Cisco Routing Protocol – Basic Stuff

CCNA Routing

Layer-Three gadgets are generically known as routers. Routers principally have two purposes:

1. To discover a layer-Three trail to a vacation spot community

2. To transfer packets from one interface to some other to get a packet to its vacation spot

In order to perform the primary serve as, a router will want to:

– Learn about routers it is attached to with the intention to be told the networks that they learn about

– Find places of vacation spot community numbers

– Choose a easiest trail to every vacation spot

– Maintain essentially the most up-to-date routing details about how to succeed in vacation spot networks

Types of Routes

A router can be told a direction by the use of certainly one of two strategies: static and dynamic. A static direction is a direction this is manually configured at the router. Dynamic routes are routes {that a} router learns via operating a routing protocol. Routing protocols will find out about routes from different neighboring routers operating the similar routing protocol.

Administrative Distance

Administrative distance ranks the IP routing protocols, assigning a price, or weight, to every protocol. Distances can vary from Zero to 255. A smaller distance is extra plausible via a router, with the most efficient distance being Zero and the worst, 255.

Static Route Configuration

Router(config)# ip direction destination_network_# [subnet_mask] IP_address_of_next_hop_neighbor



Router(config)# ip direction destination_network_# [subnet_mask] interface_to_exit [administrative_distance]

Default Route Configuration

A default direction is a different form of static direction.Where a static direction specifies a trail a router will have to use to succeed in a selected vacation spot, a default direction specifies a trail the router will have to use if it does not understand how to succeed in the vacation spot.

Router(config)# ip direction 0.0.0.Zero 0.0.0.Zero IP_address_of_next_hop_neighbor [administrative distance]


Router(config)# ip direction 0.0.0.Zero 0.0.0.Zero interface_to_exit [administrative_distance]

Dynamic Routing Protocols

Dynamic routing protocols find out about vacation spot networks from neighboring routers. Dynamic routing protocols fall underneath certainly one of 3 classes: distance vector, hyperlink state, and hybrid. Because of the diversities between the quite a lot of routing protocol varieties, every has benefits and downsides.

Distance Vector Protocols

Distance vector protocols are the most simple. Distance vector routing protocols use the space and path (vector) to search out paths to locations. Most distance vector protocols use the Bellman-Ford set of rules for locating paths to networking locations. Some examples of IP routing protocols which can be distance vector are RIPv1 and IGRP. Distance vector protocols periodically use native proclaims with a vacation spot IP cope with of to percentage routing knowledge.

Processing Updates

When a distance vector protocol receives a routing replace, it plays those steps:

1. Increment the metrics of the incoming routes within the commercial (for IP

RIP, upload 1 to the hop depend).

2. Compare the community numbers within the routing replace from the neighbor

to what the router has in its routing desk.

3. If the neighbor’s knowledge is best, position it within the routing desk and take away the outdated access.

4. If the neighbor’s knowledge is worse, forget about it.

5. If the neighbor’s knowledge is precisely the similar because the access already within the desk, reset the timer for the access within the routing desk (in different phrases, the router already discovered about this direction from the similar neighbor).

6. If the neighbor’s knowledge is a unique trail to a recognized vacation spot community, however with the similar metric as the present community within the routing desk, the router will upload it to the routing desk along side the outdated one. This assumes you haven’t exceeded the utmost selection of equal-cost paths for this vacation spot community quantity.

The six steps are typically known as the Bellman-Ford set of rules.

Link State Protocols

Link state protocols use an set of rules known as the Shortest Path First (SPF) set of rules, invented via Dijkstra, to search out the most efficient trail to a vacation spot. Examples of hyperlink state protocols come with IP’s OSPF. Whereas distance vector protocols use native proclaims to disseminate routing knowledge, hyperlink state protocols use multicasts. A distance protocol will ship out its routing desk religiously on its periodic period whether or not there are adjustments or no longer. Link state protocols are smarter. They multicast what is named a Link State Advertisement (LSA), which is a work of routing knowledge that incorporates who originated the commercial and what the community quantity is. LSAs are in most cases generated best when there are adjustments within the community. hyperlink state routers ship out best incremental updates. Advantage that hyperlink state protocols have over distance vector protocols is they improve direction classless routing [VLSM].

Hybrid Protocols

A hybrid protocol takes some great benefits of each distance vector and hyperlink state protocols and merges them into a brand new protocol. Typically, hybrid protocols are in keeping with a distance vector protocol however include lots of the options and benefits of hyperlink state protocols. Examples of hybrid protocols come with RIPv2, EIGRP.

Problems with Distance Vector Protocols

Problem: Convergence

The time period convergence, in routing phrases, refers back to the time it takes for the entire routers to grasp the present topology of the community. Link state protocols have a tendency to converge in no time, whilst distance vector protocols tent to converge slowly.

Solution: Triggered Updates

You in fact have two answers that you’ll use with the intention to pace convergence: trade the periodic timer period and/or use prompted updates.

Problem: Routing Loops

A routing loop is a layer-Three loop within the community. Basically, this can be a war of words about how to succeed in a vacation spot community. Packet Loops within the Network.

Solution: Maximum Hop Count: IP RIP set a hop depend restrict of 15, via default, and IGRP lets in a hop depend of 100.

Solution: Split Horizon

Split horizon is used with small routing loops. Split horizon states that if a neighboring router sends a path to a router, the receiving router won’t propagate this direction again to the promoting router at the identical interface.

Solution: Route Poisoning

Whereas cut up horizon is used to resolve small routing loop issues, distance vector protocols use two mechanisms to handle massive routing loop issues: direction poisoning and keep down timers. Route poisoning is a by-product of cut up horizon. When a router detects that certainly one of its attached routes has failed; the router will poison the direction via assigning a limiteless metric to it. In IP RIP, the direction is assigned a hop depend of 16 (15 is the utmost), thus making it an unreachable community.

When a router advertises a poised path to its neighbors, its neighbors spoil the rule of thumb of cut up horizon and ship again to the originator the similar poisoned direction, known as a poison opposite. This guarantees that everybody gained the unique replace of the poisoned direction.

Hold-Down Timers

In order to offer the routers sufficient time to propagate the poisoned direction and to make sure that no routing loops happen whilst propagation is going on; the routers put in force a hold-down mechanism. During this era, the routers will freeze the poisoned direction of their routing tables for the length of the hold-down timer, which is in most cases thrice the period of the routing broadcast replace. 180 Sec’s

RIP (Routing Information Protocol)

IP RIP (Routing Information Protocol) is available in two other variations: 1 and a couple of. Version 1 is a distance vector protocol. Version 2 is a hybrid protocol. RIPv1 makes use of native proclaims to percentage routing knowledge. These updates are periodic in nature, going on, via default, each and every 30 seconds, with a hold-down length of 180 seconds. Both variations of RIP use hop depend as a metric, which isn’t all the time the most efficient metric to make use of. RIPv1 is a classful protocol. Another fascinating function is that RIP helps as much as six equal-cost paths to a unmarried vacation spot. The default is in fact 4 paths, however this may also be greater as much as a most of six.


One primary enhancement to RIPv2 relates to the way it offers with routing updates. Instead of the use of proclaims, RIPv2 makes use of multicasts. And to hurry up convergence, RIPv2 helps prompted updates-when a metamorphosis happens, a RIPv2 router will right away propagate its routing knowledge to its attached neighbors. A 2nd primary enhancement that RIPv2 has is that this can be a classless protocol. RIPv2 helps variable-length subnet overlaying (VLSM), which lets you use multiple subnet masks for a given elegance community quantity.

Configuring IP RIP

Router(config)# router rip

Router(config-router)# community IP_network_#

Specifying RIP Version 1 and a couple of

Router(config)# router rip

Router(config-router)# model 1|2

Troubleshooting IP RIP

display ip protocols

display ip direction

debug ip rip

debug ip rip

IGRP (Interior Gateway Routing Protocol)

The Interior Gateway Routing Protocol (IGRP) is a Cisco-proprietary routing protocol for IP. Like IP RIPv1, this can be a distance vector protocol. However, it scales higher than RIP on account of those benefits:

– It makes use of an advanced metric in keeping with bandwidth and extend.

– It makes use of prompted updates to speed-up convergence.

– It helps unequal-cost load balancing to a unmarried vacation spot. IGRP makes use of a composite metric, which incorporates bandwidth, extend, reliability, load, and MTU, when opting for paths to a vacation spot. By default, the set of rules makes use of best bandwidth and extend

Configuring IP IGRP

Router(config)# router igrp autonomous_system_#

Router(config-router)# community IP_network_#

Unlike RIP, IGRP understands the idea that of an independent device and calls for you to configure the independent device quantity within the routing procedure. For routers to percentage routing knowledge, they will have to be in the similar AS.

Load Balancing

With RIP, you do not want to configure anything else to allow equal-cost load balancing; and RIP does not improve unequal-cost load balancing. IGRP helps each equal- and unequal-cost paths for load balancing to a unmarried vacation spot. Equal-cost paths are enabled via default, the place IGRP helps as much as six equal-cost paths (4 via default) to a unmarried vacation spot within the IP routing desk. IGRP, then again, additionally helps unequal-cost paths, however this selection is disabled via default.

The variance function permits you to come with equal- and unequal-cost IGRP routes within the routing desk.

Router(config-router)# variance multiplier

Troubleshooting IP IGRP

display ip protocols

display ip direction

debug ip igrp occasions

debug ip igrp transactions

OSPF(Open Shortest Path First)

The Open Shortest Path First (OSPF) protocol is a hyperlink state protocol that handles routing for IP visitors. Because it’s in keeping with an open usual, OSPF may be very standard in lots of company networks as of late and has many benefits, together with those:

– It will run on maximum routers, since it’s in keeping with an open usual.

– It makes use of the SPF set of rules, advanced via Dijkstra, to supply a loop-free topology.

– It supplies rapid convergence with prompted, incremental updates by the use of Link State Advertisements (LSAs).

– It is a classless protocol and lets in for a hierarchical design with VLSM and direction summarization.

Given its benefits, OSPF does have its percentage of disadvantages:

– OSPF maintains 3 kinds of tables. So it calls for extra reminiscence to carry the adjacency (record of OSPF neighbors), topology (a hyperlink state database containing the entire routers and their routes), and routing tables.

– It calls for additional CPU processing to run the SPF set of rules, which is particularly true whilst you first flip to your routers and they’re to begin with development the adjacency and topology tables.

– For massive networks, it calls for cautious design to get a divorce the community into an acceptable hierarchical design via setting apart routers into other spaces.

– It is complicated to configure and tougher to troubleshoot.

Hierarchical Design: Areas

Areas are used to supply hierarchical routing. Basically, spaces are used to regulate when and what kind of routing knowledge is shared throughout your community. In flat community designs, reminiscent of IP RIP, if a metamorphosis happens on one router, most likely a flapping direction drawback, it impacts each and every router in all of the community. With a appropriately designed hierarchical community, those adjustments may also be contained inside a unmarried house. OSPF implements a two-layer hierarchy: the spine (house 0) and spaces off of the spine. Through a proper IP addressing design, you will have to be capable to summarize routing knowledge between spaces. By appearing this summarization, the routers have a smaller topology database (they know best about hyperlinks in their very own house and the summarized routes) and their routing tables are smaller (they know best about their very own house’s routes and the summarized routes).

Metric Structure

Unlike RIP, which makes use of hop depend as a metric, OSPF makes use of charge. Cost is in fact the inverse of the bandwidth of a hyperlink: the quicker the rate of the relationship, the decrease the fee. The maximum most popular trail is the only with the bottom charge. By the use of charge as a metric, OSPF will select extra clever paths than RIP. OSPF helps load balancing of as much as six equal-cost paths to a unmarried vacation spot. The default dimension that Cisco makes use of in calculating the fee metric is: charge = 10 ^8/(interface bandwidth).

Router Identities

Each router in an OSPF community wishes a novel ID. The ID is used to supply a novel identification to the OSPF router. This is incorporated in any OSPF messages the router generates. The router ID is selected consistent with one of the most two following standards:

– The very best IP cope with on its loop again interfaces (it is a logical interface on a router)

– The very best IP cope with on its lively interfaces

It is very beneficial that you simply use a loop again interface as a result of it’s all the time up and thus the router can download a router ID.

Finding Neighbors

OSPF learns about its neighbors and builds its adjacency and topology tables via sharing LSAs. There are various kinds of LSAs. When finding out in regards to the neighbors {that a} router is attached to, in addition to protecting tabs on recognized neighbors, OSPF routers will generate hi LSAs each and every 10 seconds. When a neighbor is came upon and an adjacency is shaped with the neighbor, a router expects to peer hi messages from the neighbor. If a neighbor isn’t observed throughout the useless period time, which defaults to 40 seconds, the neighbor is asserted useless. When this happens, the router will put it up for sale this knowledge, by the use of an LSA message, to different neighboring OSPF routers. In order for 2 routers to grow to be neighbors, the next will have to fit on every router:

– The house quantity and its sort

– The hi and useless period timers

Let’s suppose that you simply grew to become on your entire routers concurrently on a section. In this example, the OSPF routers will undergo 3 states known as the change procedure:

1. Down state the brand new router has no longer exchanged any OSPF knowledge with another router.

2. Init state A vacation spot router has gained a brand new router’s hi and provides it to its neighbor record (assuming that positive values fit). Note that communique is best unidirectional at this level.

3. Two-Way state the brand new router receives a unidirectional respond to its preliminary hi packet and provides the vacation spot router to its neighbor database. Once the routers have entered a two-way state, they’re thought to be neighbors. At this level, an election procedure takes position to elect the designated router (DR) and the backup designated router (BDR).

Designated and Backup Designated Routers

An OSPF router won’t shape adjacencies to simply any router. Instead, a consumer/server design is carried out in OSPF. For every community multi-access section, there’s a DR and a BDR in addition to different routers. When an OSPF router comes up, it paperwork adjacencies with the DR and the BDR on every multi-access section that it is attached to. Any change of routing knowledge is between those DR/BDR routers and the opposite OSPF neighbors on a section (and vice versa). An OSPF router talks to a DR the use of the IP multicast cope with of The DR and the BDR communicate to all routers the use of the 224.0.0.Five multicast IP cope with.

The OSPF router with the very best precedence turns into the DR for the section. If there’s a tie, the router with the very best router ID will grow to be the DR. By default, all routers have a concern of one (priorities can vary 0-255). If the DR fails, the BDR is promoted to DR and some other router is elected because the BDR. OSPF makes use of incremental updates. This implies that every time adjustments happen, best the trade is shared with the DR, which can then percentage this knowledge with different routers at the section. OSPF routers percentage details about their attached routes with the DR, which incorporates the link-state sort, the ID of the promoting router, the price of the marketed hyperlink, and the collection selection of the hyperlink.

Configuring OSPF

Router(config)# router ospf process_ID

Router(config-router)# community IP_address wildcard_mask house area_#

Note that procedure ID don’t want to fit between other routers and that they’ve not anything to do with independent device numbers.

A wildcard masks is 32 bits in size. A zero in a little place method there will have to be a fit, and a 1 in a little place method the router does not care. Actually, a wildcard masks is an inverted subnet masks, with the 1’s and zero’s switched.

Class A:

Class B:

Class C:

Loopback Interfaces

A loopback interface is a logical, digital interface on a router. By default, the router does not have any loopback interfaces, however they are able to simply be created. Here are some causes chances are you’ll need to create a loopback interface:

– To assign a router ID to an OSPF router

– To use for trying out functions, since this interface is all the time up

To create a loopback interface, use the next command:

Router(config)# interface loopback port_#

Router(config-if)# ip cope with IP_address subnet_mask

Troubleshooting OSPF

• display ip protocols

• display ip direction

• display ip ospf interface

• display ip ospf neighbor

• debug ip ospf adj

• debug ip ospf occasions

• debug ip ospf packet

EIGRP (Enhanced Interior Gateway Routing Protocol)

The Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco-proprietary routing protocol for IP. It’s in fact in keeping with IGRP, with many improvements constructed into it. These traits come with:

– Fast convergence

– Loop-free topology

– VLSM and direction summarization

– Multicast and incremental updates

– Routes for a couple of routed protocols (Supports IP, IPX, APPLE TALK)

Characteristics of EIGRP

Both be offering load balancing throughout six paths (equivalent or unequal).

– They have an identical metric constructions.

– EIGRP has quicker convergence (prompted updates and saving a neighbor’s

routing desk in the community).

– EIGRP has much less community overhead, because it makes use of incremental updates.

EIGRP and IGRP use the similar metric construction. Both can use bandwidth, extend, reliability, and MTU when computing a easiest metric trail to a vacation spot. By default, best bandwidth and extend are used within the metric computation. One fascinating level about those protocols is that in case you have some routers for your community operating IGRP and others operating EIGRP, and each units have the similar independent device quantity, routing knowledge will routinely be shared between the 2.

EIGRP makes use of the Diffusing Update Algorithm (DUAL) to replace the routing desk. This set of rules can allow very rapid convergence via storing a neighbor’s routing knowledge in an area topology desk. If a number one direction within the routing desk fails, DUAL can take a backup direction from the topology desk and position this into the routing desk with out essentially having to speak to different EIGRP neighboring routers to search out another trail to the vacation spot.

Interaction with Other EIGRP Routers

EIGRP makes use of hi packets to find and deal with neighbor relationships, a lot as OSPF does. EIGRP generates hi packets each and every Five seconds on LAN, point-to-point, and multipoint connections with speeds of a minimum of T1/E1 speeds. Otherwise, hellos are generated each and every 60 seconds. The useless period length is thrice the hi period. EIGRP makes use of the multicast cope with of for the vacation spot within the hi packets.

For EIGRP routers to grow to be neighbors, the next knowledge will have to fit:

– The AS quantity

– The Ok-values (those allow/disable the other metric parts)

When two routers decide whether or not they are going to grow to be neighbors, they undergo the next procedure:

1. The first router generates a Hello with configuration knowledge.

2. If the configuration knowledge suits, the second one router responds with an Update message with topology knowledge.

3. The first router responds with an ACK message, acknowledging the receipt of the second one’s ACK.

4. The first router sends its topology to the second one router by the use of an Update message.

5. The 2nd router responds again with an ACK.

At this level, the 2 routers have converged.

Here are the message varieties for which an EIGRP router expects an ACK again:

– Update Contains a routing replace

– Query Asks a neighboring router to validate routing knowledge

– Reply Responds to a question message

If an EIGRP router does not obtain an ACK from those 3 packet varieties, the router will take a look at a complete of 16 instances to resend the guidelines. After this, the router broadcasts the neighbor useless. When a router sends a hi packet, no corresponding ACK is anticipated.

Configuring EIGRP

Setting up EIGRP is sort of so simple as configuring IGRP:

Router(config)# router eigrp autonomous_system_#

Router(config-router)# community IP_network_#

You will have to specify the AS quantity when configure EIGRP. Even regardless that EIGRP is classless, you will have to

configure it as a classful protocol when specifying your community numbers with the community command.

Troubleshooting EIGRP

– display ip protocols

– display ip direction

– display ip eigrp neighbors

– display ip eigrp topology

– display ip eigrp visitors

– debug ip eigrp