EIGRP & OSPF Questions

Quick OSPF Overview

OSPF router ID selection:

OSPF uses the following criteria to select the router ID:
1. Manual configuration of the router ID (via the “router-id x.x.x.x” command under OSPF router configuration mode).
2. Highest IP address on a loopback interface.
3. Highest IP address on a non-loopback and active (no shutdown) interface.

OSPF forms neighbor relationship with other OSPF routers on the same segment by exchanging hello packets. The hello packets contain various parameters. Some of them should match between neighboring routers. These include:

+ Hello and Dead intervals
+ Area ID
+ Authentication type and password
+ Stub Area flag
+ Subnet ID and Subnet mask

When OSPF neighbor relationship is formed, a router goes through several state changes before it becomes fully adjacent with its neighbor. The states are Down -> Attempt (optional) -> Init -> 2-Way -> Exstart -> Exchange -> Loading -> Full. Short descriptions about these states are listed below:

Down: no information (hellos) has been received from this neighbor

Attempt: only valid for manually configured neighbors in an NBMA environment. In Attempt state, the router sends unicast hello packets every poll interval to the neighbor, from which hellos have not been received within the dead interval

Init: specifies that the router has received a hello packet from its neighbor, but the receiving router’s ID was not included in the hello packet

2-Way: indicates bi-directional communication has been established between two routers

Exstart: Once the DR and BDR are elected, the actual process of exchanging link state information can start between the routers and their DR and BDR

Exchange: OSPF routers exchange and compare database descriptor (DBD) packets

Loading: In this state, the actual exchange of link state information occurs. Outdated or missing entries are also requested to be resent

Full: routers are fully adjacent with each other

When OSPF is run on a network, two important events happen before routing information is exchanged:
+ Neighbors are discovered using multicast hello packets.
+ DR and BDR are elected for every multi-access network to optimize the adjacency building process. All the routers in that segment should be able to communicate directly with the DR and BDR for proper adjacency (in the case of a point-to-point network, DR and BDR are not necessary since there are only two routers in the segment, and hence the election does not take place).
For a successful neighbor discovery on a segment, the network must allow broadcasts or multicast packets to be sent.

In an NBMA network topology, which is inherently nonbroadcast, neighbors are not discovered automatically. OSPF tries to elect a DR and a BDR due to the multi-access nature of the network, but the election fails since neighbors are not discovered. Neighbors must be configured manually to overcome these problems

Each OSPF area only allows some specific LSAs to pass through. Below is a summarization of which LSAs are allowed in each OSPF area:

Or this table will help you grasp it:

OSPF Summarization
OSPF offers two methods of route summarization:
1) Summarization of internal routes performed on the ABRs
2) Summarization of external routes performed on the ASBRs

1) To summarize routes at the area boundary (ABRs), use the command:
area area-id range ip-address mask [advertise | not-advertise] [cost cost]

An internal summary route is generated if at least one subnet within the area falls in the summary address range and the summarized route metric is equal to the lowest cost of all the subnets within the summary address range. Interarea summarization can only be done for the intra-area routes of connected areas, and the ABR creates a route to Null0 to avoid loops in the absence of more specific routes.

2) To summarize external routes on the domain boundary (ASBRs), use the command:
summary-address {{ip-address mask} | {prefix mask}} [not-advertise] [tag tag]
The ASBR will summarize external routes before injecting them into the OSPF domain as type 5 external LSAs.

Note: An exception of using the “summary-address” is at the boundary of a NSSA area.

In both methods of route summarization described above, a summarized route is only generated if at least one subnet in the routing table falls in the summary address range.

OSPF point-to-point network type

Setting OSPF to point-to-point mode results in advertised routes containing the actual subnet mask instead of the default behavior of advertising /32 for a loopback interface.

Summarization in EIGRP and OSPF

Unlike OSPF where we can summarize only on ABR or ASBR, in EIGRP we can summarize anywhere.

Manual summarization can be applied anywhere in EIGRP domain, on every router, on every interface via the ip summary-address eigrp as-number address mask [administrative-distance ] command (for example: ip summary-address eigrp 1 192.168.16.0 255.255.248.0). Summary route will exist in routing table as long as at least one more specific route exists. If the last specific route disappears, summary route will also fade out. The metric used by EIGRP manual summary route is the minimum metric of the specific routes. The example below shows how to configure EIGRP manual summarization:

R1(config)#interface fa1/0
R1(config-if)#ip summary-address 10 172.16.0.0 255.255.254.0

If you are not sure about OSPF LSA Types, please read our OSPF LSA Types Tutorial.

OSPF area filtering

The command “area area-number filter-list prefix … in“: Prevent prefixes from entering this area (in keyword here means “into”)
The command “area area-number filter-list prefix … out“: Prevent other areas that the ABR is connected to receive the prefix.