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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