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Zorluk: ZorFloating Static Routes

An enterprise router receives its default route via an eBGP session established with ISP-A. A network administrator attempts to configure a secondary floating static backup default route pointing to ISP-B using the following command:

`ip route 0.0.0.0 0.0.0.0 198.51.100.2 15`

Assuming the eBGP session remains active and continues advertising the default route, which outcome will occur on the router?

  1. The static route will immediately enter the routing table and replace the eBGP default route as the primary path.Cevap
  2. B
    The static route will remain inactive in the routing table until the primary eBGP neighbor session drops.
  3. C
    The router will perform equal-cost load balancing between the eBGP peer and the static route next-hop.
  4. D
    Cisco IOS will reject the command because administrative distance values below 20 are reserved for internal system protocols.

Cevap

The static route will immediately enter the routing table and replace the eBGP default route as the primary path.
In Cisco IOS, route selection between different protocol sources is determined by Administrative Distance (AD), where lower values are preferred. The default AD for External BGP (eBGP) is 20. Because the engineer configured the static route with an AD of 15, the router considers the static path more trustworthy than the eBGP path (15<2015 < 20). Consequently, the static route immediately takes precedence in the routing table, failing to function as a floating backup.

Adım Adım Çözüm

1
Identify the default administrative distance (AD) of the primary routing source.
External BGP (eBGP) has a default administrative distance of 20.
Cisco IOS assigns specific default AD values to judge route trustworthiness among different routing sources.
2
Compare the configured static route AD against the primary route's AD.
The configured static route AD is 15, which is lower than the eBGP AD of 20 (15<2015 < 20).
Lower administrative distance values indicate higher trustworthiness in Cisco IOS forwarding selection logic.
3
Determine which route is installed into the routing table.
The static route with AD 15 overrides the eBGP route with AD 20 and is actively installed into the routing table.
To create a valid floating static backup, the backup route's AD must be strictly greater than the primary route's AD (e.g., AD > 20).

Anahtar Kavram

Floating Static Route Administrative Distance Requirements
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