In most cases a prudent approach would be to introduce the main theme of the strengths and weaknesses of Route Redistribution between different Autonomous Systems running different IGP (Interior Gateway protocols).
In this instance I intend to rather explain my selection of the Internal Routing Protocols of EIGRP (Enhanced Interior Gateway Protocol), OSPF (Open Shortest Path First) and then explore Route Redistribution using the selected protocols.
RATIONALE OF CHOICE OF INTERIOR GATEWAY PROTOCOLS
The choice was driven by the course materials main focus in comparing RIP (Routing Information Protocol) and OSPF when discussing Route Redistribution. However RIP uses a basic metric of hop count whereas Cisco proprietary EIGRP, and IGRP (Interior Gateway Routing Protocol) which is now deprecated since CISCO IOS 12.2, uses a composite metric based on bandwidth and delay (Cisco Networking Academy., 2014). Therefore RIP has been replaced by EIGRP as a complex Distance Vector Protocol. OSPF remains since it is the most widely used link state protocol in large enterprise networks; its’ only nearest challenger is IS-IS which is mainly used by ISPs but because of limited reference material and knowledge pool, OSPF has dominated.
Redistribution is viewed as a temporary solution to allow the exchange of routing information between different Autonomous Systems using different IGP protocols. Possibilities could range from company mergers to protocol migrations and hardware upgrades. In a study of 1600 networks Route Redistribution was employed on 90% of 1600 businesses surveyed which concluded that they are becoming intrinsic to the construction of modern networks ((1).Le,F.,Xie,G,G,.Zhang,.2010).
The Redistribution protocol does not use any information from the independent Protocols Topology Tables as they incorporate unique metrics with the exception of redistributing routes between IGRP and EIGRP. For example OSPF calculates the shortest path using cost and EIGRP uses bandwidth and delay making the metrics incompatible. Routing metrics are key in the injecting protocols configuration because it must be manually configured otherwise a default seed metric will be used and for EIGRP this seed metric is infinite resulting in the route not being advertised but for OSPF it is 20.
A minimum of a single Redistribution Point needs to exist before redistribution will inject the route with the lowest Administrative Distance from the IP Routing Table. The Route is advertised in an outbound direction to a neighboring routing domain and these routes are marked as external routes.
REDISTRIBUTION INTO EIGRP
EIGRP uses a composite metric of bandwidth and delay as its distance metric also including Reliability, Load and MTU which during the Cisco Dual calculation they are negated.
The five metrics or K values are important to specify when configuring the seed metric for redistribution as omitting a K-value will lead to EIGRP assuming an infinite metric resulting in the routes not being advertised. The default values when configuring a seed metric should be 255 indicating a 100% reliability, 1 for the lowest load and 1500 for the lowest MTU, but bandwidth and delay are dependent on the link so a 100 Mbits Fast Ethernet link will have a corresponding 100 micro seconds delay calculated by Cisco IOS. Any modifications to the K values do not alter the physical interface but do influence the routing protocol.
Example of how the default-metric is set out when applied to all redistributed routes.
RouterBR(config-router)# defaut-metric 10000 (bandwidth) 100(delay) 255(reliability) 1(load) 1500 (MTU)
In older Cisco Technical Documents the default recommended metric was 1 1 1 1 1 which has since been revised to use the above values but there are still no published standards when calculating the seed metric (Cisco.,2012).
REDISTRIBUTION INTO OSPF
OSPF uses cost and the OSPF router performing redistribution will automatically become the ASBR (Autonomous System Boundary Router). Routes advertised into OSPF will have been either manually configured or a default metric of 20 will be injected, by default OSPF will only redistribute classful routes so a subnet parameter is required when configuring.
The routes are marked as either an external Type-1 or Type-2 and can be implemented at the same time, but Type-1 will take precedence over Type-2 as it adds the external cost to the destination as well as the cost to reach the AS Boundary Router, but Type-2 will remain static throughout the autonomous system (Juniper,2016).
REDISTRIBUTION WEAKNESSES AND STRENGTHS
Advantages to redistribution are the ability to divide a large network into separate autonomous systems thereby creating different management and or administrative divisions as well as security boundaries which reduces the size of the topology table and the workload performed by the routers. When migrating between routing protocols, or through company mergers redistribution allows a period of time to determine the best overall design and implement changes to the network accordingly.
By design a single homed link is more stable, prone to fewer errors in configuration and has a faster convergence. However in a multilink redundancy, problems are harder to troubleshoot and configuration errors may lead to Routing Loops, Suboptimal Routing and a slower convergence.
Suboptimal Routing occurs when the seed metric assigned to an external route redistributed into an internal protocol, loses its’ metric possibly resulting in the returning data being routed over a slower redundant link. A similar description is Route Feedback but the only difference is that it may develop into a Routing Loop (Balchunus,A.,2007).
The greatest impact are Routing Loops which occur when a routing protocol redistributes routes into another protocol and then receives the same routes back, so the data will oscillate until the TTL (Time to Live) has been reached (Lapukhov,P.,2008).
By implementing Redistribution-lists, changing the Administrative Distance, modifying the metric and using passive interfaces with static routes, the problems associated with Route Redistribution can be resolved or at least limited.
By designing a redistribution policy it is possible to mitigate the inherent weakness of routing loops and suboptimal routing, this policy might encompass investment in developing a skilled workforce along with replacing or upgrading hardware thereby future proofing an organization. The policy should also incorporate a physical topology map of the network and have an accurate inventoried database of hardware and specification thereby minimising configuration errors.
Route Redistribution has been brought about by pressure applied on manufacturers by various organizations requiring this feature. Whilst Route Redistribution is a complex routing protocol incorporating different policies and being more akin to BGP in complexity, it still does not have any published standards or even an RFC ((2).Le,F.,Xie,G,G,.Zhang,H,.2007).
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