EIGRP Administrative Distance, EIGRP Route Summarization, EIGRP Stub

The default administrative distance - AD of EIGRP internal route is 90 which is marked by the "D" symbol. The AD of an external route or considered as an external route using route distribution is 170 which is marked by the "D EX" symbol. There is also an eigrp summary route with the AD of 5. All of this can be shown with the #show ip route eigrp command.

To change the AD of EIGRP use the command:
(config)#router eigrp
(config-router)#distance eigrp

Auto summarization is a default feature at EIGRP. The purpose of this feature is used to automatically summarize the routes that are advertised through classfull network boundaries (EIGRP and RIPv2). It is therefore almost always necessary to turn this feature off in order make the router available to advertise classless networks routes by using the no auto-summary command. It is important to configure this route on the router sending the route. Remember that when applying the summarization, the adjecency goes down for a moment before it goes up again.

There are several advantages when applying a route summarization. A crucial advantage is that the route tables are getting smaller, because several routes are summarized. Another advantage is that CPU processing will be lighter, as the routes in the table are smaller.

To configure auto summarization use the following command:
(config)#int
(config-if)#ip summary-address eigrp <>

There are several ways to summarize routes where each route needs to have a specific bandwidth:

1. If there are multiple routes with the same CIRs, then add the amount of the CIRs together and use that as the minimum bandwidth needed. To set this use the command:
(config-if)#bandwidth

2. If there are multiple routes with different CIRs, then configure the route using a point to point (sub-)interface mode and assign each VC with the required CIR bandwidth value. To set this use the command on each sub-interface:
(config)#int serial point-to-point
(config-subif)#ip address
(config-subif)#bandwidth

3. There is also a possibility to configure a multipoint sub-interfaces. This can be done by adding all the CIRs from the routers. To set this use the command on the main router:
(config)#int serial multipoint
(config-subif)#bandwidth

The EIGRP can also be configured as a stub. However the stub has to be set only in the hub router. The spoke router does not need to be configured for stub (although possible) because the spoke router does not need to keep the routing table. It simply forwards all the routers to the hub router. This feature can also combat the SIA (stuck in active) problem because in stub networks, spoke routers will not be queried for routes when the hub does not have a feasible successor. To configure stub use the command:
(config)#router eigrp 100
(config-router)#eigrp stub

-- 10 February 2011 --

OSPF network

The first thing to know when designing an OSPF network is that each area should have a router that connects to area 0 (backbone area) either physically or logically.

Area border router (ABR) are routers that has more than 1 ospf area. To see this run the #show ip ospf command or use the #show ip ospf border-router command to see adjecent router area state is. ASBR are basically ABRs that has more than 1 routing protocol.

Route redistribution means placing a route that is learned in one routing domain into another routing domain that has different routing protocols. This is needed in order to syncronize values such as metrics. If this is not set, then there is a big chance that the 2 networks at each side of the router will not detect each other and therefore cannot communicate. The metric value in RIP and OSPF is definitely different because of the difference in obtaining a metric value (OSPF uses cost with max = 100,000,000, and RIP uses hops with max = 16), so ospf routes need to be redistributed/translated in order to be understood by RIP. This redistribution must be done in 2 way (to each other).

To redistribute RIP to OSPF and in reverse, the command is shown below.

To redistribute RIP to OSPF:

(config)#router ospf

(config-router)#redistribute connected subnets (Redistribute connected devices including subnetted routes)

(config-router)#redistribute subnets

To redistribute OSPF routing protocol to RIP:

(config)#router rip

(config-router)#redistribute connected metric

(config-router)#redistribute ospf metric

RIP requires seed metric (convert to understandable metric 0-15) be applied at the time of redistribution. The metric or the path being redistributed must be set from routers to RIP, IGRP and EIGRP. OSPF does not need to have this, because the default redistributed route seed metric of OSPF is 20. Without setting the seed metric, the routes that has a higher value than 15 will not be shown on the RIP network because it is considered as a disabled network.

When seeing the configuration, there is an "O E2" symbol which means an external route originally learned by redistribution. The cost of this route is from ASBR to dest network only.

There is a possibility where the destination routes has the same ip next hop address. This table can be shortened by using stub networks which must be configured on both end of the links (2 routers). The symbol will be changed to O*IA symbol. The command is:

(config)#router ospf

(config-router)#area stub

To change the default cost of the stub use the following command on 1 router only:

(config)#router ospf

(config-router)#area default-cost

To configure a total stub area (all external and internal area routes are replaced to a single default route) use the command on the ABR router only with :

(config)#router ospf

(config-router)#area stub no-summary

To show the neigbor configuration state use the "#show ip ospf neigbor" command

NSSA stands for "not so stubby area" which is a stub area that contains a limited number of external routes instead of all external routes connecting to OSPF. The purpose of this area is to inject other router protocols to OSPF with the redistribution command to the stub area via NSSA stub using type 7 LSAs, because type 5 LSA is blocked in NSSA area. The type 7 LSA will be translated to type 5 LSA by the NSSA ABR router in the NSSA area that connects to OSPF area 0.

The command to make an NSSA stub is :
(config)#router ospf
(config-router)#area nssa

There is a possibility to make NSSA area a total stub NSSA which will block LSA type 3, 4 and 5 and single default routes.

The command to make an NSSA total area stub is :
(config)#router ospf
(config-router)#area nssa no-summary

There are different LSA types and typical states on the router when

the show ip ospf database is run:

- LSA Type 1 : Sent by all Routers --> Router Link States
- LSA Type 2 : Sent by all DRs --> Net Link States
- LSA Type 3 : Sent by all ABRs --> Summary Net Link States
- LSA Type 4 : Sent by all ABRs --> Summary ASB Link States, describes how to get to the ASBR
- LSA Type 5 : Sent by ASBRs only --> Type-5 AS External Link States to all areas except stub and total stub areas
- LSA Type 7 : Sent by ASBRs only --> Type-7 AS External Link States, found only in NSSA area
- LSA Type 6 : Reserved for Multicast OSPF


There is a possibility to make a route summarization except using stub. This method is used when the summarized routes is implementen on routers that is in area 0, which does not allow stub configurations. There are 2 methods in summarizing routes which are area range and summary address.

Area range is used on ABRs where it summarizes routes from 1 area to another. The command is:
(config)#router ospf
(config-router)#area range


Summary address is used on ASBR routers to summarize routes that are redistributed from other protocols to OSPF. The command is:
(config)#router ospf
(config-router)#summary address


There is also a possibility to protect connections to the neighbors with authentication. The authentication can be set with passwords in either clear text or MD5. However, once set, the password must be configured the same on other neighboring routes involved.

The command for clear text password authentication is:
(config)#int
(config-if)#ip ospf authentication-key // password max 8 characters!
(config-if)#ip ospf authentication

The command for MD5 password authentication is:
(config)#int
(config-if)#ip ospf authentication message digest
(config-if)#ip ospf message-digest-key md5 MD5>

To see adjecencies use the command #debug ip ospf adj, to stop debugging use #u all

-- 26 January 2011 --