This is part 1 of 4. My shortish summarized version of Cisco BGP Basics and Best Path Selection.
- 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.xmust 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
synchronizationenabled, there must be a match for the prefix in the IP routing table to be considered valid path.
Best Path Algorithm:
- MOST specific route
- Administrative distance
|“We Love Oranges AS Oranges Mean Pure Refreshment”|
|O||ORIGIN Code (IGP > EGP > Incomplete)|
|P||Paths (External > Internal)|
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Basic Show Commands:
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