Routing and Routing Protocols CCNA 2 – Chapter 6

Routing Types

Static Route Operation Hoboken#show ip route Codes: C - connected, S - static, S 172.16.1.0/24 [1/0] is directly connected, Serial0 C 192.168.2.0/24 is directly connected, Ethernet0

ip route command RTR(config)# ip route prefix mask {address | interface} [distance] prefix IP route prefix for the destination. mask Prefix mask for the destination. address IP address of the “next hop” that can be used to reach that network. interface Network interface to use (exit-interface) distance (Optional) An administrative distance.

Static Route Operation If the router cannot reach the outgoing interface that is being used in the route, the route will not be installed in the routing table. This means if that interface is down, the route will not be placed in the routing table. If the exit interface (gateway) is “down” the static route will not be put in the routing table.

Administrative Distance and Metric Hoboken#show ip route Codes: C - connected, S - static, S 172.16.1.0/24 [1/0] is directly connected, Serial0 C 192.168.2.0/24 is directly connected, Ethernet0 [ administrative distance / routing metric (or cost) ] The cost for all static routes is “0” The default administrative distance for static routes is “1”

Administrative Distance Administrative Distance is the “trustworthiness” of the routing information. Lower the administrative distance the more trustworthy the information. If the router hears about a route to the same network from more than one source it will use the administrative distance to decide which route to put in the routing table.

Examples

Examples The network 0.0.0.0 and mask 0.0.0.0 are known as a “default route” Can be written 0.0.0.0/0 Known as a “quad zero” route”

Static Routing Configuring static routes Routers do not need to configure static routes for their own directly connected networks. We need to configure static routes for networks this router needs to reach. We will need to configure static routes for the other routers as well, as “routing information about a path from one network to another does not provide routing information about the reverse, or return path.” Convergence – When all the routers in the network (AS) have accurate and consistent information, so that proper routing and packet forwarding can take place. Convergence will not happen until all the routers have complete and accurate routing information, meaning we must configure static routes on all the routers before packets will be correctly delivered.

Static Routing 2 1 Recursive Lookup RTA(config)#ip route 192.168.1.0 255.255.255.0 172.16.0.2 RTA#show ip route Codes: C - connected, S - static, C 172.16.0.0/16 is directly connected, Serial0 S 192.168.1.0/24 [1/0] via 172.16.0.2 C 192.168.2.0/24 is directly connected, Ethernet0 2 1 It does a recursive lookup – first (1) by looking up the 192.168.1.0/24 network and finding it needs to forward the packet to 172.16.0.2 – the router then (2) looks up the 172.16.0.0 network and sees it can forward it out the interface Serial 0. Recursive Lookup The router knows it can get to 192.168.1.0/24 network by forwarding the packets to the router at the ip address of 172.16.0.2 How does the router know how to get to the ip address 172.16.0.2?

Common uses for Static Routes 10.1.1.1/24 ISP ip route 172.16.0.0 255.255.0.0 10.1.1.2 10.1.1.2/24 ABC 172.16.0.0/16 Static routes in the real-world In the example above, there is only one route, link, between ABC’s network and the ISP. When there is only a single route to a network, this is known as a stub network. It is very common for the ISP to have a static route pointing to it’s customers’ networks, in this case ABC.

Common uses for Static Routes Default 10.1.1.1/24 ISP ip route 172.16.0.0 255.255.0.0 10.1.1.2 10.1.1.2/24 ABC ip route 0.0.0.0 0.0.0.0 10.1.1.1 172.16.0.0/16 Static routes in the real-world (continued) What about ABC and sending packets to the ISP – packets going to the Internet? It is also common for customer networks to use a special kind of static route, known as a default static route. Of course we will examine this later throughout the rest of this course, but for now we specify the network and mask as “0.0.0.0 0.0.0.0” (pronounced “quad-zero”). This tells the router to forward all packets to this next-hop address (or exit interface) that do not have an explicit route in the routing table.

Common uses for Static Routes Default 10.1.1.1/24 ISP ip route 172.16.0.0 255.255.0.0 10.1.1.2 10.1.1.2/24 ABC ip route 0.0.0.0 0.0.0.0 10.1.1.1 172.16.0.0/16 RTB#show ip route Gateway of last resort is 10.1.1.1 to network 0.0.0.0 C 172.16.0.0/16 is directly connected, Ethernet0 10.0.0.0/24 is subnetted, 1 subnets C 10.1.1.0 is directly connected, Serial1 S* 0.0.0.0/0 [1/0] via 10.1.1.1 Any packets not matching the routes 172.16.0.0/16 or 10.1.1.0/24 are sent to the router 10.1.1.1 – where it is now their “problem.”

Summarizing static routes There are many times when a single static route can replace several static routes. In other words, summarizing several static routes into a single static route. 172.16.0.0/24

Summarizing static routes Baypointe The three static routes can be summarized into a single route: Baypointe(config)# ip route 172.16.1.0 255.255.255.0 192.168.1.2 Baypointe(config)# ip route 172.16.2.0 255.255.255.0 192.168.1.2    Baypointe(config)# ip route 172.16.3.0 255.255.255.0 192.168.1.2 Summarized route: Baypointe(config)# ip route 172.16.0.0 255.255.0.0 192.168.1.2 The summarized route will now include all three subnets! Be sure to use the proper mask – 255.255.0.0! Using a 255.255.255.0 mask will only route for 172.16.0.0/24 subnet and not 172.16.1.0/24, 172.16.2.0/24 or 172.16.3.0/24.

Summarizing static routes 172.16.0.0/24 Baypointe Summarized route:   Baypointe(config)# ip route 172.16.0.0 255.255.0.0 192.168.1.2 Advantages: Fewer routes in the routing table – faster routing table lookup. Subnets can be added and deleted on 172.16.0.0 network without having to change static route on Baypointe router.

Verify static routes Copy running-config startup-config

Dynamic Routing Protocols

Routed Protocols vs. Routing Protocols

Autonomous Systems An autonomous system (AS) is a collection of networks under a common administration sharing a common routing strategy. To the outside world, an AS is viewed as a single entity. The AS may be run by one or more operators while presenting a consistent view of routing to the external world. The American Registry of Internet Numbers (ARIN), a service provider, or an administrator assigns an identifying 16-bit number to each AS.

Routing Protocols The goal of a routing protocol is to build and maintain the routing table. This table contains the learned networks and associated ports for those networks. Routers use routing protocols to manage information received from other routers, information learned from the configuration of its own interfaces, along with manually configured routes.

Types of Routing Protocols Distance Vector: RIP, IGRP, EIGRP Link State: OSPF, IS-IS Path Vector: BGP Note: IGRP and EIGRP are Cisco Proprietary

Distance Vector Routing Protocols Router B receives information from Router A. Router B adds a distance vector number (such as a number of hops), which increases the distance vector. Then Router B passes this new routing table to its other neighbor, Router C. This same step-by-step process occurs in all directions between neighbor routers. “Routing by rumor” Each router receives a routing table from its directly connected neighbor routers.

Distance Vector Routing Protocols

Distance Vector Routing Protocols Routing table updates occur when the topology changes. As with the network discovery process, topology change updates proceed step-by-step from router to router. With some routing protocols routing tables updates happen on a periodic basis.

Link State Routing Protocol Operations Link-state advertisements (LSAs) – A link-state advertisement (LSA) is a small packet of routing information that is sent between routers. Topological database – A topological database is a collection of information gathered from LSAs. SPF algorithm – The shortest path first (SPF) algorithm is a calculation performed on the database resulting in the SPF tree. Routing tables – A list of the known paths and interfaces.

Link State Routing Protocol Operations

Path Determination A router determines the path of a packet from one data link to another, using two basic functions: A path determination function A switching function

Path Determination The switching function is the internal process used by a router to accept a packet on one interface and forward it to a second interface on the same router. A key responsibility of the switching function of the router is to encapsulate packets in the appropriate frame type for the next data link.

Configuring Dynamic Routing

Configuring Dynamic Routing GAD(config)#router rip GAD(config-router)#network 172.16.0.0

Network command two things: Router(config)#router rip Router(config-router)#network 172.16.0.0 Router(config-router)#network 160.89.0.0 The network command is used on only directly connected networks. With RIP and IGRP, only need to use the classful address (no subnets). Network command two things: Tells the router which interfaces that will participate in this dynamic routing protocol, which interfaces it will send and receive routing updates on. Tells other routers the networks in its routing updates that it is directly connect to.

Distance Vector vs. Link State Requests routing information from directly connected neighbors Slower Convergence Decisions based upon information provided by neighbors   Flood routing information to all routers Event-triggered updates, so convergence is fast Complete view of the internetwork topology