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BGP Basics and Best Path Selection - part1

This is part 1 of 4. My shortish summarized version of Cisco BGP Basics and Best Path Selection.






BGP Rules:

  • Path Vector protocol that routes AS to AS (AS-Path).
  • “You CAN’T tell someone else what to do whith their traffic!”
  • Use TCP port 179 to establish neighbor relationships.
  • Private AS range = 64512 – 65535
  • By default ONLY advertise best path.
  • Triggered updates ONLY (default 5 seconds INTERNAL and 30 seconds EXTERNAL).
  • Neighbor relationships form/converge in approximately 30 – 60 seconds (SLOW)
  • Default Hold timers: 180 seconds.
  • Loopback routes get default weight of 32768.
  • network x.x.x.x must be EXACT match as in routing table otherwise BGP will not advertise it.
  • iBGP does NOT modify any attributes like AS-Path or Next-hop, therefore has rules like split-horizon to prevent loops. (neighbor x.x.x.x next-hop-self).
  • iBGP, use loopbacks to form neighbor relationships.
  • By default sends BGP messages to eBGP neighbors with a TTL of 1
  • Supports up to 6 paths load-balanced.
  • If BGP synchronization enabled, there must be a match for the prefix in the IP routing table to be considered valid path.

Best Path Algorithm:

  1. MOST specific route
  2. Administrative distance
“We Love Oranges AS Oranges Mean Pure Refreshment”
W Weight (Highest)
L LOCAL_PREF (Highest)
O Originate (local)
AS AS_PATH (shortest)
O ORIGIN Code (IGP > EGP > Incomplete)
M MED (lowest)
P Paths (External > Internal)
R RID (lowest)

Basic Config:

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neighbor x.x.x.x remote-as <AS_NUM>
neighbor x.x.x.x update-source loopback0
neighbor x.x.x.x description <WHATEVER>
no synchronization
no auto-summary

Basic Show Commands:

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show ip bgp summary
show ip bgp

NOTES:

If eBGP neighbor is NOT directly connected (iBGP does NOT use same rules): neighbor x.x.x.x ebgp-multihop <HOP_COUNT_1-255>

Removing private AS number outbound: neighbor x.x.x.x remove-private-as

Authentication: (MUST match both sides of neighbor relationship)

  • neighbor x.x.x.x password 0 cisco (clear text)
  • neighbor x.x.x.x password 7 DFEAF10390E560AEA745CCBA53E044ED (MD5 hashed)

Advertise default route outbound: neighbor x.x.x.x default-originate

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