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Re: OSPF and wild card bits confusion posted 04/29/2000
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Thanks for the explanation, Bob. Just to check my own sanity, I went back to the routers and changed the wildcard bits on that ospf config entry to see if anything would change. Here's what I found:

"network 137.20.25.0 0.0.0.255 area 2" gave me this in another router's route table:
O IA 137.20.25.0/24 [110/128] via 137.20.100.34, 00:33:12, Serial0


and

"network 137.20.25.2 0.0.0.0 area 2" gave me this in another router's route table:
O IA 137.20.25.0/24 [110/128] via 137.20.100.34, 00:00:05, Serial0


I can see how that works now - the route goes in the table based on the subnet mask of the interface - not based on your ospf config entry.

But now I have to ask - why would you ever specify anything but 0.0.0.0 for wildcard bits? In the same example I mentioned previously, why didn't they just use 0.0.0.0 for all the following entries?:

> router ospf 1
>   network 137.20.25.2 0.0.0.0 area 2
>   network 137.20.64.0 0.0.15.255 area 0
>   network 137.20.100.32 0.0.0.31 area 1
>   network 137.20.224.5 0.0.0.0 area 0
>   network 137.20.240.1 0.0.0.0 area 0

Ok, I just finished reading a little in Doyle's TCP/IP book and he cleared it up for me some more. But I still have the question: what's the best practice to use - when I'm in the lab, should I just use 0.0.0.0 for everything?


Thanks,

Kinton


At 4/29/00, you wrote:
The format "network ip-address wildmask" is used to specify which interfaces
are in which areas. This is different from RIP, IGRP, EIGRP which use the
network statement to say which addresses they are routing.

By using the /32 format, there is no question of which interface you
specify.  Remember that the order of the network statements can affect the
outcome when you are using the masks as other than /32. This is because for
each interface, the network statements are scanned until a match is found,
or there are no more.

If you had an interface address of 132.10.100.1 and used the following,

network 132.10.100.0 0.0.0.255 area 3
network 132.10.100.1 0.0.0.0 area 0

the interface would be in area 3, not 0.

Bob Reed


----- Original Message ----- From: Kinton Connelly <kinton@xxxxxxxxxxxx> To: <ccielab@xxxxxxxxxxxxxx> Sent: Saturday, April 29, 2000 5:23 PM Subject: OSPF and wild card bits confusion


> I've been going through the CCIE Boot Camp practice labs and just ran into > something that has me confused. On lab 8a, I don't understand where they're > getting the wild card bits for the OSPF areas. I've included a bit of > sample code from router 5 below. > > Here's what I don't understand: normally, if you give me an interface like > Serial1 below and tell me to put it in OSPF Area 2, I'll take the interface > address: > > 137.20.25.2/24 > > and turn it into this OSPF statement: > > network 137.20.25.0 0.0.0.255 area 2 > > But as you can see below, sometimes they do it this way and sometimes they > don't. Why? Why put it in there as "network 137.20.25.2 0.0.0.0 area 2" - > that would be the entry for a /32 network. > > Thanks for any help, > > Kinton > > > R5 > -- > interface Loopback0 > ip address 137.20.240.1 255.255.240.0 > ! > interface Ethernet0 > ip address 137.20.64.5 255.255.240.0 > ! > interface Serial0.1 multipoint > ip address 137.20.100.34 255.255.255.224 > ! > interface Serial0.2 point-to-point > ip address 137.20.200.17 255.255.255.240 > ! > interface Serial1 > ip address 137.20.25.2 255.255.255.0 > ! > interface BRI0 > ip address 137.20.224.5 255.255.240.0 > ! > router ospf 1 > network 137.20.25.2 0.0.0.0 area 2 > network 137.20.64.0 0.0.15.255 area 0 > network 137.20.100.32 0.0.0.31 area 1 > network 137.20.224.5 0.0.0.0 area 0 > network 137.20.240.1 0.0.0.0 area 0 > > _______________________________________________________ > To unsubscribe from the CCIELAB list, send a message to > majordomo@xxxxxxxxxxxxxx with the body containing: > unsubscribe ccielab >

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