Comparing Interior Routing Protocols

Two types Distance Vector

◦ Examples: RIP v1 and RIPv2 (Routing Information Protocol) IGRP (Interior Gateway Routing Protocol)

Link State◦ Examples

OSPF (Open Shortest Path First) IS-IS (Intermediate System - Intermediate System) NLSP (Netware Link Services Protocol)

Path Vector◦ Example

BGP (Border Gateway Protocol)

Review: Interior Gateway Protocols

Link State vs. Distance VectorLink State (LS) advantages:

More stable (aka fewer routing loops)

Faster convergence than distance vector

Easier to discover network topology, troubleshoot network.

Routing table shows the◦ route source

Directly connected networksStatic routesDynamic routing protocols

Parent and Child Routes◦ A Level 1Parent route does not contain any next-hop IP address or exit interface information

Level 2 child routes contain route source & the network address of the route

Diagram illustrates 2 child networks belonging to the parent route 172.16.0.0 / 24

Review: Routing Table Structure

level 1 route

level 2 route

Longest Match: Level 1 Network Routes–Best match is also known as the longest match –The best match is the one that has the most number of left most bits matching between the destination IP address and the route in the routing table.

Routing Table Lookup Process

Classful routing protocols do not send subnet mask information with their routing updates.

A router running a classful routing protocol will react in one of two ways when receiving a route:◦ If the router has a directly connected interface belonging to the

same major network, it will apply the same subnet mask as that interface.

◦ If the router does not have any interfaces belonging to the same major network, it will apply the classful subnet mask to the route.

◦ Example: 10.3.1.0 and 10.5.5.0 belong to the same major network (10.0.0.0)

What happens when Router B sends routing update to Router A?◦ What subnet mask will be used by Router A?

Classful and Classless Routing

When using classful routing protocols, the subnet mask must remain consistent throughout your entire network

Scenario: Routing Behaviour- Classful/Classless Routing

What happens to a packet with destination 172.16.4.0/24 for • Classful routing and • Classless routing?

If no match is found in child routes of previous slide then router continues to search the routing table for a match that may have fewer bits in the match

Routing Behavior: Classless Routing

why the router drops the Packet destined to 172.16.4.0/24None of the child routes left most bits match the first 24 bits.

Classful Routing Behavior

Route Summarization

Figure 1

The use of CIDR and VLSM not only reduces address waste, but it also promotes route aggregation, or route summarization.

route summarization reduces the burden on upstream routers.

Example: Figure 1 variable-sized networks and

subnetworks is summarized at various points using a prefix address until the entire network is advertised as a single aggregate route of 192.168.48.0/20

Route Flapping

Figure 1

Route flapping occurs when a router interface alternates rapidly between the up and down states. This can be caused by a number of factors, including a faulty interface or poorly terminated media.

Summarization can effectively insulate upstream routers from route-flapping problems.

Example: Figure 1 If the RTC interface connected

to the 200.199.56.0 network goes down, RTC removes that route from its table

What if routers were not configured to summarise?

Steps to calculate a route summary

List networks in binary format

Count number of left most matching bits to determine summary route’s mask

Copy the matching bits and add zero bits to determine the summarized network address

Classless Inter-Domain Routing (CIDR)

Default routes ◦ Packets that are not defined specifically in a

routing table will go to the specified interface for the default route

◦ Example: Customer routers use default routes to connect to an ISP router.

◦ Command used to configure a default route is

◦ #ip route 0.0.0.0 0.0.0.0 s0/0/1

Default Route

Default Route and Static route

When network topology changes, network traffic must reroute quickly. The phrase "convergence time" describes the time it takes a router to start using a new route after a topology changes.

Routers must do three things after a topology changes:

Detect the change

Select a new route

Propagate the changed route information

Comparing Routing Protocols: Convergence

EASE OF IMPLEMENTATION

SPEED OF IMPLEMENTATION

Comparing Routing Protocols: ADMINISTRATIVE CRITERIA

Three key issues determine the amount of bandwidth a routing protocol consumes:

1. When routing information is sent---1. Periodic updates are sent at regular intervals.

Flash updates are sent only when a change occurs.

2. Complete updates contain all routing information. Partial updates contain only changed information.

3. Flooded updates are sent to all routers. Bounded updates are sent only to routers that are affected by a change.

Note: These three issues also affect CPU sage.

Comparing Routing Protocols BANDWIDTH REQUIREMENTS

CPU usage is protocol dependent. Some protocols use CPU cycles to compare new

routes to existing routes. Other protocols use CPU cycles to regenerate

routing tables after a topology change. In most cases, the latter technique will use

more CPU cycles than the former. ◦ Example: For link-state protocols, keeping areas small

and using summarization reduces CPU requirements by reducing the effect of a topology change and by decreasing the number of routes that must be recomputed after a topology change.

Comparing Routing Protocols:CPU REQUIREMENTS

Routing protocols use memory to store routing tables and topology information.

Route summarization cuts memory consumption for all routing protocols.

Keeping areas small reduces the memory consumption for hierarchical routing protocols

Comparing Routing Protocols: MEMORY

The ability to extend your internetwork is determined, in part, by the scaling characteristics of the routing protocols used and the quality of the network design.

Network scalability is limited by two factors: ◦ operational issues and

Operational scaling concerns encourage the use of large areas or protocols that do not require hierarchical structures.

◦ technical issues When hierarchical protocols are required, technical

scaling concerns promote the use of small areas

Comparing Routing Protocols: SCALABILITY

Some routing protocols provide techniques that can be used as part of a security strategy.

Some routing protocols allow filter on the routes being advertised so that certain routes are not advertised in some parts of the network.

 Some routing protocols can authenticate routers that run the same protocol. Authentication mechanisms are protocol specific

Authentication can increase network stability by preventing unauthorized routers or hosts from participating in the routing protocol, whether those devices are attempting to participate accidentally or deliberately.

Comparing Routing Protocols: SECURITY

RIP: Routing Information Protocol Uses hop count as metric (max: 16 is infinity) Tables (vectors) “advertised” to neighbors every

30 s. Each advertisement: upto 25 entries

No advertisement for 180 sec: neighbor/link declared deadroutes via neighbor invalidatednew advertisements sent to neighbors

(Triggered updates)neighbors in turn send out new advertisements

(if tables changed)link failure info quickly propagates to entire netpoison reverse used to prevent ping-pong

loops (infinite distance = 16 hops)

RIPv1 Problems (Continued) Split horizon/poison reverse does not

guarantee to solve count-to-infinity problem◦ 16 = infinity => RIP for small networks only!◦ Slow convergence

Broadcasts consume non-router resources RIPv1 does not support subnet masks

(VLSMs) ◦ No authentication

RIPv2 Why ? Installed base of RIP routers Provides:

◦ VLSM support◦ Authentication◦ Multicasting◦ “Wire-sharing” by multiple routing domains,◦ Tags to support EGP/BGP routes.

Uses reserved fields in RIPv1 header. First route entry replaced by authentication

info.

E-IGRP (Interior Gateway Routing Protocol)

CISCO proprietary; successor of RIP (late 80s) Several metrics (delay, bandwidth, reliability, load

etc) Uses TCP to exchange routing updates Loop-free routing via Distributed Updating Alg.

(DUAL) based on diffused computation Freeze entry to particular destination Diffuse a request for updates Other nodes may freeze/propagate the

diffusing computation (tree formation)Unfreeze when updates received.Tradeoff: temporary un-reachability for some

destinations

Link State Protocols Key: Create a network “map” at each node. 1. Node collects the state of its connected links

and forms a “Link State Packet” (LSP) 2. Flood LSP => reaches every other node in the

network and everyone now has a network map. 3. Given map, run Dijkstra’s shortest path

algorithm (SPF) => get paths to all destinations 4. Routing table = next-hops of these paths. 5. Hierarchical routing: organization of areas, and

filtered control plane information flooded.

Link State Issues Reliable Flooding: sequence #s, age LSA types, Neighbor discovery and

maintainence (hello)◦ Efficiency in Broadcast LANs, NBMA, Pt-Mpt

subnets: designated router (DR) concept Areas and Hierarchy

◦ Area types: Normal, Stub, NSSA: filtering◦ External Routes (from other ASs), interaction with

inter-domain routing. Advanced topics: incremental SPF

algorithms

OSPF

OSPF Network Topology   OSPF Addressing and Route Summarization

OSPF Route Selection

OSPF Convergence

OSPF Network Scalability

OSPF Security

Next Week..