notes/docs/lectures/acn/14_bgp.md

Border Gateway Protocol

Routing Protocols

The main job is to distribute the data to build forwarding tables

• These are intra-domain routing protocols
  • Or Interior gateway protocols
  • When the source and destination are inside the same network

It is important to distinguish between local and global protocols

• Interior vs Exterior Gateway Protocol (IGP vs EGP)

BGPv4

The internet inter-domain routing protocol

• Derives from GGP & EGP
• Deals in IP prefixes and autonomous systems
  • autonomous systems are purely administrative
  • Purpose is to enable policy to be applied
  • Only prefixes matter in the data-plane
• Internet policy domains
  • Logical construct only
  • No meaning outside BGP
  • Do not map simply onto ISPs or networks
• Currently ~493,000 prefixes & ~46,000 ASs
  • Because we have less ASs, the routing is easily -> less complex
  • Reduces complexity
  • Speeds up performance

BGP uses TCP as transport

• OPEN, UPDATE, KEEPALIVE, NOTIFICATION

Sessions between peers have:

• Simple capability negotiation
• Manage simultaneous OPEN connections
• Lose everything on session failure

Sessions and Routing Information Base (RIBs)

A BGP peer typically has many sessions

• Logically, for each peer, it receives the information about routing from peers, this is sorted into Adj-RIB-in table.
• After processing, it produces a Adj-RIB-out table which it sends to other peers
  • Advertisements received and to be sent
• Generates a local RIB table from Adj-RIB-in
  • Routes to use and potentially distribute
  • Resolved into per-port forwarding tables
• Generate Adj-RIB-out from Loc-RIB and policy

Update messages

• Incremental - indicate changes to state
  • These updates could be:
  • Withdrawn routes
  • Path attributes, common to all advertised routes
  • Advertised routes, known as NLRI
• There are ~27 path attributes
  • Only ~12 are in common use
  • Communicate information about prefixes
  • Used to apply policy in BGP decision process

Path Attributes

Mandatory - every AS has to inform the other ASs about these 3 attributes:

• Next hop
• AS Path
• Origin

Discretionary

• Local preferences
  • Allows prioritisation of ASs

Optional & transitive

• Aggregator
• Community
• Extended Communities

Optional & non-transitive

• Multi-exit discriminator
• Originator ID

Path Vectors

Distance Vector - prefer lowest cost path (not always)

Path Vector

• How do we know if an AS has seen this advert before
• Store the list of ASs in the packet
  • This is called the AS_PATH
• This way loops can be broken
  • If our ASN appears in a received AS_PATH, drop the advert

Decision Process

Drop prefix if:

• NEXT_HOP is unreachable via local routing table
• Local AS appears in AS_PATH (packet in a loop)

Then (commonly) apply following preference:

1. Higher weight (local to this router)
2. Highest LOCAL_PREF
3. Shortest AS_PATH
4. Lowest ORIGIN
5. Lowest MED
6. EGP to IGP (hot potato)
7. Shortest internal path
8. Prefer oldest route
   • Oldest routes are often most stable
9. Lowest interface IP address

Consistency

Learn external routes on EBGP sessions

• EBGP defined as peers having different ASNs
• Must ensure every router knows all external routes
• Redistribute external routes inside the network
  • Via IGP - only in small networks
  • via IBGP - gives full control over route distribution

Scaling

Can distribute IBGP routes on IBGP sessions

• Have to maintain N\cdot \frac{(N-1)}{2} IBGP sessions
  • Each carrying up to 490k routes x2 tables
• Two standard solutions
  1. Route Reflectors
     • Super nodes re-advertising IBGP routes
     • Allows for hierarchy
  2. AS Confederations
     • split AS up into mini-ASs

Failures

• Handling link failures
  • Bind to loopback
    • If it cant talk to other nodes, will only support communication internally
  • Flap damping
    • A warning message saying don't send traffic to me
    • This can make things worse if this message is delayed
• Process failures
  • Out of memory error due to too many routes

Network Inter-connection

• Networks interconnect via EBGP sessions
  • POPs - points of presence or IX internet exchanges
• Multi-homing
  • This is all logical
  • http://0x0.st/ooCs.png