This question is for Keith Bogart ONLY .it`s about an issue in your lecture

@Network_Eric
let`s start with RFC 1583
R1) #router ospf 1
R1(config-router)#compatible rfc1583

R1#show ip ospf | section comp
Supports NSSA (compatible with RFC 3101)

why this command is not configured ?
also, what the rfc 1583 mentioned about the NSSA external lsa?because i don`t see anything about it in rfc

Major,

There seems to be some confusion about the RFCs, so I’ll give some clarity.

  • RFC 1583 - This is the first RFC defining the OSPFv2 standard, released in 1994
  • RFC 1587 - This is an expansion to OSPFv2 for NSSA areas, so this is the RFC that defines NSSA. Released in 1994
  • RFC 2328 - This is an update to OSPFv2, this changes routes to ASBRs to prefer intra-area routes to avoid backbone area congestion. This was released in 1998
  • RFC 3101 - This is an update to RFC 1587 and changes the behavior of NSSA areas as we previously discussed. Released in 2003.

Now, it’s worth noting, the changes in RFC 2328 have a conflict with NSSA pathing as described in RFC 1587, if a device is configured for compatibility with both RFC 1587 and RFC 2328, 2328 will take precedence and change the expected pathing.

Cisco is compatible with RFC 1583 by default, not RFC 2328. Other vendors, like Palo, are compatible with the more modern RFC 2328 by default. Cisco is also compatible with RFC 3101 by default.

So your direct question is, you told it to be compatible with an RFC it is already compatible with by default, and did not modify the NSSA RFC.

A couple of things of note though. I am definitely not an expert when it comes to RFCs and the minor changes in each iteration, etc. I’ve gotten a chance to look some stuff up and I’ve learned some interesting things, but it’s worth noting that while this can be interesting, it’s pretty much useless trivia. Since Cisco defaults to 1583 and 3101 by default, you only really need to know how those two behave (or whichever vendor platform you are using). Yes, you can change compatibility and the behavior, but you don’t really get anything by doing so.

So, if you have further questions I can certainly try to help, but keep in mind this is outside of the scope of general network engineering, the CCNP, and even the CCIE. My advice is, while it’s good to read RFCs and have a general idea on how they interact, they are mostly of use when something doesn’t behave the way you think it should. There’s either an obscure rule in it you weren’t aware of, or in the examples provided earlier in this thread, they are using different versions of the RFC.

Hope the above was helpful.

1 Like

@Network_Eric
thanks eric for all your efforts . i love you man . btw,my real name is Mohamed i`m 36 years old from Egypt.
you said

RFC 2328 - This is an update to OSPFv2, this changes routes to ASBRs to prefer intra-area routes to avoid backbone area congestion. This was released in 1998

Q1. what do you means when you said “this changes routes to ASBRs to prefer intra-area routes to avoid backbone area congestion” ?
are you referring to in RFC2328 they mentioned if the ABR R1 received external LSA and received the same LSA from the intra-area he would prefer the inter-area LSA ? and if that is the case ,in RFC 2328 they said they modified things in RFC 1583 and put it in RFC 2328 , but what did they said in RFC1583 about that same situation ?

========================

you said

Now, it’s worth noting, the changes in RFC 2328 have a conflict with NSSA pathing as described in RFC 1587, if a device is configured for compatibility with both RFC 1587 and RFC 2328, 2328 will take precedence and change the expected pathing.

why did you mentioned both RFC2328 and RFC 1587 if both are away from each others and both are not supporting the same thing . one is supporting NSSA only (which is RFC 1587) and the other is supporting another different thing (which is RFC 2328) i`m stilll not understanding this point.

=========================
you said

Cisco is compatible with RFC 1583 by default, not RFC 2328. Other vendors, like Palo, are compatible with the more modern RFC 2328 by default. Cisco is also compatible with RFC 3101 by default.

Q2. also the same thing , you compared somthings is not related to each other .
look, what i understand is
RFC 1583 has been deprecated by RFC 2328 . both are talking about the SPF and how the intra,inter,external LSAs are working .but not including the NSSA topics.
RFC 1587 has been deprecated by RFC 3101 . both are talking about NSSA things only .
so, let`s say RFC 1583 is not related to RFC 2328 .
and the RFC 1587 is not related to RFC3101.

==========================
Q3. you said

Cisco is compatible with RFC 1583 by default, not RFC 2328

but how did you know that ?
R1(config)#router ospf 1
R1(config-router)#compatible ?
rfc1583 compatible with RFC 1583
rfc1587 compatible with RFC 1587

i dont see RFC 2328 in these choice .also i dont see RFC 3101 in the choice .why ?

Hey Major,

Q1. I am referring to 16.4.1 in the RFC, this changes how systems prefer routes to the ASBR when 1583 is disabled.

Q2. The comparison I’m making is between vendors. Cisco supports 1583 by default, while Palo supports 2328 by default. This was said to highlight the point that the behavior is only really important depending on the platform you are using at any given time. So if you have a Cisco device connected to a Palo Alto running OSPF they might behave differently in how they handle routing.

For the issue with compatibility, that’s because if both of them are enabled, they have a conflict. 2328 says that if there is an intra-area route to the ASBR that it will always be preferred, 1587 does not. This means that if both are enabled, the Intra-area will be preferred despite what it says in 1587. This is not overly relevant, as cisco does not have 1587 or 2328 enabled by default, but I thought it interesting to note the order.

Q3. Unfortunately OSPFv2 is less descriptive. If you do show ip OSPF it will show you the RFC3101, but does not mention the 1583. If however you are running OSPFv3, show ospfv3 will show you all of the RFCs that are currently in use by the process, including 1583. To “enable” 2328, you actually disable 1583

1 Like

@Network_Eric

Thanks eric

Per RFC 2328 , the following rules indicate which paths are preferred when multiple intra-AS paths are available to ASBRs or forwarding addresses:
 
1.	Intra-area paths using non-backbone areas are always the most preferred.
2.	The other paths, intra-area backbone paths and inter-area paths, are of equal preference.

could you please give me example for each bullet ?
i want to ask you a question before , in RFC 2328 section 16.4.1 , is it talking about the external routes ONLY ? or it`s talking about how ospf behaves if he received internal OR external routes from different areas ?

Hey Major,

This applies only to external routes, so LSA type 5 and Type 7.

For examples, if you are in area 1 and the ASBR redistributing in to OSPF is also in Area 1, it will be preferred to go directly to that ASBR. If you have the same route being redistributed in Area 2 you will always use the one in Area 1, even if the cost to get to that ASBR is higher than the one in Area 2.

If you are in the Backbone Area (Area 0) then intra-area and inter-area routes are treated equally and so you will use cost to determine which ASBR you forward to. So, if a network is being redistributed in Area 2 and the same network gets redistributed in Area 0, if you are in Area 0 (or any other area that is not area 2), you will not have a preference for which you choose and instead choose based on cost.

Having said the above, I haven’t take the time to lab it, as I’ve only used the default compatibility on the Cisco devices of my labs. Hope you find that helpful!