Overlapping Protocols: Backdoors

With different IGPs and EGPs working together to achieve routing, routes can be learned via different protocols; choosing one protocol over another affects how the traffic flows. For example, if traffic follows a RIP route, it might traverse one link, whereas if it follows an external BGP route, it might end up on another link. Backdoor links offer an alternative IGP path that can be used instead of the external BGP path. IGP routes that can be reached over the backdoor link are called backdoor routes. With the existence of such alternative routes, a mechanism that gives one protocol preference over other protocols is needed. Cisco Systems offers a preference parameter called the administrative distance of a protocol. The lower a routing protocol's administrative distance, the higher the preference for the protocol.

It should be noted that administrative distance is a parameter that is relative only to the locally configured router and is not known by or communicated to any other routers in the AS. Thus, if you intend to modify the administrative distance of one router in the AS, it is highly recommended that the administrative distance be changed similarly on all routers in the AS in order to guarantee a consistent routing decision. Table 6-1 lists distances according to the Cisco implementation.


Table 6-1. Protocol Distance Default Values
ProtocolDistance
Directly connected0
Static1
EBGP20
EIGRP (internal)90
IGRP100
OSPF110
ISIS115
RIP120
EGP140
EIGRP (external)170
IBGP200
BGP local200
Unknown255

Table 6-1 indicates that a directly connected route is generally preferred over a static route, which in turn is preferred over an EBGP route, and so on. Note that EBGP routes with a distance of 20 are preferred over all the other IGP routes.

Figure 6-7 illustrates the use of backdoor routes. In the figure, AS1 is receiving updates about NetA from two different sources. AS1 is receiving routes via EBGP on the link to AS3 and via the backdoor link running RIP between AS1 and AS2. According to Table 6-1, the router will automatically give a distance of 20 to the EBGP route and a distance of 120 to the RIP route. In AS1, the routers that learn the route via EBGP (AS border routers) will install the lower distance in the routing table. Hence, traffic toward NetA will follow the indirect BGP route via AS3 and then AS2, rather than the direct RIP route via AS2.

Figure 6-7. Backdoor Routing Conflicts


Cisco provides a way to force IGP routes to take precedence over EBGP routes. The concept is simple. Specific EBGP routes can be tagged as backdoor routes, which sets the distance of these routes to be the same as the "BGP local" route's distance (the default is 200). According to Table 6-1, this distance is higher than any IGP learned route, and the backdoor IGP route will be preferred.

Alternatively, as previously discussed, another option is to use the distance BGP subcommand to alter the administrative distance of all the BGP prefixes learned by the router.

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