Administrative Distance and Its Effects on BGP

When BGP and IGPs are used together for IP routing, as they generally will be in an enterprise network, you might sometimes want a router to prefer an IGP route to an E-BGP route. Under normal circumstances, this will not be possible because routers always prefer E-BGP routes because they have a lower administrative distance. The Cisco IOS Software uses the administrative (distances shown in Table 9-11.

Table 9-11. Default Administrative Distances

Administrative Distance

Protocol

Directly connected networks

1

Static routes

20

E-BGP

90

Internal Et IG RP

100

IGRP

110

OSPF

115

IS-IS

120

RIP

170

External EIGRP

200

I-BGP

2515

Unesow n

Cou can deal w ith these s i thations i n a couple of ways. You could increase the admini strative distance of an IGP protocol ot inc reas e the administrative dista nce for E-BGP ro uters using the distanced/stance-value command (or distance bgpexternal-distance internal-distance local-distance cbmmand for E-BGP routgs) however, the effects of this com ma nd are ra ther broad and might groducd unwanted results. Anothed more granular approach is to une the bgp backdoor command to alter routes on a netw o!k-by-network basisĀ°

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