id 10.10.10.10

auto-cost reference-bandwidth 10000

redistribute bgp 110 metric-type 1 subnets

network 10.1.40.0 0.0.0.255 area 2

network 10.1.50.0 0.0.0.255 area 2

!

router bgp 110

bgp log-neighbor-changes

bgp redistribute-internal

network 10.1.111.0 mask 255.255.255.0

redistribute ospf 1 match internal external 1 external 2

neighbor 10.1.111.11 remote-as 110

neighbor 10.1.111.11 description "External routes to ASBR"

neighbor 10.1.111.11 next-hop-self

Something to point out when redistributing iBGP into an IGP (OSPF in this case) is that the iBGP learned route (R10 learned about the 11.11.11.11 network from R11) will NOT by default be advertised to another iBGP peer OR, to an IGP. We need to add the bgp redistribute-internal command to our bgp instance. We don’t have another iBGP peer here, but we do want the 11.11.11.11 net to be advertised into OSPF at the R10 ASBR.

router bgp 110

bgp log-neighbor-changes

bgp redistribute-internal

When distributing into EIGRP, we need to have various values set to base EIGRP’s final metric on – as shown in the config on R5

router eigrp 11

redistribute ospf 1 metric 1000000 10 255 255 1500

The values are bandwidth(kb), delay, reliability, effective bandwidth (loading) and MTU

OSPF Path Selection

Administrative Distance being equal, OSPF will choose the best path first by route type, and then path\interface cost.

Route Type:

• Note the use of specifying type 1 external OSPF routes vs type 2 routes (the default). A type 1 route will give you the total cost, including the external (redistributed) and INTERNAL cost, while a type 2 route will just account for the external cost.
• Also, a type 1 (O E1) route will be chosen over a type 2 route (O E2) regardless of cost. See the difference below – from R8 – the route to 6.6.6.6 is a E2 route, while the route to 11.11.11.11 is a E1

Further, here is the order for OSPF route types – again, chosen over cost.

Intra-Area (O)

Inter-Area (O IA)

External Type 1 (E1)

NSSA Type 1 (N1)

External Type 2 (E2)

NSSA Type 2 (N2)

Also, I want R10 to be the next hop for all routes learned by R11, so the next-hop-self line was added to the neighbor statement on R10

router bgp 110

neighbor 10.1.111.11 next-hop-self

Cost:

OSPF bases cost on the link bandwidth with the formula:

Reference bandwidth value (Mb) / interface bandwidth

For example, in this lab, I set the reference bandwidth to 10,000 on each router and all interfaces are 1 Gbps.

auto-cost reference-bandwidth 10000

10000 Mbps (or 10 Gbps) \ 1 Gbps = 10 - so all links in this lab have a cost of 10.

Loopback interfaces here, since the reference bandwidth on all routers was modified, have a cost of 1. By default it would be cost 0.

From the perspective of R9, R8’s loopback, is 2 links and a loopback interface away ( 10+10+1=21)

I hope this OSPF reference was useful – in the next post I’ll cover OSPF area types and LSAs.

Thanks for reading!!