{"id":814,"date":"2021-12-30T10:14:28","date_gmt":"2021-12-30T10:14:28","guid":{"rendered":"http:\/\/feralpacket.org\/?p=814"},"modified":"2022-03-01T16:02:46","modified_gmt":"2022-03-01T16:02:46","slug":"__trashed","status":"publish","type":"post","link":"https:\/\/feralpacket.org\/?p=814","title":{"rendered":"BGP Best Path Selection Algorithm"},"content":{"rendered":"

BGP Best Path Selection Algorithm<\/span><\/p>\n

http:\/\/www.cisco.com\/c\/en\/us\/support\/docs\/ip\/border-gateway-protocol-bgp\/13753-25.html<\/a><\/p>\n

BGP Cost Community<\/span><\/p>\n

https:\/\/www.cisco.com\/c\/en\/us\/td\/docs\/ios\/12_0s\/feature\/guide\/s_bgpcc.html<\/a><\/p>\n

Why Routers Ignore Paths:<\/span><\/p>\n

– path are marked \u201cnot synchronized\u201d in sh ip bgp long-prefixes<\/span> output<\/p>\n

– next hop is inaccessible<\/p>\n

– path from eBGP neighbor has local AS in AS_PATH<\/p>\n

– bgp enforce-first-as<\/span> is set and the update doesn\u2019t list the neighbor AS as the first AS<\/p>\n

– paths are marked \u201creceive only\u201d in sh ip bgp long-prefixes<\/span> output<\/p>\n

Best Path Algorithm:<\/span><\/p>\n

Note: if extended community cost is configured for pre-bestpath<\/span>, compare before anything else<\/p>\n

-> lowest is best<\/p>\n

-> set extcommunity cost pre-bestpath <community-id> <cost><\/span><\/p>\n

-> bgp bestpath cost-community ignore<\/span><\/p>\n

-> this command was intended only to assist in troubleshooting path selection<\/p>\n

– Important, next hop needs to be valid<\/span><\/p>\n

– It\u2019s not that difficult to remember whether higher or lower is best.  With the first two (weight and local_preference), highest is best.  Everything else, lowest, shortest, or oldest is best.<\/span><\/p>\n

– The steps below include the configuration commands that can modify the default behavior of each step.<\/span><\/p>\n

 1.  weight<\/span><\/p>\n

-> highest is best<\/p>\n

-> Cisco proprietary<\/p>\n

-> locally significant<\/p>\n

-> range 0 to 65,535<\/p>\n

-> default is 0 for received routes<\/p>\n

-> default is 32768 for locally generated routes<\/p>\n

-> route-map WEIGHT permit 10<\/span><\/p>\n

->   set weight 40000<\/span><\/p>\n

-> neighbor 1.1.1.1 route-map WEIGHT in<\/span><\/p>\n

-> neighbor 1.1.1.1 <\/span>default-originate <\/span>route-map WEIGHT in<\/span><\/p>\n

-> network 1.1.1.0 mask 255.255.255.0 route-map WEIGHT<\/span><\/p>\n

-> This works.  As does applying the route-map to the redistribute and aggregate-address commands.  But why would you do this to locally originated routes since it\u2019s only locally significant.<\/p>\n

 <\/span><\/p>\n

2.  local_preference<\/span><\/p>\n

-> highest is best<\/p>\n

-> significant only in the local AS<\/p>\n

-> range 0 to <\/span>4294967295<\/span><\/p>\n

     -> default 100 for every route<\/span><\/p>\n

-> bgp default local-preference 200<\/span><\/p>\n

-> route-map LP permit 10<\/span><\/p>\n

->   set local-preference 200<\/span><\/p>\n

-> neighbor 1.1.1.1 route-map LP in<\/span><\/p>\n

-> neighbor 1.1.1.1 <\/span>default-originate <\/span>route-map LP in<\/span><\/p>\n

-> redistribute eigrp 1 route-map LP<\/span><\/p>\n

-> redistribute ospf 1 route-map LP match internal external 1 external 2<\/span><\/p>\n

-> network 1.1.1.0 mask 255.255.255.0 route-map LP<\/span><\/p>\n

-> aggregate-address 1.1.2.0 255.255.255.252 attribute-map LP<\/span><\/p>\n

 3.  locally originated path<\/span><\/p>\n

-> locally generated routes are preferred over received routes<\/p>\n

-> network<\/span> command<\/p>\n

-> aggregate-address<\/span> command<\/p>\n

-> redistribute<\/span> command<\/p>\n

 4.  AS_PATH<\/span><\/p>\n

-> shortest is best<\/p>\n

-> bgp bestpath as-path ignore<\/span><\/p>\n

-> route-map PREPEND permit 10<\/span><\/p>\n

->   set as-path prepend 100 100<\/span><\/p>\n

-> neighbor 1.1.1.1 route-map PREPEND out<\/span><\/p>\n

 5.  ORIGIN<\/span><\/p>\n

-> lowest type is best<\/p>\n

-> IGP < EGP < INCOMPLETE<\/p>\n

 <\/span><\/p>\n

6.  multi-exit discriminator (MED)<\/span><\/p>\n

-> metric<\/p>\n

-> lowest is best<\/p>\n

-> range 0 to 65,535<\/p>\n

-> default is 0 for received routes<\/p>\n

-> when MED is not assigned<\/p>\n

-> set to the value of the IGP metric for routes injected into BGP<\/p>\n

-> network or redistribute commands<\/p>\n

-> used to influence a neighboring AS with multiple entry points<\/p>\n

-> or a neighboring AS may be influencing the local AS with multiple entry points<\/p>\n

     -> <\/span>compared only if the first AS in the AS_SEQUENCE is the same for multiple paths<\/span><\/p>\n

     -> confederation sub-ASs are ignored unless configured<\/span><\/p>\n

     -> <\/span>bgp deterministic-med<\/span><\/p>\n

         -> compare routes advertised by different peers in the same AS<\/span><\/p>\n

-> bgp always-compare-med<\/span><\/p>\n

         -> <\/span>compare routes advertised by different ASs.<\/span><\/p>\n

-> if used, must be configured in the entire AS to prevent routing loops<\/p>\n

     <\/span>-> bgp bestpath med confed<\/span><\/p>\n

         -> <\/span>compare all paths that consist only of AS_CONFED_SEQUENCE<\/span><\/p>\n

-> bgp bestpath med confed missing-as-worst<\/span><\/p>\n

-> bgp bestpath med missing-as-worst<\/span><\/p>\n

-> route-map MED permit 10<\/span><\/p>\n

->   set metric 100<\/span><\/p>\n

-> neighbor 1.1.1.1 route-map MED out<\/span><\/p>\n

-> neighbor 1.1.1.1 <\/span>default-originate <\/span>route-map MED out<\/span><\/p>\n

-> redistribute eigrp 1 metric 100<\/span><\/p>\n

-> redistribute ospf 1 metric 100 match internal external 1 external 2<\/span><\/p>\n

-> redistribute eigrp 1 route-map MED<\/span><\/p>\n

-> redistribute ospf 1 route-map MED match internal external 1 external 2<\/span><\/p>\n

-> network 1.1.1.0 mask 255.255.255.0 route-map MED<\/span><\/p>\n

-> aggregate-address 1.1.2.0 255.255.255.252 attribute-map MED<\/span><\/p>\n

 7.  eBGP over iBGP<\/span><\/p>\n

-> paths that contain as_confed_sequence and as_confed_set are considered internal<\/p>\n

 8.  IGP metric<\/span><\/p>\n

-> lowest is best<\/p>\n

-> bgp bestpath igp-metric ignore<\/span><\/p>\n

9.  extended community cost<\/span><\/p>\n

-> IGP point of insertion (POI)<\/p>\n

-> lowest is best<\/p>\n

-> route-map EXTCOMMUMITYCOST permit 10<\/span><\/p>\n

->   set extcommunity cost 1 1<\/span><\/p>\n

-> route-map EXTCOMMUMITYCOST permit 10<\/span><\/p>\n

->   set extcommunity cost igp 1 1<\/span><\/p>\n

-> neighbor 1.1.1.1 route-map <\/span>EXTCOMMUMITYCOST<\/span> out<\/span><\/p>\n

-> neighbor 1.1.1.1 <\/span>default-originate <\/span>route-map <\/span>EXTCOMMUMITYCOST<\/span> out<\/span><\/p>\n

-> redistribute eigrp 1 route-map <\/span>EXTCOMMUMITYCOST<\/span><\/p>\n

-> redistribute ospf 1 route-map <\/span>EXTCOMMUMITYCOST<\/span> match internal external 1 external 2<\/span><\/p>\n

-> network 1.1.1.0 mask 255.255.255.0 route-map <\/span>EXTCOMMUMITYCOST<\/span><\/p>\n

-> aggregate-address 1.1.2.0 255.255.255.252 attribute-map <\/span>EXTCOMMUMITYCOST<\/span><\/p>\n

10.  multipath<\/span><\/p>\n

-> if multiple paths match the above steps,<\/p>\n

-> determine if multiple paths require installation into the routing table<\/p>\n

-> default, only one best path is installed into the routing table<\/p>\n

-> maximum-paths 2<\/span><\/p>\n

-> maximum-paths eibgp 2<\/span><\/p>\n

-> maximum-paths ibgp 2<\/span><\/p>\n

 <\/span><\/p>\n

11.  oldest route<\/span><\/p>\n

-> when multiple paths are external, prefer the path that was received first<\/p>\n

-> oldest is best<\/p>\n

-> this step can be skipped by forcing BGP to compare router-ids<\/p>\n

-> bgp bestpath compare-router-id<\/span><\/p>\n

 12.  router-id<\/span><\/p>\n

-> lowest is best<\/p>\n

 <\/span><\/p>\n

13.  cluster length<\/span><\/p>\n

-> lowest is best<\/p>\n

-> router reflectors only<\/p>\n

 <\/span><\/p>\n

14.  neighbor address<\/span><\/p>\n

-> lowest is best<\/p>\n","protected":false},"excerpt":{"rendered":"

BGP Best Path Selection Algorithm<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[6],"tags":[39,31,10],"class_list":["post-814","post","type-post","status-publish","format-standard","hentry","category-ccie","tag-bgp","tag-published","tag-service-provider"],"_links":{"self":[{"href":"https:\/\/feralpacket.org\/index.php?rest_route=\/wp\/v2\/posts\/814","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/feralpacket.org\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/feralpacket.org\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/feralpacket.org\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/feralpacket.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=814"}],"version-history":[{"count":3,"href":"https:\/\/feralpacket.org\/index.php?rest_route=\/wp\/v2\/posts\/814\/revisions"}],"predecessor-version":[{"id":1000,"href":"https:\/\/feralpacket.org\/index.php?rest_route=\/wp\/v2\/posts\/814\/revisions\/1000"}],"wp:attachment":[{"href":"https:\/\/feralpacket.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=814"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/feralpacket.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=814"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/feralpacket.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=814"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}