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)
Link 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 networks Static routes Dynamic 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 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.
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? When using classful routing protocols, the subnet mask must remain consistent throughout your entire network
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
why the router drops the Packet destined to 172.16.4.0/24 None of the child routes left most bits match the first 24 bits.
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
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 188.8.131.52 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
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
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
EASE OF IMPLEMENTATION SPEED OF IMPLEMENTATION
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.
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.
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
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
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.
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 dead routes via neighbor invalidated new advertisements sent to neighbors (Triggered updates) neighbors in turn send out new advertisements (if tables changed) link failure info quickly propagates to entire net poison reverse used to prevent ping-pong loops (infinite distance = 16 hops)
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
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.
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
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.
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