In this
topic, we wanna talk about some common reasons
that Routes might not be appearing as expected in our IP Routing Table when we running OSPF on
that Router. One Reason is that
the Area to which an interface belongs where we trying to advertise the network
for that interface maybe that interface is in an Area that is not adjacent to
Backbone Area
Remember, with OSPF we supposed to have a Backbone Area to which all other Areas connect we call it
Area 0 or sometimes people configured as Area 0.0.0.0 but we
just call it Area 0, any other Area needs to be
adjacent that Area for example,
we had an interface participating in Area 2 and in order to get from Area 2 to
Area 0, we had to go through Area 1 Area 1 was a
transit Area that’s not a valid Configuration because Area 2 is discontigues from the Backbone, that
it’s not a best practice is not something you
want to design in from the beginning but it’s a work around, i
want you to know about we can have something
called a Virtual Link
“A
Virtual link allow us to logically span a Transit Area” we can
go into the ABR’s, the Area Border Routers on
each side of this Transit Area and we can configure them for a Virtual Link,
so in example i was giving Area 2 can appear to
be adjacent to Area 0 it’s gonna go over this Virtual Link which makes its look
like a Direct Connection into Area 0 Virtual
Link is a work around but not a best practice.
Maybe we have a more believable Routing Source if OSPF Says,
here’s how to get to this network and EIGRP say’s
here is how to get to this same network, the Router by default is gonna believe EIGRP because
it has a Lower Administrative Distance remember, the default Administrative
Distance of EIGRP 90 with OSPF it’s 110 it’s
not as believable and
Ø We might have a Directly Connected
network
Ø We might have a Statically
configured network
That can also be more believable then OSPF, now keep in
mind that we can go in and alter the Administrative
Distance of Routing Protocol if
we want to, we can go into Router EIGRP
Configuration Mode or Router OSFP Configuration Mode and we can give the Distance
command and we could set the Administrative Distance to non-default value but that’s one reason that Route might not be showing up as expected it’s
not is believable as another Route pointing to very same network.
Another reason we might
have OSPF Routing issue is we Filtering Routes
And Filtering works a bit different with OSPF that it does with EIGRP, now it’s good time to discuss OSPF Route Filtering and then we jump into a Trouble Ticket.
Let’s think about, why we
might want to do OSPF Route Filtering we might want to do it for security reasons for
example
Ø Maybe we don’t want a
Router to have a reachability to a particular network
Ø Maybe we trying to reduce
the size of the IP Routing Table in a Router by eliminating any unnecessary
Routes
Ø Maybe we trying to prevent a
Routing Loops
And the question comes up, how do we Filter these Routes and there are 3 primary ways we can do this and to
illustrate these approach’s, let’s imagine a topology where we got a Backbone Area, Area 0 we got Area 1 and
one of the Routers in Area1 is also connected to
EIGRP Autonomous System
And remember, couple of these Routers have special Names, the Router
that has one interface in Area 0 and another
interface in Area 1 it’s called an ABR
It’s an Area Border Router, we also have a Router
that’s an ASBR, an Autonomous System Boundary
Router.
“It has at least one interface in an OSPF Area and at least
one interface in another Autonomous System in this case its EIGRP Autonomous
System” and when we doing Route Filtering with OSPF, typically we do either at the ASBR as a Routes coming in that external Autonomous System or we do at the ABR between
our Areas, lets imagine that we got
a network in Area 1
And that network is being
advertised into Area0
And let’s ask yourself, how is that
network going to show up in Area 0?
its gonna show up as “Type 3 LSA” which
generated by ABR, let’s say we got a network in EIGRP Autonomous System
And it’s being advertised into OSPF, it’s
gonna be advertised via “Type 5 LSA”
That’s generated by the ASBR and
because of the way this works and ABR might be
very appropriate place to Filter a “Type 3 LSA”, we can do that
with a Filter List and ASBR it could be an appropriate place to Filter “Type 5 LSA” and that’s gonna be done is part
of Redistribution Configuration
Something we talk
more about in next our module but there is another approach let’s say that we want a Filter a Route from just one of those Router inside
of Area 0, is that even possible with OSPF, can we tell OSPF to
have a different Link-State Database for just 1
Router in Area.
NO! we cannot, that’s not allowed infect that violates one of basic concepts of OSPF
we were saying that every Router in an Area needs to share the same Link-State Database about that Area, they need to agree on what to topology looks like,
what’s the network on that Area, we not going to able selectively
Filter Routes out of an OSPF Database for just 1 Router in an Area,
however but if we did this
What we, let OSPF go head and learn that network, that we running
to Filter Out, OSPF knows about in its RIB
(Routing information base) remember
that just because OSPF knows the Routes that’s no
guaranty that Routes is gonna be injected into the Router IP Routing Table,
the Route learn by OSPF is just a candidate to be
injected into the IP Routing Table, so what if did this before OSPF could inject that Route into the IP Routing Table
we blocked it, specifically we can configure something called a Distribute List and that can be used to block that Route from
being injected into the IP Routing Table
Well letting that Route stay in the OSPF Database, OSPF knows about the Routes but the IP Routing Table does not.
Those are 3 Primary approaches to OSPF Route
Filtering that we might need to keep in mind, is we doing a
Troubleshooting if we trying to determined whether
or not a Routes is Filtered now let’s go to interface and Troubleshoot on OSPF Routing issue.
If we got neighborship, that’s not
a guaranty that my Routing is working as expected for example, i notice on that Router R3 i am unable to ping the
fastetherent 0/0 interface on Router R1, let’s try
Ø R3#ping 192.168.1.1
That’s not looking
good, we do seem to have a path to R1, i can get to its other interface lets
confirm that
Ø R3#ping 192.168.2.1
We do indeed have a connectivity to Router R1 so what’s going on here, we have got OSPF configured on all if my Routers,
i thought that i told OSPF to make all of my
interfaces participate in the OSPF Routing Process, here is the
way in Router R3 that we can peer into the OSPF
RIB, the Routing Information Base
to see exactly what it has learned, what network is it know
about, let’s do
Ø R3#show ip ospf rib
We actually do know about that, we know about
that, we don’t seem to able to get to it,
let’s look at IP Routing Table
Ø R3#show ip route
There
is a different Route in the IP Routing Table that says, here is how you get to 192.168.1.0/24 network it’s directly connected to Null0,
which goes nowhere Oh! that’s the statically configured Routes
Ø Maybe we did that to
prevent traffic
Ø Maybe for Security Reasons
Ø Maybe we wanna to prevent
traffic from R3 to that network
But let’s say
that we determined, that’s what we don’t want to do, we actually want to be able to get that network we need to
delete that Static Route remember, those reasons we discussed earlier, we might not be able to learn a Route because we were
Filtering a Route we talked about some
other reason, one of those reasons
was there is more believable Routing Source in this case that more believable
Routing Source is this Static Route, lets fix this, let’s look
at Static Routes
Ø R3#show run | include ip
route
That’s the command we want to remove
off
Ø R3(config)#no ip route
192.168.1.0 255.255.255.0 null0
Now let’s see if we know the better Route
via OSPF
Ø R3#show ip route
That looks
much better, its say’s that we now know how to get there via OSPF, do we have
connectivity now
Ø R3#ping 192.168.1.1
And we resolved our
Connectivity issue, our Routing issue
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