Networking with Cisco

Had a nice week everyone? I’ve been writing a lot and working a lot, but anyways here is part 2 of 2!

Link State Database / Topology Table
What’s that? you may ask – maybe only because I forgot to mention it in the previous article, well it’s a database which contains data on how the routers ’see’ the network topology (link states), all the routers in an AS will have a copy of this table.
This table is getting changed as the network topology changes, as for example if a prefix is moved or an interface goes down.

VLSM
One time during this week I suddenly realized that I should probably mention that OSPF supports VLSM (Variable length Subnet Masks), that some people still stick to the usage of routing protocols that does not support VLSM is way beyond my understanding.

LSAs and LSA Types
There are 7 types of LSA (Link state advertisements) in OSPF;

1. Router Link Advertisements, generated by each router and is flooded in a single area.
2. Network Link Advertisements, flooded throughout the network and is generated by the DRs. Describes a set of routers connected to a network.
3. Type 3 are summary link advertisements. These are generated by the Area Border Routers and describes Inter-area routes, generating a quad zero route by the command default information originate also generates a type 3 LSA.
4. Type 3 and type 4 is very often described at the same time, the type 4 LSA describes routes to an ASBR.
5. These are generated by the ASBR, and describes routes that are redistributed into OSPF from AS’s or routing protocols.  These are flagged in the routing table with O E1 and O E2 (external type 1 or 2) and are flooded to all areas except for stub areas.
6. Group membership link entry LSAs are generated by multicast OSPF routers.
7. Type 7 LSAs are only flooded to not-so-stubby-areas and are generated by ASBRs. When external routes are injected to areas other than the backbone area 0 are type 7, these are converted to type 5 by area border routers before they are injected into the backbone area.

Route summarization
My feeling is that at least once (a day?) in every network administrators life they’d wish the routing table was smaller and had a bunch of fewer prefixes, but what can we do?

We can use route summarization to make the routers summarize all routes in an area.

The configuration is as follows

Router(config-router)# network 10.0.0.0 0.0.0.255 area 0
Router(config-router)# network 10.0.1.0 0.0.0.255 area 1
Router(config-router)# area 0 range 10.0.0.0 255.255.255.0
Router(config-router)# area 1 range 10.0.1.0 255.255.255.0

This router will act as an area border router (ABR) between area 0 and area 1, the area areaid range command tells the router to summarize all routes that area to that summary address before advertising them in another area.

Multiarea OSPF Configuration on Cisco IOS

The scenario are 4 routers, preconfigured with IP addresses and daisy chained.
R1: Area 0
R2: Area 0
R3: Area 0 and area 1
R4: Area 1

Area 0 = 10.0.0.0/24
Area 1 = 10.0.1.0/24

We will use route summarization.

To configure R3 to be both in area 0 and area 1, let us say we use /30-ranges for connecting the routers.

R3(config-router)#network 10.0.0.0 0.0.0.3 area 0
R3(config-router)#network 10.0.1.0 0.0.0.3 area 1
R3(config-router)#area 0 range 10.0.0.0 255.255.255.0
R3(config-router)#area 1 range 10.0.1.0 255.255.255.0

Configure all the other routers as usual, but R4 should be configured as only area 1.
I configured all routers to redistribute connected and static subnets.

To verify that you see the area 1 as 10.0.1.0/24 instead of (now) 10.0.1.0/30.

R1#sh ip route
Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.0.0.0/30 is directly connected, FastEthernet0/0
O IA 10.0.1.0/24 [110/2] via 10.0.0.2, 00:01:56, FastEthernet0/0

There you go!