Study Question 2 3

[SnowNinja]

Okay, I'm having a hard time with OSPF and designation of DRs and BDRs. My understanding is that the router ID is determined first by highest IP of the loopback interfaces, THEN the active physical interfaces, and that the virtual interfaces no matter the ip addresss always have priority over any active physical interfaces irregardless of the IP address. The answer to this question tells me that my understanding is not correct. Can someone answer this one for me?

You are an administrator and you’ve just configured OSPF on a router with both physical and logical interfaces. Which of the following factors determine the router ID?

a. The lowest IP address of any interface.
b. The highest IP address of any interface.
c. The highest IP address of any logical interface
d. The middle IP address of any logical interface.
e. The lowest IP address of any physical interface.
f. The highest IP address of any physical interface.
g. The lowest IP address of any logical interface.


-SnowNinja

 

[GrnEyedLdy]

C

The way I understand it is when the router has physical and logical interfaces then the OSPF Router ID will be choosen from the highest IP address of the logical interface.


Patty

 

[jabw]

According to the books I'm using to prepare for the CCNA exam:
1: Router identities:
- prerequisite is an active interface.
- The highest IP address is chosen.
- First from the logical interfaces (loopback interface)
- Second choice is the physical interface.

Source: McGrawHill CCNA Study Guide p463.

 

[SnowNinja]

So, really, it sounds like the determining factor is the highest IP on ANY interface. It always sounded like loopback interfaces with any address will always have priority over any physical interface irrespective of address. Am I understanding this properly?

-SnowNinja

 

[jabw]

Looks right to me. But keep in mind that for all interfaces to participate in this game they must be activated (no shut)

 

[CiscoGuy33]

DR Election
DR and BDR election is done via the Hello protocol. Hello packets are exchanged via IP multicast packets on each segment. The router with the highest OSPF priority on a segment will become the DR for that segment.

The same process is repeated for the BDR. In case of a tie, the router with the highest RID (router ID) will win. The default for the interface OSPF priority is one. Remember that the DR and BDR concepts are per multiaccess segment. Setting the ospf priority on an interface is done using the ip ospf priority <value> interface command.

A priority value of zero indicates an interface which is not to be elected as DR or BDR.

If all OSPF priorities are the same then Router ID decides the elections.

For Router ID recommended Configuration -

When the Router ID is not configured, it is selected in the following manner:

1) If the loopback interface is configured, it is selected for Router ID

2) If the loopback interface is NOT configured, the IP address of the highest active interface is selected as Router ID.

I hope this helps!

E.A. Broda
CCNA, CCDA, CCAI, Network +

 

[SnowNinja]

Okay, I guess the point being made in the question then is that the highest IP of any virtual interface will be the prmary determining factor, then the "middle", then the lowest. Then, if would be the highest IP of the physical interfaces, and so forth. Right

-Snow Ninja

 

[Cluebird]

There is no "middle" with OSPF. If loopbacks are configured on the box BEFORE the OSPF process starts, the router will pick a router ID based on the highest primary address on any loopback interface regardless of whether there is a higher IP address on an active physical interface. If you want to change the router ID AFTER you've started OSPF, you either have to reboot the router or use clear ip ospf process * to reinitialize OSPF.
For example, I have two loopbacks on the router, LO1 192.168.10.10/24 and LO2 192.168.5.10/24, as well as a physical FA0/0 192.168.100.10/24. What is router ID when I start OSPF? Ans: 192.168.10.10
Now I add LO3 192.168.200.100 after OSPF is running, will the RID change? Ans: NO

 

[jabw]

The most important prerequisite is an active interface.
And a loopback interface always can be up. If you give it the highest IP address and apply the no shut command to it than it's expected to work as mentioned.

 

[CiscoGuy33]

Loopback does not have to have the highest address!

If a loopback has been configured when the router boots and a router ID has not been configured then the router will use the loopback address as the router ID.

It only looks at the physical IP addresses of the interfaces if no loopback is configured.

Since loopbacks do not go down as physical interfaces can, this is why you want to configure loopbacks when using OSPF, you will always know which will be DR and BDR. You can also control this by setting priorites as stated in my above post.

E.A. Broda
CCNA, CCDA, CCAI, Network +

 

[SnowNinja]

Right! THAT'S what makes me nuts about this question. So, loopback will ALWAYS have priority no matter what the address is. This I knew. But the answer for this question in the study material make it look like it HAS to be the highest IP for the LO. So, let's say there are multiple LO set up for whatever reason. I guess then it would use to highest IP address. Right? Which answer in the question would you choose?

-SnowNinja

 

[CiscoGuy33]

I would choose "C" - if you had more then one loopback (and I can not think of many reasons why you would) but if you had more then one loopback then the loopback with the highest address will be the router ID.

I have seen questions where they give you 4 routers and list priorities, loopback addresses(not on all)and multiple physical addresses on each router and ask which would be DR and which BDR.

Yes it can be crazy!!!!

I hope this helps!

E.A. Broda
CCNA, CCDA, CCAI, Network +

 

[Cluebird]

I agree with CiscoGuy33. C is the best answer for the given information.

FYI, in production systems there are often several loopbacks configured. They are used for: tunnel interfaces (sometimes I have to create GRE tunnels for non-IP traffic and then send that through IPSEC); fixing NAT/PAT routing issues; preventing host routes from dial-in clients from affecting the routing table; readdressing networks; IP unnumbered; RID for OSPF, EIGRP, BGP to name a few.
In labs we use loopbacks to populate routing tables and play with multiple routing processes. They are quite useful, like a Swiss army knife!

However, with OSPF, "Thar' be dragons, thar'"