geekywiki

I created this post to refresh myself with the basic topics of BGP as it’s been quite a while since ive tinkered with it.

Attributes
Origin – Must Pass to Everyone
* IGP = i (network)
* EGP = E (ebgp)
* Incomplete = ? (redistributed)

as_path – Must Pass to Everyone
* List of traveled AS through.
* May prepend own AS to influence routing

next_hop – Must Pass to Everyone
* Diff AS NLRI = EGP IP IP
* Same AS IGP Learned = IGP IP
* Diff AS IGP Learned = EGP IP

Local_Pref – Not Passed to other AS. Pased to same AS
* influence outgoing routes (higher is preferred)

MED – Passed to other AS if supported, but only once
* influence incoming routes (lower preferred)

Atomic_Aggregate – May pass
* Notifies downstream that a loss of path information has occurred
* downstream routers cannot make NLRI information of that route more specific

Aggregate – May Pass many times
* provides information about where aggregation was performed by including AS number and IP address of router that originated aggregate route

Community – May pass many times
* identifies a destination as a member of some community that share one or more properties.
* Default format of NN:AA with cisco. AA:NN with everyone else. (ip bgp community new-format)
*Internet – does not have community. all routes use this community by default
*NO_EXPORT – cannot be exported to EBGP peers or outside of the confederation
* NO_ADVERTISE – cannot be advertised to EBGP or IBGP peers
* LOCAL_AS – also called NO_EXPORT_SUBCONFED – cannot be advertised to EBGP, including peers in other AS within confederation

ORIGINATOR_ID – May pass, but once
* Used in route reflectors
* this is router ID of the originator of route in local AS
* Update dropped if router sees its own ID in this attribute

Cluster_List – May pass, but once
* Used in route reflectors
* sequence of cluster IDs through which route has passed
* Update dropped if router sees its own ID in this attribute

Weight – May not pass to any other router
* Used to influence outgoing on the local router only
* 0-65535. Highest preferred. Default of 0

AS_SEQUENCE – ordered list of AS numbers
AS_SET – unordered list of AS numbers. Used when aggregating addresses to avoid loops

BGP Criteria
# Weight (highest)
# Local_Pref (highest)
# Originated on this router (if possible)
# AS_Path (shortest)
# Origin Code (IGP > EGP > ?)
# MED (lowest – if same AS number to both routes)
# EBGP routes > Confed EBGP routes > IBGP routes
# Next_hop (shortest path to – lowest IGP metric)
# If multipath is enabled, install both routes
# Router_ID (lowest – only if multipath not enabled)

Cisco default route dampening penalties
Penalty – 1000 per flap
Suppress Limit – 2000 (when route becomes unadvertised)
Reuse Limit – 750 (when route becomes advertised once again)
Half Life – 15 mins
Max supress time – 60 mins or 4 times the half life (max time route is unadvertised)