Category Archives: Cisco


Have you ever questioned your self, why on earth do we need OSPF type-4 LSA ? well, you are not the only one. Welcome to this blog post where we’ll try to discover the actual worth of that LSA.

First let’s examine the below graph, where we have a basic implementation of multi-area OSPF. R1 RID is, R2 RID is, R3 RID is, and we have network redistributed to OSPF area 1 by area 1 ASBR; R1.


We’ll start by examining the contents of the Link State Data Base (LSDB) of R3 for the external prefix 200.200.200/24;



When we look extensively at the type-5 LSA of prefix 200.200.200/24 in the LSDB of R3, we can see that it has an advertising router of and a FW addr:

The is the RID of the advertising router, while the forwarding address is a way to that can be used to route traffic in another direction than to the router that originated the LSA. Having a value of as the FW addr essentially tells other routers to forward packets for that destination to the router that advertised the LSA.

However when looking closely, we can see that R3 LSDB also has a type-3 LSA for,  so the question that popped out of my mind was “If there is a valid reachable type-3 for, then why do I need Type-4 LSA?”.

After taking a while looking into the issue I realized what went wrong. Well, I made a mistake, the summary LSA was not the same that advertised the external LSA. So what is the difference ?

Here’s were everything starts to get clear, we all network engineers have the convention that OSPF RID is driven from the highest loopback interface IP address present on the router and in case of the absence of loopback interfaces, the highest physical interface IP address well be chosen as the OSPF RID of the router, here is the catch, OSPF RID is a node-identifier. It usually uses an IPv4 address format, but it is an ID and does not need to match any interface present on the router.

Ok then, let’s try something, we’ll change the IP address of the lo0.0 of R1 to and see what happens.


If we examined the external LSA of prefix we notice that it still has the as an advertising router, but wait, where is the LSA type-3 that was advertising, it’s no longer there and was replaced with that is the new lo0.0 address of R1 that was already included under protocol ospf hierarchy of the configuration stanza. So the only way to reach the RID of R1 and consequently the external prefix 200.200.200/24 is by following the information contained in the ASBRSum LSA.

As you notice, our small lab was fruitful and we can conclude the following:

  • Routers in other areas see a Type-5 with a meaningless RID. They might have a type-3 prefix that looks like the ASBRs RID, but this doesn’t help it find the ASBR.
  • The type-4 LSA generated by the ABR is needed as a glue record, so that routers can ‘anchor’ the unknown ASBR RID against as reachable via a well-known node, the local ABR.

I hope this was informative and thank you for viewing.


Understating Cisco IOS v15 Licensing

IOS 15 is a single software package for the hardware platform that your are using. Now that flash is cheap and large, there doesn’t need to be a different IOS image for the different versions with the advanced features. So you get one software image that has all the features for that platform.

Prior to IOS 15 , we used to have 8 IOS images, all routers came with IPBase image and according to your needs you can choose what IOS image to install on your router as each image had different features than others as illustrated below.

wilkins_iosver_fig03 Read the rest of this entry

Use of EEM Scripting for Special Hot Backup Solutions

In this blog I would like to share some information about some special Hot backup solutions that we are forced to deal with due to customer requirements.


In the above figure, we have a customer that’s connected to the ISP through 2 Links, the Main link is a Pre-WiMax connection that’s carrying customer’s Internet and VPN traffic and configuration on the PE router is as below.

PE#show run int Fast1/0.100
Building configuration…
Current configuration : 184 bytes
interface FastEthernet1/0.100
description “Main Internet Link Pre-WiMax”
encapsulation dot1Q 100
ip address secondary
ip address

PE#show run int Fast1/0.200
Building configuration…

Current configuration : 156 bytes
interface FastEthernet1/0.200
description “Main VPN Link Pre-WiMax”
encapsulation dot1Q 200
ip vrf forwarding VPN-A
ip address
end Read the rest of this entry

OSPF Domain-ID || Domain-Tag

OSPF Domain ID

When OSPF is used as the routing protocol on a provider edge to customer edge (PE-CE) link in a multiprotocol label switching (MPLS) VPN. PE routers mark OSPF routes with the domain attribute derived from the OSPF process number to indicate whether the route originated within the same OSPF domain or from outside it.

Importance of Domain-ID

In MPLS-VPN network ISP cloud is treated as a super backbone area, and PEs are considered as ABRs or ASBRs depending on the domain-id value, then routes redistributed to CEs would be OSPF inter- area routes or would be OSPF external routes.

Why would we care to have OIA or E or O routes in our OSPF database?

The answer is simply that OSPF prefers intra-area routes then inter-area routes and finally external routes to be installed in routing table. And we don’t want to come in to the case that routes are leaked from a back door in then being redistributed to the ISP cloud creating a loop in the network and disrupting traffic.

So if we had a different processes on our PEs then so we have to explicitly configure the domain-id value under the OSPF process.

OSPF loop prevention in PE-CE routing

  • DN bit

o    When a type 3 LSA is sent from a PE router to a CE router, the DN bit [OSPF-DN] in the LSA Options field MUST be set. This is used to ensure that if any CE router sends this type 3 LSA to a PE router, the PE router will not redistribute it further.

  • Domain-Tag

o   PE routers originate Type 5 LSAs reporting the extra-domain routes as AS-external routes. Each such Type 5 LSA MUST contain an OSPF route tag. This tag identifies the route as having come from a PE router. The VPN Route Tag MUST be used to ensure that a Type 5 LSA originated by a PE router is not redistributed through the OSPF area to another PE router.

Read the rest of this entry