From: Victor Cappuccio (cvictor@protokolgroup.com)
Date: Thu Oct 13 2005 - 15:18:00 GMT-3
Daniel, Many Thanks for this invaluable email, this is always useful to
know, when you are away from the DoCCD and someone asks you
Regards
Victor.
Chacko, Raj wrote:
>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" <cvictor@protokolgroup.com>
>To: "Cisco certification" <ccielab@groupstudy.com>
>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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