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IS-IS Hello PDUs (IIH) through which the adjacency is initially established. If IS-IS authentication is enabled, hello authentication is automatically enabled.

If hello authentication is available, it is likely that all IS-IS PDUs can be authenticated using MD5. In this case, it is sensible to authenticate all IS-IS protocol exchanges.

5.2.8 IS-IS GRACEFUL RESTART

As with OSPF, when an IS-IS router (an IS) restarts, it causes SPF calculations to be performed on all routers within its area or, in the case of a level 2 router, throughout the entire domain. In a relatively unstable network, this can cause a significant load on the processor calculating the routing table.

In common with graceful restart for OSPF, graceful restart for IS-IS is optional. Unlike OSPF, there is no negotiation of the capability between the neighbours. The restarting router merely includes a restart TLV with the RR bit set in the IIH PDUs. This informs the neighbour that it should not re-initialize the adjacency. It also requests a complete sequence number PDU (CSNP) from any neighbours with adjacencies via point-to-point links and requests that any neighbour sets the SRM flags (used to indicate that an LSP needs to be refreshed) for all of the LSPs learned from the restarting router.

If a router is operating an interface on a shared network in both level 1 and level 2, then it must send and receive IIH PDUs for each level with the RRs set. Over a point-to-point link, a single IIH PDU can be sent for both levels. Irrespective of the type of network, separate CSNPs must be sent for each level. IIH PDUs are sent until the exchange of CSNPs is complete, which ensures that the restarting router is completely updated by all its neighbours following the restart.

5.2.9 ROUTING INFORMATION PROTOCOL (RIP)

RIP is a distance vector routing protocol. There are currently two versions of RIP. Version 1 is classful. It assumes the legacy classful prefix lengths (0.0.0.0 through 127.0.0.0 are all /8, 128.0.0.0 through 191.255.0.0 are all /16 and 192.0.0.0 through 223.255.255.0 are all /24). Version 2 provides extensions to support classless routing. It associates an arbitrary prefix length between 0 and 32 with each network address.

Being a distance vector protocol, RIP suffers from the ‘counting to infinity’ problem. The ‘counting to infinity’ problem is based on the fact that an unreachable route is always given a metric of ‘infinity’. It is patently not possible to actually have a metric which is equal to infinity, so some absolute value has to be assigned to be equal to infinity. In RIP, this value is 16, which constrains RIP networks to a diameter of 15 hops. If a path is longer than 15 hops, it is effectively equal to infinity and therefore unreachable.

RIP’s default timers also mean that it is exceptionally slow to converge. Each node announces its complete routing table to its neighbours every 30 seconds so, in the worst possible case, a change experienced on one side of a maximum diameter network would take seven minutes to propagate to the other side of the network. In addition, routes

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learned from a neighbour will, by default, not expire for 180 seconds. If a router fails on a switched network, shortly after announcing a set of prefixes, it will take up to 180 seconds before the neighbouring routers even register the loss of the neighbour and its routes. Only then will it start propagating information to its neighbours saying that the prefixes are no longer reachable via that path.

Using RIP for anything other than one or two hop networks is, in reality, too slow to converge. However, there are still a few peripheral products in use on service providers’ networks, which support no dynamic routing protocols other than RIP. In this situation, the only alternatives are RIP or static routing. It is often preferable to use static routing in this case rather than use RIP.

5.2.10INTERIOR GATEWAY ROUTING PROTOCOL (IGRP) AND ENHANCED INTERIOR GATEWAY ROUTING PROTOCOL (EIGRP)

IGRP and EIGRP are Cisco proprietary routing protocols. Although the details of the protocol behaviour have been published, no other vendor may implement IGRP or EIGRP. Therefore, their use is constrained to Cisco-only networks.

5.2.10.1 IGRP Specifics

Like RIP, IGRP is a classful, distance vector routing protocol. It operates in a very similar way to RIP, periodically broadcasting its entire routing table via all interfaces. It also requests information from all neighbours when it starts up to obtain a complete routing table as quickly as possible.

IGRP uses the concept of autonomous systems as process identifiers. IGRP autonomous systems do not necessarily correspond to the concept of autonomous systems in OSPF or BGP. In IGRP an autonomous system represents a set of routers using a common IGRP routing process to exchange information. It is possible, within a single IGP, to define multiple IGRP instances between which it is possible to tightly control the exchange of routing information.

Unlike other routing protocols, IGRP uses a composite metric system that allows it to overcome, to a certain extent, the constraints of RIP. IGRP still uses the hop count to a destination as an element of the metric and can be used in networks up to 255 hops in diameter. Composite metrics are derived from four link characteristics: bandwidth, delay, load and reliability. By default, IGRP uses just the bandwidth and delay, only using load and reliability if specifically configured to do so.

Bandwidth and delay are static values not directly linked to the actual bandwidth and delay of the associated link. By default, each link type has a particular value. However, these values can be manually configured. Bandwidth is transmitted as a three octet number derived from the bandwidth in kilobits per second scaled by a factor of 107. Delay is also used to represent unreachability. Delay is measured in tens of milliseconds and transmitted as a three octet number. An unreachable destination has a delay of 0xFFFFFE

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(approximately 167 seconds). The total metric is calculated as the minimum bandwidth plus the sum of the delays, end to end.

The importance of bandwidth and delay in deriving the IGRP metric means that it is essential that they are chosen carefully, accurately and consistently across the entire network. Changes to either bandwidth or delay should be made with extreme care.

In addition to the extended network diameter, IGRP has some other benefits over RIP. Primary among these is the ability to do unequal cost load sharing. Unequal cost load sharing uses a value called variance. This is simply a multiplier that indicates a range within which a higher metric must fall in order for it to be considered for use in unequal cost load sharing. If the higher metric is less than or equal to variance multiplied by the lower metric, then it is a feasible route.

Like RIP, IGRP is easy to configure. It is this simplicity that made it attractive to operators of networks running exclusively Cisco equipment.

5.2.10.2 EIGRP Specifics

Despite its name, EIGRP is not really a direct derivative of IGRP. It is Cisco proprietary but it is classless and is neither a pure distance vector nor a link state protocol. The routing algorithms employed by EIGRP, described in more detail below, incorporate some features of distance vector and some features of link-state algorithms.

EIGRP does provide compatibility with IGRP and uses the same concept of an autonomous system representing a collection of routers using the same routing process. In fact, EIGRP uses the same algorithm as IGRP scaled by 256 to provide greater granularity in the metric values.

There are mechanisms for NLRI to be exchanged automatically between EIGRP and IGRP. In fact, it is (almost) as simple as assigning the same autonomous system number to both protocols on a router. The main constraint is that being classful, IGRP is unable to carry any information relating to the length of a particular prefix and simply assumes that it has learned an entire classful prefix. If care is not taken, this can lead to routing loops and black holes.

EIGRP uses the Reliable Transport Protocol (RTP) to manage the delivery of EIGRP protocol packets. Packets are multicast to the well-known multicast group 224.0.0.10 using a Cisco proprietary mechanism known as reliable multicast. Responses to multicast packets are unicast. Two sequence numbers are used to ensure ordered delivery of packets. Each transmitted packet contains the sequence number from the source that increments with each new transmitted packet and the sequence number of the last packet received from the destination.

Unlike IGRP or RIP, which use the periodicity of updates to identify the loss of an adjacent router and its associated routes, EIGRP does not have periodic updates. Therefore, it is necessary to use a neighbour discovery and maintenance mechanism to keep track of neighbours. EIGRP uses periodic Hello packets to maintain its neighbours. EIGRP Hello packets also inform the neighbours how long they should wait without hearing any Hellos before declaring the sending node dead and updating the routing information.

Unlike a pure distance vector protocol in which a single shortest route via one next hop router is chosen and all other routing information relating to that destination is discarded,

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EIGRP maintains a complete topology of the network. This is somewhat similar to a linkstate protocol and provides incredible benefits with respect to the speed of convergence. Rather than a router having to wait for invalid routes to expire and for discarded routing information to be retransmitted by alternative next hops, EIGRP provides a mechanism for an alternative path to be identified in advance of any failure and, if no such alternative path exists, a router can query its neighbours to find alternatives.

Having described how much EIGRP resembles a link-state protocol, it is necessary to point out that EIGRP still transmits routes as distances and next hops (vectors) to directly connected neighbours. However, unlike IGRP and RIP, EIGRP only sends updates when a routing change occurs, only sends the changed routing information and only sends those updates to other routers impacted by the changed routes.

Particularly in enterprise networks, where bandwidth in wide area networks is generally more restricted and for whom bandwidth is usually more expensive, this reduction in protocol traffic can be a major benefit, particularly when the network is moderately unstable.

As stated earlier, EIGRP does not use a pure link-state algorithm. Link-state algorithms rely on each router to individually calculate the topology of the network from flooded routing information. EIGRP, on the other hand, uses a distributed mechanism whereby calculations are made in a coordinated way throughout the network. This is known as diffusing computations.

5.2.11DIFFUSING UPDATE ALGORITHM (DUAL)

DUAL is the algorithm through which updates for the diffusing computations are exchanged. In order for DUAL to function, it is essential that the following conditions are met:

•Neighbour state is recognized and recorded within a fixed time.

•Updates are received complete and in the correct order within a fixed time.

•Updates are processed in the order in which they are received within a fixed time.

DUAL uses the term feasible distance (FD) to identify the lowest calculated metric to a destination. The feasibility condition (FC) is met when a neighbour advertises a distance to a destination that is lower than the FD for that destination. If the FC is met, then the advertising neighbour becomes the feasible successor (i.e. the next hop). The feasible successor is always closer to the destination and cannot have a path to the destination back via the router with the FD. This makes it impossible to create a routing loop.

For every destination, for which at least one valid feasible successor exists, the following information is stored:

•the FD to the destination;

•all feasible successors;