Hour 428: EIGRP Leak-map’s

I want talk a little bit about a less known feature of EIGRP called leak-maps. Similarly to the BGP unsupress-map feature, the EIGRP leak-map is a method in which we allow routes that have been summarized to be “leaked” to the routing table. This technique is also commonly referred to as route-leaking. Today, I will try to explain a couple of cases where you would want to use this feature.

Case A: Route leaking to control routing

Route-leaking can help you control packet flow by using the shortest prefix match rule. Let’s first examine the following diagram.

CaseA Continue reading

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Hour 40: OSPF an advanced link-state protocol

EIGRP is often called an advanced distance vector protocol because it has distance vector routing protocol AND link-state protocol properties. But wait a second… OSPF also has a mix of link-state and distance-vector protocol properties… So why is OSPF not called an “advanced link-state routing protocol”?

Let’s determine the properties of EIGRP that makes it an advanced distance protocol and then compare it to OSPF.

EIGRP distance-vector properties:

  • An EIGRP router only advertises its best route to its neighbor, not every route that it is aware of.
  • An EIGRP router does not have a complete map of the topology , it is only aware of what its neighbors have told it ( routing by rumor )

Now to prove that OSPF also has distance-vector properties, let’s look at OSPF’s Network Summary LSA (Type 3) characteristics:

In OSPF, when an ABR originates a Type 3 LSA and knows multiple paths to a destination, it will only advertise the lowest cost route into the backbone. When a router receives a Type 3 LSA from an ABR, it does not run the SPF algorithm. Rather, it simply adds the cost of the route to the ABR and the cost included in the LSA. Depending on another router instead of determining the full route to the destination is a distance-vector protocol behavior.

Now let’s compare these characteristics to the EIGRP distance-vector properties:

  • Type 3 LSA’s in an OSPF router only advertise its best route to the backbone, not every route that it is aware of.
  • Type 3 LSA’s in an OSPF router do not have a complete map of the topology, since it does not run the SPF algorithm. It is only aware of what the ABR originating the type 3 LSA told it.

These distance-vector properties are similar and we can conclude that OSPF also has distance-vector properties. By this logic, if we can call EIGRP an advanced distance vector protocol because it has link-state properties, can we then not also call OSPF an “advanced link-state protocol” since OSPF has distance vector properties?

Hour 31: IGP Review EIGRP

EIGRP

Consult the symbols legend at the end of the post for information on symbols.

Metric:

  • AD of 90 internal, AD of 170 external, AD of 5 summary
  • [(RTR)[metric weights <tos #> <k1> <k2> <k3> <k5>] TOS=0 (always) sets what K values are used to calculate the metric
  • Default Metric weights: K1 (BW) = 1; K2 (Load) = 0; K3 (DLY) = 1; K4 (Reliab) = 0, K5 (MTU) = 0
  • Default Metric = Default Metric = 256*(10000000/BW + Delay/10)
  • Delay 1 = 10 microseconds. Delay is cumulative
  • Router uses own interface bandwidth if it’s lower than advertised by peer (Lowest path BW is used) Bandwidth
  • Internal paths are preferred over external paths regardless of metric (because of AD)
  • [(RTR) offset-list <acl> {in | out} <offset> <IF>] Offset-list can be used to manipulate inbound and outbound metric, delay is added
  • [(RTR)metric maximum-hops <1-255>] Changes TTL,  Default Hop Count (TTL) is 100
  • [(ROUTE-MAP)match metric <400> +- <100>] Matches metric from 300 to 500, using a Route-map statement

Neighbors:

  • Hello (keepalive) not acknowledged
  • Must be in the same AS and K-Values must match to form an adjacency
  • Source of Hello is primary subnet on interface
  • Sends Hello multicast to 224.0.0.10, protocol is 88
  • [passive interface <IF>] Stops outbound and inbound hellos on specified interface
  • [(RTR)neighbor <ip> <IF>] Send hellos as unicast, suppress sending any hellos via 224.0.0.10 on specified interface. Static configuration is required on all other peers on the same interface too.
  • Will not form an adjacency with a secondary ip address Continue reading

Hour 15: Distance Vector vs Link State analogy

One of the questions people new to networking often ask me is the difference between Distance Vector routing and Link State routing. There are many differences between them and you can research the details if you’d like to but I like to use two analogies to explain them. Here they are:

Distance Vector Routing: You are wandering in the Banff mountains of northern Alberta in Canada, a wonderful place to wander if you aren’t lost. But you are lost. You come upon a fork in the trail and you see a sign pointing west, reading “Edmonton 140km”. You have no choice but to trust the sign. You have no clue how the terrain is for the next 140km and you don’t know whether there is a better route or even if the sign is correct. In similar way, distance vector protocols provide road signs to networks. They provide the direction and the distance, but no details about what lies along the route.

Link-state Routing: You are in the biggest mall in Canada; the West Edmonton mall. You need to get to a computer store. You find the map of the mall and there’s a “you are here” dot on it, indicating where you are. From there you plan your way to the nearest computer store.

Just like every map in the mall is the same, the link-state database is the same in all routers within an area of a link-state routing protocol. The one difference between all the maps in the shopping mall is the “you are here” dot and so, the best path to a specific store will be different from each location in the mall. Link-state routing functions the same way, it calculates the best way to every network within the area, from their own perspective, using its own map; the Link-State Database.

I hope you liked these analogies as they’ve been really helpful to me.

On a side note: today I received my Cisco 2511 Access Server. I need to make a couple of roll-over cables to connect them to my home lab. I’m still waiting on 2x NM-1FE-2W WIC modules to be delivered. I need these for my 3600’s series Frame Relay switching router because the NM-1E-2W version does not support WIC-2T interface cards. So be careful if you plan on purchasing a 3600’s series router as only the FE modules support the WIC-2T interface cards.