Hi,
There is a bit of confusion in these questions so I'll try to make the best explanation possible :) .
1)
RID and OSPF Priority are related to each other but they are not directly used for the same purpose.
RID:
- used to uniquely identify an OSPF-Router inside an OSPF "domain"
- RID can be manually configured ("router-id x.x.x.x" command inside ospf router configuration mode)
- RID can be automatically configured using a loopback interface's IP address or a physical interface's IP address
- Every router in the OSPF domain should have a unique RID
OSPF Priority:
- mainly used in multiaccess segments (ex: Broadcast segments) in order to elect DR/BDR routers
- By default the value is 1
- Can be changed with the command "ip ospf priority x" interface conf command
- Router with the highest priority in the multiaccess segment will be elected as DR, the second highest will be elected as BDR (remember, no preemption exist in DR/BDR election)
- If all the routers in the segment have the same priority, the router with the highest RID will be elected as DR and the second highest will be elected as BDR
So, in order to reply to your question "When should I need to change router-id or priority in OSPF topology?"
- You do need really need to change the RID, in general. You can change it in order to use a more "readable" RID (such as 1.1.1.1 or 2.2.2.2,....) or when 2 or more routers use the same RID
- You can change the OSPF priority in order to change the DR/BDR election
2)
I don't know if there is a "technical" limit to how many routers you can have in an area but, in general, an area shouldn't have a high number of routers.
Remember that having a lot of routers in an area means a bigger LSDB, more LSAs and so on.
So when you have a lot of routers, it could be a good idea to use multi-area OSPF
3)
- OSPFv2 is used for IPv4
- OSPFv3 is mainly used for IPv6 (but you can even use OSPFv3 to propagate IPv4 information, by encapulation IPv4 information insde OSPFv3 messages, using address-families)
- In general, if you area using IPv4 you're using OSPFv2 whilst if you're using IPv6 you are using OSPFv3 (i've said in general)
4-5)
These 2 commands are used for different purposes
- show ip int interface: it mainly displays L3 information about that interface
- show ip ospf int interface: it display OSPF-related information about that interface
So, your questions:
- "Multicast reserved groups joined: 224.0.0.5 224.0.0.6" under the first command means that the interface will accept and process packets destined to that multicast address (224.0.0.5: all OSPF-routers; 224.0.0.6: all-DR/BDR-routers)
- "Network Type BROADCAST" under the second command means that OSPF will treat that interface (and the network attached to it) as an "Broadcast OSPF Network Type".
It is not related to the actual "capabilities" of that interface, it is only used by OSPF to understand the OSFP Network Type.
If you do not change the default network type, OSPF will calculate the network type based on the physical characteristics of the interface (f0/0 as a FastEthernet interface so, by default, will be treated as a OSPF Broadcast network type) - "Is the reason why Net Type is broadcast because I advertised Net 0.0.0.0 ?" No.
Rethink about this command, especially the wildcard mask...do you mean a /0 or a /32 subnet mask? If the purpose of your command is to advertise and enable all the interfaces, your wildcard should be 255.255.255.255
6)
They can help you while troubleshooting OSPF.
When you look at the LSDB where LSAs are stored (show ip ospf database) you can undestand for example why you can't see a route in routing table or why somenthing isn't working (maybe because you do not have an LSA), you can undestand the OSPF configuration (one area, multi-area),...
Luca
