BGP – Notes

IBGP and EBGP peer relationships:

• Timers
  • Hold time – 180 seconds
  • Keepalive – 60 seconds
  • Default min time between advertisement runs – 30 seconds
• Peer Groups
  • R1(config)#router bgp 100
  • R1(config-router)#neighbor <PEER GROUP NAME> peer-group
  • R1(config-router)#neighbor <PEER GROUP NAME> remote-as 100
  • R1(config-router)#neighbor <PEER GROUP NAME> update-source lo0
  • R1(config-router)#neighbor <PEER GROUP NAME> next-hop-self
  • Now apply below:
  • R1(config-router)#neighbor 10.1.1.1 peer-group <PEER GROUP NAME>
  • R1(config-router)#neighbor 192.168.1.1 peer-group <PEER GROUP NAME>
    • Efficient for smaller configurations.
    • Efficient for grouping configuration settings.

• Session Templates
  • Anything relevant to the session
    • timers, remote-as, update-source, ebgp-multihop
  • R1(config-router)#template peer-session <NAME>
  • R1(config-router)#inherit peer-session <NAME OF ALTERNATE PEERSESSION>
  • R1(config-router)#remote-as 100
  • R1(config-router)#update-source lo0
  • R1(config-router)#exit-peer-session
  • R1(config-router)#neighbor 10.12.12.2 inherit peer-session <NAME>
• Policy Templates
  • Anything relevant to policy
    • next-hop-self, filter-list; route-map
  • Inheritance is more flexible – inherit from up to 7 templates directly.
  • R1(config-router)#template peer-policy <NAME>
  • R1(config-router)#inherit peer-policy <NAME OF ALTERNATE PEERPOLICY>
  • R1(config-router)#next-hop-self
  • R1(config-router)#route-map <MAP NAME> out/in
  • R1(config-router)#exit-peer-policy
  • R1(config-router)#neighbor 10.12.12.2 inherit peer-policy <NAME>

• Dynamic Neighbors
  • Allows BGP peering to a group of remote neighbors that are defined by a range of IP addresses.
  • R1(config-router)#neighbor <NAME> peer-group
  • R1(config-router)#neighbor <NAME> remote-as 100
  • R1(config-router)#bgp listen range 10.10.10.0/24 peer-group <NAME>
  • R1(config-router)#neighbor <NAME> transport connection-mode passive
    • Allows routers falling in subnet 10.10.10.0/24 to initiate BGP session with R1.

• Network Command
  • Classful
    • R1(config)#router bgp 65000
    • R1(config-router)#network 1.0.0.0 – (classful, no mask required)
  • Classless
    • R1(config)#router bgp 65000
    • R1(config-router)#network 11.11.11.0 mask 255.255.255.0

• Private AS Numbers
  • 64512-65535 – 16 bit numbers
  • 4200000000 – 4294967294 – 32 bit numbers

Cisco Best PATH SELECTION:

• Highest Weight
• Highest Local Pref
• Prefer locally originated
• Shortest AS_Path
• Lowest origin type
• Lowest MED
• Prefer eBGP over iBGP
• Lowest IGP metric to the BGP NEXT_HOP
• Oldest path
• Lowest Router ID Source
• Min cluster list length
• Lowest neighbor address

Well-Known, Mandatory Attributes:

• AS-Path
• Origin Code
  • i – IGP
  • e – EGP
  • ? – Redistribution
• Next Hop Value

Well-Known, Discretionary

• Local Preference
  • Outbound traffic decisions
  • Default – 100
  • Higher is better
  • iBGP Transmitted, does not pass anywhere else.

• Atomic Aggregate

Optional, Transitive

• Aggregator
• Community

Optional, non-transitive

• Multi-Exit Discriminator
  • Advertising Metric on prefix information.
  • Lower Value preferred

• Originator ID
• Cluster List
• Cluster ID
• Weight