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 220.127.116.11, R2 RID is 18.104.22.168, R3 RID is 22.214.171.124, and we have network 126.96.36.199.24 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 188.8.131.52 and a FW addr: 0.0.0.0.
The 184.108.40.206 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 0.0.0.0 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 220.127.116.11/32, so the question that popped out of my mind was “If there is a valid reachable type-3 for 18.104.22.168/32, 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 22.214.171.124/32 summary LSA was not the same 126.96.36.199 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 100.100.100.100/32 and see what happens.
If we examined the external LSA of prefix 188.8.131.52/24 we notice that it still has the 184.108.40.206 as an advertising router, but wait, where is the LSA type-3 that was advertising 220.127.116.11/32, it’s no longer there and was replaced with 100.100.100.100/32 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 18.104.22.168 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.