Routing is at the core of the internet; a demarcation point separates the public network from a customer’s private network whilst routing protocols determine how routers communicate with each other,
and forward packets around the networks. Routing protocols consist of Exterior Gateway Protocols (EGP), where the Border Gateway Protocol (BGP) is used predominantly to conserve public IP addresses whilst IS-IS has a niche place inside large Internet Service Provider networks where simpler programming and scalability at the Datalink layer is paramount (Doyle, 2001). While EIGRP (Enhanced Interior Gateway Routing Protocols), OSPF (Open Shortest Path First) and RIP (Routing Information Protocol) are used as Interior Gateway Protocols (IGP) in an intranet environment.
Routing protocols use the information obtained through adjacent routers to calculate an alternate best path for each affected route and recalculation enables protocols to discover alternative routes as a result of a network topology change which is referred to as routing convergence (Shamim et al., 2002). Routing convergence issues can be divided into Neighbour Adjacencies, Routes and Miscellaneous issues when Troubleshooting IGP and EGP (Lacoste et al, 2015) an exception to this are BGP issues which are mainly grouped into troubleshooting neighbour adjacencies.
TROUBLESHOOTING NEIGHBOUR ADJACANCIES
The routing protocols for both IGP and EGP apart from RIP require a neighbour relationship to form before they are able to exchange routing information and each protocol uses a different algorithm to calculate the shortest and best path. Irrespective of the protocol employed the following are some of the issues that do emerge.
A prerequisite to form any connection is for the ports to be transporting bits over the physical layer to adjoining Routers. Incorrect version numbers used by the same protocols and the adjoining link between routers using different subnetworks, will also prevent adjacencies from forming but to begin the protocol process needs to be enabled and the correct network statement applied to the links. A passive statement applied to an interface participating in the neighbour adjacencies will be equivalent to disabling the protocol but applied correctly it improves security and reduces traffic.
ACL (Access Control Lists) may deny specific traffic, Authentication failing between directly connected routers and out of sync timers, will stop adjacencies from forming but mismatched timers induce a network to flap.
Mismatched Area Numbers, Area Type and Maximum Transmission Unit (MTU) in addition to Duplicate Router Identification are issues for OSPF; mismatched values in the metric formula for EIGRP; total time to live for RIP; and no Layer 3 connectivity, misconfigured peer groups, packets sourced from the wrong IP address and no route to the local router for BGP, will all prevent these protocols from forming neighbour adjacencies.
Neighbour relationships are the cornerstone to exchanging routes which are installed in the routing table as part of complex routing protocols. Missing routes may be a product of mismatched network statements and the interface subnetwork resulting in the deactivation of the routing protocol. Whereas Route Filtering dictates which route is installed in the routing table for OSPF but for EIGRP and BGP this prevents a route from being advertised.
When a route with a higher Administrative Distance takes precedence over the shortest or superior route, it is classed as suboptimal routing. This pertains primarily to Cisco devices employing OSPF and EIGRP on the same network, as other manufacturers do not support EIGRP (Juniper, 2017). The Split Horizon rule does not advertise the route out of the same interface it was received on, thereby preventing the creation of routing loops, which are also referred to as the count to infinity problem. Both these factors will result in routes not being displayed in the routing table.
So far the focus has mainly been on the operation of the routing protocols and moving forward we advance to general troubleshooting issues which are harder to diagnose in a converged network because of the greater complexity to configure.
Path control mechanisms such as Redistribution is viewed as a temporary solution allowing the exchange of routing information between different protocols but in a study of 1600 companies it found that 90% of their networks employed Route Redistribution which indicates they are becoming intrinsic to the construction of modern networks (Le et al, 2010). Policy Based Routing allows you to statically control traffic through the use of user-defined policies but combine it with an IP Service-Level Agreement (SLA’s) and it will dynamically redirect traffic however this adds an additional layer of complexity to the Network Path Control Mechanisms
Other general issues are Autosummarization also referred to as Aggregation, where the protocol summarizes the network subnets to a classful boundary because in a discontiguous network the network separating the classful networks will have two exact routing entries and a device in the discontiguous network, returning packets to either of the two routes might have forwarded the packets inadvertently to the wrong network. Load Balancing values if misconfigured will also contribute to missing routes along with misconfigured static route injected into a protocols routing table. The various protocols will calculate load balancing differently, even between protocols for different vendors and the same applies to route injection.
Accidental router misconfiguration, network failures and system bugs hold the top spot at 51% for routing issues carried out in a recent survey whilst not far behind were customer equipment and BGP peering issues (TechValidate, 2018) and these findings apply to both Interior and Exterior Routing Protocols, so by careful design you will reduce the time spent troubleshooting.
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