Re: about OSPF router ID

From: Narbik Kocharians (narbikk@gmail.com)
Date: Sat Jan 31 2009 - 16:26:01 ARST

I totally agree with Scott, the OSPF router id is a 32 bit dotted decimal
number, it *can* be an IPv4 address, but it can also be any dotted decimal
number like "0.0.0.1" for R1 and so forth.

One problem that you may run in to (In a CCIE LAB) is when you have OSPF and
BGP with synchronization enabled in an AS, the router that redistributes the
BGP routes into OSPF must have the same router-id configured on both routing
protocols (meaning OSPF and BGP router-id must be identical on that router)
and in this case *you won't* be able to use anything other than a valid IP
address, because BGP's router-id must be a valid IP address.

Hope this helped.

On Sat, Jan 31, 2009 at 3:57 AM, ciscozest <ciscozest@gmail.com> wrote:

> Dear Scott, Jared, Roy and Jason,
>
> Thank you all for the input. Really appreciate that. Have a nice weekend :)
>
> -----Original Message-----
> From: swm@emanon.com [mailto:swm@emanon.com]
> Sent: Saturday, 31 January 2009 3:23 PM
> To: Jared Scrivener; 'Jason Madsen'; 'ciscozest'
> Cc: 'Cisco certification'; 'Cisco certification'
> Subject: RE: about OSPF router ID
>
> Actually, it not only doesn't need to be pingable, but it doesn't even need
> to be a valid IPv4 address. It's simply a 32-bit number.
>
> If you're bored, make your router-id's 240.1.1.1, 240.1.1.2, 240.1.1.3,
> etc. Definitely can't put that on an IP interface... Definitely can't ping
> it. But it works just fine.
>
> Jared's got a point about name lookups, but on the other hand, if you
> properly populate your DNS lookups you'll be good on that one!
>
> Real life, most people use a loopback, whatever your main management
> interface happens to be. Just keeps things simple. But it's just a 32-bit
> number, so the fact that it relates to an actual IP address is for OUR
> benefit, not the routers'!
>
> Scott
>
>
> ---- Message from "Jared Scrivener" <jscrivener@ipexpert.com> at
> 2009-01-30 21:25:18 ------
> >Whilst it is true that an OSPF Router ID doesn't have to be pingable, it
> >generally makes life easier to use a reachable IP (normally Loopback 0).
> >
> >Let's say that you are asked to also turn on "ip ospf domain-lookup" which
> >will translate your neighbor's Router-ID into a DNS name (which will
> either
> >be defined by a hosts file or received via DNS).
> >
> >If you do it via hosts entries and your are ALSO a DNS server then your
> DNS
> >clients would receive an unreachable IP address when they ping via DNS
> name.
> >
> >I know that's a rare case, but given the nature of question
> interdependency
> >on the lab (and the evil nature of workbook vendors) I personally use L0
> as
> >my OSPF Router-ID (and set it manually using the "router-id" command)
> every
> >time unless otherwise directed.
> >
> >Cheers,
> >
> >Jared Scrivener CCIE3 #16983 (R&S, Security, SP), CISSP
> >Technical Instructor - IPexpert, Inc.
> >Telephone: +1.810.326.1444
> >Fax: +1.810.454.0130
> >Mailto: jscrivener@ipexpert.com
> >
> >-----Original Message-----
> >From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> >Jason Madsen
> >Sent: Friday, 30 January 2009 9:01 PM
> >To: ciscozest
> >Cc: Cisco certification; Cisco certification
> >Subject: Re: about OSPF router ID
> >
> >OSPF Router IDs can be any UNIQUE IPv4 address...they don't have to be
> >addresses assigned to an interface.
> >
> >On Fri, Jan 30, 2009 at 6:56 PM, ciscozest <ciscozest@gmail.com> wrote:
> >
> >> Hi all,
> >>
> >>
> >>
> >> I read Cisco press book stated that OSPF router ID do not need to be
> >> pingable. In this case if I use a router ID which is not assigned to
> any
> >> interface on that router, would this cause any issue such as OSPF
> >> adjacency,
> >> LSA table advertisement, etc? Has anyone do this before and can
> enlighten
> >> me? Thanks.
> >>
> >>
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-- 
Narbik Kocharians
CCSI#30832, CCIE# 12410 (R&S, SP, Security)
www.MicronicsTraining.com
www.Net-Workbooks.com
Sr. Technical Instructor

Blogs and organic groups at http://www.ccie.net

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