From: dusth@comcast.net
Date: Thu Oct 13 2005 - 14:58:04 GMT-3
Howdy, I don't know if I still want to dump all of this stuff in my brain when I'm in the lab. This bgp selection is under the first main link of the bgp documentation.
-------------- Original message --------------
> Man, I love this group!
> Thanks.
> Raj
>
> -----Original Message-----
> From: Daniel Kutchin [mailto:daniel@kutchin.com]
> Sent: Thursday, October 13, 2005 2:54 AM
> To: ccielab@groupstudy.com
> Subject: Re: Mnemonics for BGP Best Path Algorithm
>
> Victor,
>
> The last time I asked the group here are the 2 good ones that I got
>
> #--- 1 ---
>
> We - weight
> Love - local preference
> Algorithms - as-path
> On - Origin
> My - MED
> Router - router-id
>
>
>
> #--- 2 ---
>
> Discard all Worries before Leaving Rome As the Original Mis-information
> Sound's like a Neighbor's Idea.
>
> Discard = DISCARD unreachable next hop.
> Worries= highest WEIGHT
> Leaving=highest LOCALpreference
> Rome=Originated on this ROUTER
> As=shortest AS_PATH
> Original=ORIGIN code
> Mis-information=lowest MED
> Sound=SOURCE (external or internal)
> Neighbor's=Closet IGP NEIGHBOR
> Idea=lowest router ID
>
>
> Daniel
>
>
> ----- Original Message -----
> From: "Victor Cappuccio"
> To: "Cisco certification"
> Sent: Thursday, October 13, 2005 7:15 AM
> Subject: Mnemonics for BGP Best Path Algorithm
>
>
> Hello I'm looking for a Mnemonics for the BGP Best Path Algorithm, like
> for example when redistributing some routing protocol in EIGRP I always
> use this one BIG (bandwidth) Dogs (Delay) Really (Reliability) Like
> (Load) Me (Metric) (I read it some where and I always remember it), can
> you think one for BGP?
>
> http://www.cisco.com/univercd/cc/td/doc/cisintwk/ics/icsbgp4.htm
>
> Thanks ;)
> Victor.
>
>
> ---------
>
> 1._ Prefer the path with the highest WEIGHT.
>
> *Note: *WEIGHT is a Cisco-specific parameter, local to the router on
> which it's configured.
>
> 2._ Prefer the path with the highest LOCAL_PREF. Note the following:
>
> o_ Path without LOCAL_PREF is considered as having the value set with
> the *bgp default localpreference*
>
> command, or 100 by default.
>
> 3._ Prefer the path that was locally originated via a *network *or
> *aggregate *BGP subcommand, or through
>
> redistribution from an IGP. Local paths sourced by *network *or
> *redistribute *commands are preferred
>
> over local aggregates sourced by the *aggregate-address *command.
>
> 4._ Prefer the path with the shortest AS_PATH. Note the following:
>
> o_ This step is skipped if *bgp bestpath as-path ignore *is configured.
>
> o_ An AS_SET counts as 1, no matter how many ASs are in the set.
>
> o_ The AS_CONFED_SEQUENCE and AS_CONFED_SET are not included in the AS_PATH
>
> length.
>
> 5._ Prefer the path with the lowest origin type: IGP is lower than EGP,
> and EGP is lower than
>
> INCOMPLETE.
>
> 6._ Prefer the path with the lowest multi-exit discriminator (MED). Note
> the following:
>
> o_ This comparison is only done if the first (neighboring) AS is the
> same in the two paths; any
>
> confederation sub-ASs are ignored. In other words, MEDs are compared
> only if the first AS in the
>
> AS_SEQUENCE is the same for multiple paths. Any preceding
> AS_CONFED_SEQUENCE is
>
> ignored.
>
> o_ If *bgp always-compare-med *is enabled, MEDs are compared for all
> paths. This option needs to
>
> be enabled over the entire AS, otherwise routing loops can occur.
>
> o_ If *bgp bestpath med-confed *is enabled, MEDs are compared for all
> paths that consist only of
>
> AS_CONFED_SEQUENCE (paths originated within the local confederation).
>
> o_ Paths received from a neighbor with a MED of 4,294,967,295 will have
> the MED changed to
>
> 4,294,967,294 before insertion into the BGP table.
>
> o_ Paths received with no MED are assigned a MED of 0, unless *bgp
> bestpath missing-as-worst *is
>
> enabled, in which case they are assigned a MED of 4,294,967,294.
>
> o_ The *bgp deterministic med *command can also influence this step as
> demonstrated in the How
>
> BGP Routers Use the Multi-Exit Discriminator for Best Path Selection.
>
> 7._ Prefer external (eBGP) over internal (iBGP) paths. If bestpath is
> selected, go to Step 9 (multipath). 7.
>
> *Note: *Paths containing AS_CONFED_SEQUENCE and AS_CONFED_SET are local
> to the
>
> confederation, and therefore treated as internal paths. There is no
> distinction between Confederation
>
> External and Confederation Internal.
>
> 8._ Prefer the path with the lowest IGP metric to the BGP next hop.
> Continue, even if bestpath is already
>
> selected.
>
> 9._ Check if multiple paths need to be installed in the routing table
> for BGP Multipath. Continue, if
>
> bestpath is not selected yet.
>
> 10._When both paths are external, prefer the path that was received
> first (the oldest one). This step
>
> minimizes route-flap, since a newer path will not displace an older one,
> even if it would be the
>
> preferred route based on the next decision criteria (Steps 11, 12, and 13).
>
> Skip this step if any of the following is true:
>
> o_ The bgp best path compare-routerid command is enabled.
>
> *Note: *This command was introduced in Cisco IOS. Software Releases
> 12.0.11S, 12.0.11SC,
>
> 12.0.11S3, 12.1.3, 12.1.3AA, 12.1.3.T, and 12.1.3.E.
>
> o_ The router ID is the same for multiple paths, since the routes were
> received from the same router.
>
> o_ There is no current best path. An example of losing the current best
> path occurs when the neighbor
>
> offering the path goes down.
>
> Prefer the route coming from the BGP router with the lowest router ID.
> The router ID is the highest IP
>
> address on the router, with preference given to loopback addresses. It
> can also be set manually using
>
> the bgp router-id command.
>
> *Note: *If a path contains route-reflector (RR) attributes, the
> originator ID is substituted for the router ID
>
> in the path selection process.
>
> 12._If the originator or router ID is the same for multiple paths,
> prefer the path with the minimum cluster
>
> list length. This will only be present in BGP route-reflector
> environments. It allows clients to peer with
>
> RRs or clients in other clusters. In this scenario, the client must be
> aware of the RR-specific BGP
>
> attribute.
>
> 13._Prefer the path coming from the lowest neighbor address. This is the
> IP address used in the BGP
>
> neighbor configuration, and corresponds to the remote peer used in the
> TCP connection with the local
>
> router.
>
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