Week Ten. Attendance Announcements Data Center Design update Final exam 150 points Review Week Nine Information Current Week Information Upcoming Assignments Mimic Simulator Lab Assignment 4-1-2, Basic Routing and LAN Switching Configuration. Week Ten Topics. Routing Protocols
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Data Center Design update
Final exam 150 points
Three principles regarding routing tables:
Using a dynamic routing protocol in such a case does not present any substantial benefit.
There is no need to use a dynamic routing protocol across this link because the ISP represents the only exit point to the Internet
Routers in complex networks must adapt to topology changes quickly and select the best route from multiple candidates
A hub-and-spoke topology consists of a central location (the hub or switch) and multiple branch locations (spokes), with each spoke having only one connection to the hub or switch.
Using dynamic routing would be unnecessary because each branch has only one path to a given destination-through the central location.
Dynamic routing of TCP/IP can be implemented using one or more protocols which are often grouped according to where they are used.
An interior gateway protocol (IGP) is a routing protocol that is used within an autonomous system (AS). Two types of IGP.
Distance-vector routing protocols each router does not possess information about the full network topology. It advertises its distances to other routers and receives similar advertisements from other routers. Using these routing advertisements each router populates its routing table. In the next advertisement cycle, a router advertises updated information from its routing table. This process continues until the routing tables of each router converge to stable values.
Distance-vector routing protocols make routing decisions based on hop-by-hop. A distance vector router’s understanding of the network is based on its neighbors definition of the topology, which could be referred to as routing by RUMOR.
Route flapping is caused by pathological conditions (hardware errors, software errors, configuration errors, intermittent errors in communications links, unreliable connections, etc.) within the network which cause certain reach ability information to be repeatedly advertised and withdrawn.
In networks, with distance vector routing protocols flapping routes can trigger routing updates with every state change.
Cisco trigger updates are sent when these state changes occur. Traditionally, distance vector protocols do not send triggered updates.
Link-state routing protocols, each node possesses information about the complete network topology. Each node then independently calculates the best next hop from it for every possible destination in the network using local information of the topology. The collection of best next hops forms the routing table for the node.
This contrasts with distance-vector routing protocols, which work by having each node share its routing table with its neighbors. In a link-state protocol, the only information passed between the nodes is information used to construct the connectivity maps.
An exterior routing protocol is designed for use between different networks that are under the control of different organizations
Factors affecting the convergence time include the following:
Metric is a numeric value used by routing protocols to help determine the best path to a destination.
RIP uses the metric hop count number . The lower the numeric value, the closer the destination.
OSPF uses the metric bandwidth.
EIGRP uses bandwidth
EIGRP is an advanced distance vector protocol that employs the best features of link-state routing.
OSPF is the standardized protocol for routing IPv4. Since it’s initial development, OSPF has been revised to be implemented with the latest router protocols.
When the OSPF topology table is fully populated, the SPF algorithm calculates the shortest path to the destination. Triggered updates and metric calculation based on the cost of a specific link ensure quick selection of the shortest path to the destination.
OSPF is link-state routing protocol
RIP and EIGRP are distance-vector (routing by rumor) routing protocols, susceptible to routing loops, split-horizon, and other issues.
OSPF has fast convergence
RIP hold-down timers can cause slow convergence.
OSPF supports VLSM and CIDR
RIPv1 does not
RIP, simple but very limited, or
OSPF, robust but more sophisticated to implement.
EIGRP is Cisco proprietary
Autonomous System Boundary Router (ASBR) is an area border router placed between an OSPF autonomous system and a non-OSPH network that operates both OSPF and an additional routing protocol, such as RIP. ASBRs must be located in a non-stub OSPF area.
When all routers are configured into a single area, the convention is to use area 0(zero)
If OSPF has more than one area, it must have an area 0
Multi-area OSPF becomes more complicated to configure and understand
OSPF Routing Domain
1. Flooding of link-state information
The first thing that happens is that each node, router, on the network announces its own piece of link-state information to other all other routers on the network. This includes who their neighboring routers are and the cost of the link between them.
Example: “Hi, I’m Router A, and I can reach Router B via a T1 link and I can reach Router C via an Ethernet link.”
Each router sends these announcements to all of the routers in the network.
2. Building a Topological Database
Each router collects all of this link-state information from other routers and puts it into a topological database.
3. Shortest-Path First (SPF), Dijkstra’s Algorithm
Using this information, the routers can recreate a topology graph of the network.
Believe it or not, this is actually a very simple algorithm and I highly suggest you look at it some time, or even better, take a class on algorithms.
4. Shortest Path First Tree
This algorithm creates an SPF tree, with the router making itself the root of the tree and the other routers and links to those routers, the various branches.
5. Routing Table
Using this information, the router creates a routing table.
Large link-state table
Each router maintains a LSDB for all links in the area
The LSDB requires the use of memory
Frequent SPF calculations
A topology change in an area causes each router to re-run SPF to rebuild the SPF tree and the routing table.
A flapping link will affect an entire area.
SPF re-calculations are done only for changes within that area.
Large routing table
Typically, the larger the area the larger the routing table.
A larger routing table requires more memory and takes more time to perform the route look-ups.
Solution: Divide the network into multiple areas
Link-State Advertisement (LSA) is contained inside the link-state packets (LSPs), these advertisements are usually multicast packets, containing information about neighbors and path costs, that are employed be link-state protocols. Receiving routers use LSAs to maintain their link-state databases and, ultimately, routing tables.
Hierarchical routing enables you to separate large internetworks (autonomous systems) into smaller internetworks that are called areas.
With this technique, routing still occurs between the areas (called inter-area routing), but many of the smaller internal routing operations, such as recalculating the database –re-running the SPF algorithm, are restricted within an area
Changes in one area are generally not propagated (spread) to another
Route summarization is extensively used in multi-area OSPF
Internal: Routers with all their interfaces within the same area
Backbone: Routers with at least one interface connected to area 0
ASBR:(Autonomous System Boundary Router): Routers that have at least one interface connected to an external internetwork (another autonomous system)
ABR: (Area Border Router): Routers with interfaces attached to multiple areas.
The Link State Database is similar to a map in a shopping mall. All maps in the mall is the same, just as the LSDB is the same in all routers within an area. The only real physical difference in the maps is “you are at” what dot. By looking at your position in relation to where you want to go, you can determine the best way to get to your destination. Link state routers calculate the best path to every network within the area, from their own perspective.
An OSI network is a hierarchy of these entities:
There are four levels of routing:
By default, EIGRP uses only these:
If these are the default:
When are these used?
These values are used when the administrator manually enters them
The following three tables are maintained by EIGRP:
BGP is a path vector routing protocol.
Defined in RFC 1772
BGP is a distance vector routing protocol, in that it relies on downstream neighbors to pass along routes from their routing table.
BGP uses a list of AS numbers through which a packet must pass to reach a destination.
In contrast, RIP updates use UDP port 520
OSPF, IGRP, EIGRP does not use a Layer 4 protocol
To guarantee loop free path selection, BGP constructs a graph of autonomous systems based on the information exchanged between BGP neighbors.
BGP views the whole internetwork as a graph, or tree, of autonomous systems.
The connection between any two systems forms a path.
The collection of path information is expressed as a sequence of AS numbers called the AS Path.
This sequence forms a route to reach a specific destination
When two routers establish a TCP-enabled BGP connection between each other, they are called neighbors or peers.
Each router running BGP is called a BGP speaker.