Day 10 - Routing Concepts
Distance Vector Routing Protocols
Distance Vector is a routing protocol that uses distance or hop count as its primary metric for determining the best forwarding path. Distance Vector routing protocols are primarily based on the Bellman-Ford algorithm. Distance Vector routing protocols periodically send their neighbour routers copies of their entire routing tables to keep them up to date on the state of the network. While this may be acceptable in a small network, it increases the amount of traffic that is sent across networks as the size of the network grows. All Distance Vector routing protocols share the following characteristics: ▪ Counting to infinity ▪ Split horizon ▪ Poison reverse ▪ Hold-down timers
Basic Routing
The role of routing protocols is to learn about other networks dynamically, exchange routing information with other devices, and connect internal and/ or external networks. It is important to note that routing protocols DO NOT send packets across the network. Their role is to determine the best path for routing. Routed protocols actually send the data, and the most common example of a routed protocol is IP.
Split horizon
Split horizon mandates that routing information cannot be sent back out of the same interface through which it was received. This prevents the re-advertising of information back to the source from which it was learned. While this characteristic is a great loop prevention mechanism, it is also a significant drawback, especially in hub-and-spoke networks.
IP Addressing and Address Summarisation
Summarisation reduces the amount of information that routers must process, which allows for faster convergence within the network. Summarisation also restricts the size of the area that is affected by network changes by hiding detailed topology information from certain areas within the network.
Routing Protocol Classes
- Distance Vector and Link State. Distance Vector routing protocols traditionally use a one-dimensional vector when determining the most optimal path( s) through the network, while Link State routing protocols use the Shortest Path First (SPF) when determining the most optimal path( s) through the network.
o Next hop
=Next hop is a routing term that refers to the next closest router a packet can go through. The next hop is among the series of routers that are connected together in a network and is the next possible destination for a data packet. More specifically, next hop is an IP address entry in a router's routing table, which specifies the next closest/most optimal router in its routing path. Every single router maintains its routing table with a next hop address, which is calculated based on the routing protocol used and its associated metric.
Prefix Matching
.Longer, or more specific, routing table entries are preferred over less specific entries, such as summary addresses, when determining which entry to use to route traffic to the intended destination network or node. i will type m & ot wills how all names start with m then i type ma then iw ill show name starts wil ma so it goes & finally i will type marco and it will show name with marco. so it will not show other m name which does not have arco after . so longest match. same happens with destination network address matching of a packet with network address of routes in routing table.
Understanding Vectors
A one-dimensional vector is a directed quantity. It is simply a quantity (number) in a particular direction or course.The starting and ending points of the vector are not relevant. Instead, the only thing that actually matters is how long the vector is and how far it travels.
Internet Protocol Routing Fundamentals
A routing protocol allows a router to learn dynamically how to reach other networks. A routing protocol also allows the router to exchange learned network information with other routers or hosts. In addition to understanding the intricacies of routing protocols, it is also important to have a solid understanding of when and in what situation one routing protocol would be used versus another.
Administrative Distance
Administrative distance is the feature that routers use in order to select the best path when there are two or more different routes to the same destination from two different routing protocols. Administrative distance defines the reliability of a routing protocol. This default value is an integer between 0 and 255, with a value of 0 assigned to the most reliable source of information and a value of 255 assigned to the least reliable source of information. Any routes that are assigned an administrative distance value of 255 are considereduntrusted and will not be placed into the routing table. The administrative distance is a locally significant value that affects only the local router. This value is not propagated throughout the routing domain. Therefore, manually adjusting the default administrative distance for a routing source or routing sources on a router affects the preference of routing information sources only on that router.
o Passive interfaces (how they work)
An important routing protocol design and configuration consideration is to limit unnecessary peerings, as shown in Figure 10.10 below. This is done using passive interfaces, which prevents the router from forming routing adjacencies on the specific interface. This functions differently based on the specific routing protocol used but the behaviour usually falls within the following two categories: ▪ The router does not send routing updates on the passive interface ▪ The router does not send Hello packets on the interface, so neighbour relationships are not formed Passive interfaces are usually able to receive routing updates or Hello packets but are not allowed
Cisco Express Forwarding (CEF)
CEF operates at the data plane and is a topology-driven proprietary switching mechanism that creates a forwarding table that is tied to the routing table (i.e., the control plane). CEF was developed to eliminate the performance penalty experienced due to the first-packet process-switched lookup method used by flow-based switching. CEF eliminates this by allowing the route cache used by the hardware-based Layer 3 routing engine to contain all the necessary information to the Layer 3 switch in the hardware before any packets associated with a flow are even received.
Building the IP Routing Table
Cisco routers use the administrative distance, the routing protocol metric, and the prefix length to determine which routes will actually be placed into the routing table, which allows the router to build the routing table. The routing table is built via the following general steps: 1. If the route entry does not currently exist in the routing table, add it to the routing table. 2. If the route entry is more specific than an existing route, add it to the routing table. It shouldrouting table. It should also be noted that the less specific entry is still retained in the routing table. 3. If the route entry is the same as an existing one, but it is received from a more preferred route source, replace the old entry with the new entry. 4. If the route entry is the same as an existing one, and it is received from the same protocol, then: i. Discard the new route if the metric is higher than the existing route; or ii. Replace the existing route if the metric of the new route is lower; or iii. Use both routes for load balancing if the metric for both routes is the same.When building the RIB by default, the routing protocol with the lowest administrative distance value will always be chosen when the router is determining which routes to place into the routing table.
0 Link State vs. Distance Vector
Distance vector routing is so named because it involves two factors: the distance, or metric, of a destination, and the vector, or direction to take to get there. Routing information is only exchanged between directly connected neighbors. This means a router knows from which neighbor a route was learned, but it does not know where that neighbor learned the route; a router can't see beyond its own neighbors. This aspect of distance vector routing is sometimes referred to as "routing by rumor." Measures like split horizon and poison reverse are employed to avoid routing loops. Link-state routing, in contrast, requires that all routers know about the paths reachable by all other routers in the network. Link-state information is flooded throughout the link-state domain (an area in OSPF or IS-IS) to ensure all routers posses a synchronized copy of the area's link-state database. From this common database, each router constructs its own relative shortest-path tree, with itself as the root, for all known routes. http://packetlife.net/blog/2008/oct/2/distance-vector-versus-link-state/
Hold-down timers
Hold-down timers are used to prevent networks that were previously advertised as down from being placed back into the routing table. When a router receives an update that a network is down, it begins its hold-down timer. This timer tells the router to wait for a specific amount of time before accepting any changes to the status of that network. During the hold-down period, the router suppresses the network and prevents advertising false information. The router also does not route to the unreachable network, even if it receives information from another router (that may not have received the triggered update) that the network is reachable. This mechanism is designed to prevent black-holing traffic.
Link State Routing Protocols
Link-State Routing protocols are routing protocols whose algorithms calculate the best paths to networks differently than Distance Vector routing protocols. Whereas Distance Vector protocols know routes by measures of distance and vector(direction) as reported by neighboring routers, Link-State routing protocols calculate their network routes by building a complete topology of the entire network area and then calculating the best path from this topology or map of all the interconnected networks. There are two link-state routing protocols, OSPF and IS-ISState routing protocols create a database that comprises the complete topology of the network. This allows all routers within the same area to have the same view of the network.
Stability
Network stability, or a lack thereof, is another major objective for routing algorithms. Routing algorithms should be stable enough to accommodate unforeseen network events, such as hardware failures and even incorrect implementations.
Optimal Routing
One of the primary goals of all routing protocols is to select the most optimal path through the network from the source subnet or host to the destination subnet or host. The most optimal route depends upon the metrics used by the routing protocols.
Rapid Convergence
Rapid convergence is another primary objective of all routing algorithms. As stated earlier, convergence occurs when all routers in the network have the same view of and agree on optimal routes. When convergence takes a long time to occur, intermittent packet loss and loss of connectivity may be experienced between remote networks. In addition to these problems, slow convergence can result in network routing loops and outright network outages.
Packet Forwarding
Packet forwarding involves two processes: ▪ Determining the best path ▪ Sending the packet (switching) When the router receives a packet for a directly connected network, the router checks the routing table and then the packet is forwarded to that network If the packet is destined for a remote network, the routing table is checked and if there is a route or default route, the packet is forwarded to the next-hop router. If the packet is destined for a network not in the routing table and no default route exists then it is dropped. Firstly, the router decapsulates the Layer 3 packet by removing the Layer 2 frame header and trailer. Next, it examines the destination IP address of the IP packet to find the best path in the routing table. Finally, it encapsulates the Layer 3 packet into a new Layer 2 frame and forwards the frame out of the exit interface, so the encapsulation could change from Ethernet to HDLC. Remember in an earlier module that the source and destination IP address will never change as the packet traverses towards its final destination. The MAC address, however, will change to permit transport between intermediary devices.
Poison reverse (or route poisoning)
Poison reverse (or route poisoning) expands on split horizon. When used in conjunction with split horizon, poison reverse allows the networks to be advertised back out of the same interface on which they were received. However, poison reverse causes the router to advertise these networks back to the sending router with a metric of "unreachable" so that the router that receives those entries will not add them back into its routing table.
The two most common Distance Vector routing protocols are
RIP= The Routing Information Protocol (RIP) is one of the oldest distance-vector routing protocols, which employs the hop count as a routing metric. RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from the source to a destination. The maximum number of hops allowed for RIP is 15. IGRP=Interior Gateway Routing Protocol (IGRP) is a distance vector interior routing protocol (IGP) developed by Cisco. It is used by routers to exchange routing data within an autonomous system. IGRP is a proprietary protocol. EIGRP is an advanced Distance Vector routing protocol, using features from both Distance Vector and Link State
Flat and Hierarchical Routing Algorithms
Routing protocol algorithms operate using either a flat routing system or a hierarchical routing system. A hierarchical routing system uses a layered approach wherein routers are placed in logical groupings referred to as domains, areas, or autonomous systems. This allows different routers within the network to perform specific tasks, optimising the functionality performed at those layers. Some routers in the hierarchical system can communicate with other routers in other domains or areas, while other routers can communicate only with routers in the same domain or area. This reduces the amount of information that routers in the domain or area must process, which allows for faster convergence within the network. The primary advantage afforded by hierarchical routing systems is their scalability. A flat routing system has no hierarchy. In such systems, routers must typically be connected to every other router in the network and each router essentially has the same function. Such algorithms work well in very small networks; however, they are not scalable. In addition, as the network grows, troubleshooting becomes much more difficult because instead of just focusing your efforts on certain areas, for example, you now have to look at the entire network.
Classful and Classless Protocols
Routing protocols can be classified into different groups according to their characteristics. Specifically, routing protocols can be classified by their: Purpose: Interior Gateway Protocol (IGP) or Exterior Gateway Protocol (EGP) Operation: Distance vector protocol, link-state protocol, or path-vector protocol Behavior: Classful (legacy) or classless protocol For example, IPv4 routing protocols are classified as follows: RIPv1 (legacy): IGP, distance vector, classful protocol IGRP (legacy): IGP, distance vector, classful protocol developed by Cisco (deprecated from 12.2 IOS and later) RIPv2: IGP, distance vector, classless protocol EIGRP: IGP, distance vector, classless protocol developed by Cisco OSPF: IGP, link-state, classless protocol IS-IS: IGP, link-state, classless protocol BGP: EGP, path-vector, classless protocol The classful routing protocols, RIPv1 and IGRP, are legacy protocols and are only used in older networks. These routing protocols have evolved into the classless routing protocols, RIPv2 and EIGRP, respectively. Link-state routing protocols are classless by nature.
The Shortest Path First Algorithm
The SPF algorithm creates a shortest-path tree to all hosts in an area or in the network backbone with the router that is performing the calculation at the root of that tree. In order for the SPF algorithm to work in the correct manner, all routers in the area should have the same database information. In OSPF, this is performed via the database exchange process.
Passive Interfaces
You can use the passive-interface command in order to control the advertisement of routing information. The command enables the suppression of routing updates over some interfaces while it allows updates to be exchanged normally over other interfaces Basically, you want to eliminate unnecessary routing peering adjacencies, so you would configure the ports towards the Layer 2 switches as passive interfaces in order to suppress routing updates advertisements.
Counting to infinity characteristic
characteristic, if a destination network is farther than the maximum number of hops allowed for that routing protocol, the network would be considered unreachable. The network entry would therefore not be installed into the IP routing table.
Routing Protocal Metrics The different routing protocol metrics may be based on one or more of the following:
▪ Bandwidth=The term bandwidth refers to the amount of data that can be carried from one point to another in a given period. Routing algorithms may use bandwidth to determine which link type is preferred over another. ▪ Cost =The cost, as it pertains to routing algorithms, refers to communication cost. The cost may be used when, for example, a company prefers to route across private links rather than public links that include monetary charges ▪ Delay = traffic. In general, delay refers to the length of time required to move a packet from its source to its destination through the internetwork. ▪ Load= refers to the degree of use for a particular router interface. The load on the interface is a fraction of 255. For example, a load of 255/ 255 indicates that the interface is completely saturated, while a load of 128/ 255 indicates that the interface is 50% saturated. ▪ Path length =The path length metric is the total length of the path that is traversed from the local router to the destination network. ▪ Reliability=reliability refers to the dependability of network links or interfaces.
Routing Problems Avoidance Mechanisms
▪ Invalidation timers: These are used to mark routes as unreachable when updates for those routes are not received for a long time. ▪ Hop count limit: This parameter marks routes as unreachable whenthey are more than a predefined number of hops away. The hop count limit for RIP is 15, as it is not usually used in large networks. Unreachable routes are not installed in the routing table as best routes. The hop count limit prevents updates from looping in the network, just like the TTL field in the IP header. ▪ Triggered updates: This feature allows the update timer to be bypassed in the case of important updates. For example, the RIP 30-second timer can be ignored if a critical routing update must be propagated through the network. ▪ Hold-down timers: If a metric for a particular route keeps getting worse, updates for that route are not accepted for a delayed period. ▪ Asynchronous updates: Asynchronous updates represent another safety mechanism that prevents the routers from flooding the entire routing information at the same time. As mentioned before, OSPF does this every 30 minutes. The asynchronous updates mechanism generates a small delay for every device so they do not flood the information exactly at the same time. This improves bandwidth utilisation and processing capabilities. ▪ Route poisoning: This feature prevents routers from sending packets through a route that has become invalid. Distance Vector protocols use this to indicate that a route is no longer reachable. This is accomplished by setting the route metric to a maximum value. ▪ Split horizon: Split horizon prevents updates from being sent out of the same interface they came from because routers in that area should already know about that specific update. ▪ Poison reverse: This mechanism is an exception to the split horizon rule for the poisoned routes.
