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FTOS Configuration Guide for the C-Series - Force10 Networks

FTOS Configuration Guide for the C-Series - Force10 Networks

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• A path with no AS_PATH configured has a path length of 0.<br />

• AS_CONFED_SET is not included in <strong>the</strong> AS_PATH length.<br />

• AS_CONFED_SEQUENCE has a path length of 1, no matter how many ASs are in <strong>the</strong><br />

AS_CONFED_SEQUENCE.<br />

4. Prefer <strong>the</strong> path with <strong>the</strong> lowest ORIGIN type (IGP is lower than EGP, and EGP is lower than<br />

INCOMPLETE).<br />

5. Prefer <strong>the</strong> path with <strong>the</strong> lowest Multi-Exit Discriminator (MED) attribute. The following criteria apply:<br />

• This comparison is only done if <strong>the</strong> first (neighboring) AS is <strong>the</strong> same in <strong>the</strong> two paths; <strong>the</strong><br />

MEDs are compared only if <strong>the</strong> first AS in <strong>the</strong> AS_SEQUENCE is <strong>the</strong> same <strong>for</strong> both paths.<br />

• If <strong>the</strong> bgp always-compare-med command is entered, MEDs are compared <strong>for</strong> all paths.<br />

• Paths with no MED are treated as “worst” and assigned a MED of 4294967295.<br />

6. Prefer external (EBGP) to internal (IBGP) paths or confederation EBGP paths.<br />

7. Prefer <strong>the</strong> path with <strong>the</strong> lowest IGP metric to <strong>the</strong> BGP next-hop.<br />

8. <strong>FTOS</strong> deems <strong>the</strong> paths as equal and does not per<strong>for</strong>m steps 9 through 11 listed below, if <strong>the</strong> following<br />

criteria is met:<br />

• <strong>the</strong> IBGP multipath or EBGP multipath are configured (maximum-path command)<br />

• <strong>the</strong> paths being compared were received from <strong>the</strong> same AS with <strong>the</strong> same number of ASs in<br />

<strong>the</strong> AS Path but with different NextHops<br />

• <strong>the</strong> paths were received from IBGP or EBGP neighbor respectively<br />

9. Prefer <strong>the</strong> path originated from <strong>the</strong> BGP router with <strong>the</strong> lowest router ID. For paths containing a Route<br />

Reflector (RR) attribute, <strong>the</strong> originator ID is substituted <strong>for</strong> <strong>the</strong> router ID.<br />

10. If two paths have <strong>the</strong> same router ID, prefer <strong>the</strong> path with <strong>the</strong> lowest cluster ID length. Paths without a<br />

cluster ID length are set to a 0 cluster ID length.<br />

11. Prefer <strong>the</strong> path originated from <strong>the</strong> neighbor with <strong>the</strong> lowest address. (The neighbor address is used in<br />

<strong>the</strong> BGP neighbor configuration, and corresponds to <strong>the</strong> remote peer used in <strong>the</strong> TCP connection with<br />

<strong>the</strong> local router.)<br />

After a number of best paths is determined, this selection criteria is applied to group’s best to determine <strong>the</strong><br />

ultimate best path.<br />

In non-deterministic mode (<strong>the</strong> bgp non-deterministic-med command is applied), paths are compared<br />

in <strong>the</strong> order in which <strong>the</strong>y arrive. This method can lead to <strong>FTOS</strong> choosing different best paths from a set of<br />

paths, depending on <strong>the</strong> order in which <strong>the</strong>y were received from <strong>the</strong> neighbors since MED may or may not<br />

get compared between adjacent paths. In deterministic mode, <strong>FTOS</strong> compares MED between adjacent<br />

paths within an AS group since all paths in <strong>the</strong> AS group are from <strong>the</strong> same AS.<br />

Weight<br />

The Weight attribute is local to <strong>the</strong> router and is not advertised to neighboring routers. If <strong>the</strong> router learns<br />

about more than one route to <strong>the</strong> same destination, <strong>the</strong> route with <strong>the</strong> highest weight will be preferred. The<br />

route with <strong>the</strong> highest weight is installed in <strong>the</strong> IP routing table.<br />

<strong>FTOS</strong> <strong>Configuration</strong> <strong>Guide</strong>, version 7.7.1.0 467

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