Monday, July 16, 2012

ccnp route miscellaneous

Note: In EIGRP, default routes cannot be directly injected (as they can in OSPF with the default-information originate command. Also, EIGRP does not have the “default-information originate” command).

“To qualify as a feasible successor, a router must have an AD less than the FD of the current successor route”

EIGRP considers the load of an interface only when sending an update for some other reason (like a link failure)

When an EIGRP topology change is detected, what is the correct order of events when there is an FS?
DUAL is notified.
The neighbor adjacency is deleted.
Remove all topology entries learned from that neighbor.
The feasible route is used.

Notice that the “maximum-paths” command is used to share traffic to equal cost path while the “variance” command can share traffic to unequal cost path.

An EIGRP route will go active if there are no successors or feasible successors in the EIGRP topology table.
The stuck in the active state is caused when the wait for the query replies have timed out.

Metric values are represented in a 32-bit format for granularity.

The command “eigrp stub” is same as “eigrp stub connected summary” command because connected and summarized routes are advertised by default

Popular LSA Types are listed below:

LSA Type   Description        Details
1   Router LSA        Generated by all routers in an area to describe their directly attached links
2   Network LSA        Advertised by the DR of the broadcast network (does not cross ABR)
3   Summary LSA        Advertised by the ABR of originating area.
4   Summary LSA        Generated by the ABR of the originating area to advertise an IP address of ASBR to all other areas in the autonomous system
5   AS external LSA Used by the ASBR to advertise networks from other autonomous systems
7   Defined for NSSAs Generated by an ASBR inside a Not-so-stubby area (NSSA) to describe routes redistributed into the NSSA

Area Type           Type 1 & 2        Type 3                       Type 4 Type 5 Type 7
                                  within area         from other areas
Standard & backbone Yes                Yes                        Yes           Yes No
Stub                                 Yes                Yes                        No            No No
Totally stubby                 Yes                No                        No            No No
NSSA                         Yes                Yes                        No            No Yes
Totally stubby NSSA Yes                No                        No            No Yes

By default, Frame Relay is classified as a non-broadcast network, meaning it doesn’t send any broadcasts/multicasts like RIP, OSPF or EIGRP updates across the network (hello packets of OSPF are multicast to Hence, in NBMA network, the neighbors are not discovered automatically; they must be configured manually. There are two ways to simulate a broadcast model on an NBMA network:

+ Define the network type as broadcast with the “ip ospf network broadcast” interface sub-command
+ Configure the neighbor statements under router ospf mode (though configuring the neighbor statement on one end is sufficient to form adjacency, it is a good practice to have it configured on both the ends)

Besides these two ways, another way of making OSPF work with Frame Relay is configuring the network as a Point-to-Point network (or Point-to-Multipoint,

notice that OSPF treats Point-to-Multipoint network as a collective of point-to-point links).

Note: Point-to-Multipoint networks do not maintain a DR/BDR relationship.

The command “ip ospf network point-to-multipoint” configures this interface as point-to-multipoint for broadcast media. In broadcast networks, there is no need to specify neighbors. No DR or BDR is elected in this network type.

Note: If we configure this interface as a point-to-point non-broadcast (with the “ip ospf network point-to-multipoint non-broadcast” command) then we need to manually declare the OSPF neighbors.

To define a totally stub area, use the area area-id stub no-summary command on the ABR (in OSPF router configuration) and the area area-id stub on the totally stub router. The ABR will inject a default route into the area so routers in this type of area only see routing information local to their area, plus a default route pointing to the ABR, from which they can reach all other areas and all other networks

Passive-interface command is used to disable sending updates out from a specific interface. For OSPF it prevents Hello packets from being sent out or received through the interface. This will not allow to create a neighbor adjacency and prevent a router from learning prefixes from those neighbors.

For RIP, configuring an interface as passive will still allow the interface to listen to incoming routing updates for other RIP neighbors, but the interface will no longer send them.

Note: The passive-interface command is used in router configuration mode, not interface mode.

When using the command “ip ospf network nonbroadcast”, we turn the network into a nonbroadcast network so routers can not send broadcast or multicast. But

OSPF uses multicast address to send Hello packet periodically. Therefore we have to manually define the neighbor (via the neighbor command) to make OSPF send Hello packets to its adjacent routers 

For Multi-access network (for example Ethernet or Frame Relay), a designated router (DR) and a backup designated router (BDR) are elected

Note: By default, Frame Relay environment is nonbroadcast so it is actually not necessary to use the command “ip ospf network nonbroadcast” on Frame Relay network.

To make an area “totally stubby” we must apply the “area area-id stub no-summary” command on the ABR and area area-id stub” commands to all other routers in that area.

Note: The ABR in a totally stubby area does not create Type 3 summary LSA. It only creates a default route to outside destinations.

The “show ip ospf database external” command displays information only about external LSAs (Type 5 LSAs)
The “show ip ospf database summary” command displays information only about the summary LSAs.
The “show ip ospf database nssa-external” command displays information only about the not so stubby area (NSSA) external LSAs.

The default-information originate command advertises a default route into a normal area, provided the advertising router already has a default route. If we want the router to advertise the default route even if it does not have a default route then we can append the keyword always after the above command.

For your information, the main difference between LSA Type 7 and Type 5 is Type 7 is generated inside a NSSA while Type 5 is generated inside a standard area.

The show ip ospf interface command shows us information about the neighbor count and adjacent neighbor count:

The show ip ospf neighbor command shows us the role of each neighbor (DR, BDR, DROTHER).

The show ip ospf command displays the number of times that the OSPF Shortest Path First (SPF) algorithm has been executed?

A stub area only allows LSA Type 1, 2 and 3.

Notice that an NSSA allows LSA Type 3 & 7 while a Totally NSSA only allows LSA Type 7

Both Totally Stubby Area & Totally Stubby NSSA do not accept external AS routes or inter-area routes (LSA Types 3, 4 and 5). They recognize only intra-area routes and the default route The main difference between them is Totally Stubby NSSA accepts routes from other AS while Totally Stubby Area does not.

Note: E2 is the default external metric, but E1 is preferred over E2 if two equal-cost paths exist.

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