From: William Lijewski (ccie8642@hotmail.com)
Date: Mon Oct 06 2003 - 01:54:42 GMT-3
It's not really in area 1. The router ID is only the way that the routers
see one another (ID = identifier). By default OSPF will pick the highest
Loopback, and if there are no Loopbacks it will pick the highest active
interface. This address that it picks is just to identify itself to other
routers. You can override this router-id selection with the 'router-id'
command, which I would recommed to do. The router ID that you specify for
OSPF does not have to be routable, it does not have to be pingable, it does
not have to be in the IP routing table of any other router. It is simply an
identifier, nothing more.
Personally, when I do a lab, I use a router ID of 1.1.1.1 for R1, 2.2.2.2
for R2, 3.3.3.3 for R3, etc... I use these numbers so when I type in 'show
ip ospf neighbors' I can tell at a glance who I'm neighbored with. Again,
these router identifiers do not have to be advertised in any protocol - they
are simply the way that the other OSPF routers will identify one another.
FYI: The router ID's are important in that they are used to create
virtual-links in OSPF. If you do not statically set the router ID's you can
run into problems. By default OSPF will pick the highest Loopback. Say for
example we have Loopbacks set up on our routers of 150.5.X.X, where X is the
router number. When OSPF comes up its going to pick that 150.5.X.X IP
address as its router ID. We will use this router ID to create a virtual
link to another router - both sides of the virtual-link will point to the
150.5.X.X router ID's of the other side. This will all work fine, until,
for instance, we add higher Loopbacks for BGP, say in the 200.10.X.X range
on the same routers. Nothing will break right away. However, if we were to
clear the OSPF process, or the proctor was to reload the router before
grading it, we will have a problem. When the routers come back up, or the
OSPF process restarts, it is going to again pick the highest Loopback for
its IP address. In this case the 200.10.X.X addresses. Our virtual-links
are still pointing to the 150.5.X.X ID's. The 150.5.X.X addresses are no
longer the router ID's, the virtual-link will no longer work, and you
network will not have full connectivity. Statically setting the router ID's
will prevent any new, higher Loopback from overriding the OSPF router ID.
Bill Lijewski
CCIE #8642
Network Learning Inc
5 Day R&S CCIE Bootcamp Instructor
>From: "Richard L. Pickard" <nettable_walker@comcast.net>
>Reply-To: "Richard L. Pickard" <nettable_walker@comcast.net>
>To: <ccielab@groupstudy.com>
>Subject: what determines the area the router ID falls into when using
>redistribute connected subnets command ?
>Date: Sun, 5 Oct 2003 23:28:14 -0600
>
>R_5 --- frame relay --- R_3
>
>Router ID =
>
>145.7.5.5
>
>
>
>
>
>debug from R_3 = 00:27:12: OSPF: Rcv hello from 145.7.5.5 area 1 from
>Serial0/0 145.7.100.2
>
>
>
>partial config from R_5:
>
>
>
>router ospf 10
>
>router-id 145.7.5.5
>
>log-adjacency-changes detail
>
>redistribute connected subnets
>
>network 145.7.25.0 0.0.0.255 area 0
>
>network 145.7.100.0 0.0.0.7 area 1
>
>
>
>so why is 145.7.5.5 in area 1 (as opposed to area 0)
>
>Just wondering about this ---
>
>
>
>clear ip ospf process on both routers made no difference -
>
>
>
>Thanks,.
>
>
>
>Richard
>
>
>
>
>
>//
>
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