CCNP - need help with OSPF brainstumper

Author

need help with OSPF brainstumper

cciewannabe

2002-06-21, 5:42 pm

Hey all,
I was wondering if someone could help me with something im trying to figure out. I've read OSPF in Cisco Press and checked out their website and am still in a quandry of the responsibilities of the Designated Router vs. and Adjacent router.

maybe im having brain farts, but in the Cisco book and on the website it states that the Designated Router is responsible for maintaining... well ill copy and paste it from their website, here it is:

On multiaccess networks (networks supporting more than two routers), the Hello protocol elects a designated router and a backup designated router. Among other things, the designated router is responsible for generating LSAs for the entire multiaccess network. Designated routers allow a reduction in network traffic and in the size of the topological database.

ok im fine with that, but where i get confused is that in stating the responsibilities of adjacent router they too are responsible for synchronizing LSAs with their neighbors... so which is it, it seems to me that they'd be performing redundant tasks, and i wonder why would one need both adjacent routers and a designated router, wouldn't it be an either or being they do much of the same thing. im probably off on a tangent here.. please help me, ive got most of my hair pulled out already.

Thanks in advance

cahillrobert

2002-06-21, 10:43 pm

Hopefully this VERY GENERAL outline will help.

There are two router types Designated (DR), *primary talker*, and Backup (BR).

There are several things to determine which is which, the easiest way is by the creation of a loopback address, the highest ip address wins.

The DR (designated router) maintains the master OSPF database for it's directly connected routers.

Very Generally each broadcast network maintains a DR and BR relationship. So a DR on one network interface of 6 routers, will have 1 DR (itself), 1 BR (next highest IP address), and 4 DROTHERS.

When a route change is made the router broadcasts the update to the DR / BR. If the BR receives the update it forwards it to the DR. The BR cannot update the other routers, only the DR can.
The way the LSAs are structured the routers will only accept updates from the DR. This mechanism provides for structured and possibly authorized updates.

If the DR goes down the BR assumes DR responsibliity,,, oops to far

cciewannabe

2002-06-22, 12:34 pm

thanks for your feedback, i think i got it, and perhaps im looking too deep with my quesiton. ciscos examples are simple and perhaps my real question is in how the multicasts are sent to the DR from a lower layer neighbor. in Cisco's example they show the drother being directly connected to the DR and that's simple to understand. but what if... there are 3 routers and multiple paths between the DR and the drother. I think i understand that the hello packets are just with the directly connected routers and updates are forwarded downward or as they say flooded. if i understand this right, drothers will never share LSAs with one another but will forward LSAs that originated from the DR. and when a change happens in the network the drother doesn't inform anybody but the DR, for which then the DR will tell everybody. Adjacencies the paths of every drother to the DR and if a drother had multiple paths to the DR the DR will elimanate one of those paths. I don't know, i could be way off base, PLEASE CORRECT ME IF YOU KNOW IM WRONG... it's disconcerting to me that Cisco only gives a simple example and uses general terms such as router to explain all of these concepts. im sittting here trying to figure out what do they mean by router, does cisco mean drother to drother or dr when it speaks of adjacencies. granted if a drother isn't directly connected it will have to get forwarded or direct updates from another drother. that's what im trying to figure out. maybe i should buy a book on just ospf.
thanks for your help

cahillrobert

2002-06-22, 2:04 pm

Updates only occur on one broadcast domain and are handled by the elected DR. One router will not update every other router in one area. Only those directly adjacent to it. The LSAs do not route.

Let say that you are using 192.168.1.1-255 as loopback addresses.

The DR, BR, and the other devices are only on the same broadcast domain. eg. 172.31.254.0 255.255.255.240 so only the 14 devices are talking OSPF.
So 1 DR, 1 BR, 12 Drothers all in the same area.
Router1 f0/0 is the DR Lo1-192.168.1.14
Router2 f0/1 is the BR Lo1-192.168.1.13

However Router1 could additionally be a DROTHER on another Broadcast Domain (172.31.254.16 255.255.255.240) on a different interface f0/1 with 14 other devices. In this case the DR would have a lo > 192.168.1.14

What I am trying to get at, is there is not ONE DR for an area. There will be multiple. Either in series or in parallel.
Each broadcast domain has only 1 DR, 1 BR. If there are only two routers 1DR & 1BR.

Unicast updates occur when there is a non-broadcast environment.
Multicast updates occur when there is a broadcast environment.

I miss my white-board in cases like this.

cciewannabe

2002-06-22, 9:44 pm

thank you, thank you thank you! you should go teach claire (or is it clare) gough how to explain concepts. it seems so clear now, just one peice missing to my mental puzzle and that it's per broadcast domain. i reread claires chapter 5 again and no reference is given to the concepts of multiple DRs and BRs within an area. her examples are very simple and show only one DR and one BR, which led to my confusion. she refers to broadcast domain as simply segment and now it makes sense, but when i think of broadcast domain i refer to it as broadcast domain (albeit segment may be synonymous) but i was thinking ospf was more complicated than it is, and didn't put two and two together, duh. i was wondering how the hell there could be only one DR in an area. maybe its me, but i don't like this book of claires, i have no problem with the other books in the set, or any other cisco press book (except for typos). im dissapointed at how breif and obtuse this book is. the review on amazon concurs with my opinion. i just purchased another cisco press book on just ospf to help me further. although i have a pretty good grip now i want to know all i can for my CCIE path. Thanks again for your help. now on to EIGRP.

cahillrobert

2002-06-22, 11:13 pm

I have heard a lot about Jeff Doyle and ???
Routing TCP/IP Volume I and II from CiscoPress

I have heard so many good things about this reference material that it will probably be one of my next investements.

It may be a little overkill for the CCN/DP level but if you want a really good reference book to take you to the CCIE level this is THE book.

I have recently seen this book referene on non-Cisco vendors web site along with Henrabi (?sp). That is also supposed to be the BGP4 ref. material.

wirechild

2002-06-22, 11:31 pm

Two things come to mind...

1) the addition of a lab

2) a better book or source

I had the same difficulty with understanding some of the basics while reading Gough's book until I put it down and read Paquet's and Terre's book and then applied the reading to my lab. Things were so much clearer. The labs in that book are priceless for learning the concepts.

cahillrobert

2002-06-23, 10:59 am

According to OSPF design THEORY.

1 There should only be one DR for an OSPF area. It probably is a rare event.

2. The next goes counter to 1. There shouldn't be any more then 6 hops in an OSPF area from an ABR. Usually very realistic and doable.

The idea to try to keep convergence time to a minimum.

-----
You can change which router is the DR by adding "ip ospf priority #" TO AN INTERFACE where the number is greater than the default of 1. When the election occurs, despite the Lo Address, the highest priority wins. A priority of ZERO will not participate in an election process, both connecting interfaces should not be set at 0. Leaf routers, at the distant edge, should be set as 0. This way they will never be elected as a DR because of a higher interface or Lo address.
----

When a DR is rebooted the BR will take over the DRs responsibility; when the DR comes back on line the orignal BR will still act as the DR. Until it is also rebooted.

sukuvi

2002-06-23, 8:04 pm

What a nice course on OSPF, Cahillrobert!!!!!
You just gave me a refresher course right there.
One of the problems with these concepts is once you don't use them any more, part of it will be gone. Then when you are faced with a problem, you have to go back to the books to refresh yourself.
Well done once again.

About the books, I found some handy reference book that I jotted down a few weeks ago:

1/ Cisco OSPF Command and Configuration Handbook
Author: William Parkhurst
ISBN: 1587050714
Published: April 2002
Price: $55.00

2/ Cisco BGP-4 Command and Configuration Handbook
Author: William Parkhurst
ISBN: 158705017X
Published: April 2001
Price: $55.00

3/ CCNP Practical Studies: Routing
Author: Henry Benjamin
ISDN: 1587200546
Published: April 2002
Price: $49.95

The prices are from the publisher, Cisco Press, however I think we could find them cheaper if we shop around online, using mysimon.com, for example.

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