1. BGP

2. BGP Border Gateway Protocol Routing between Autonomous Systems (ASes)
Currently version 4
RFC 4271
TCP port 179
Routing between Autonomous Systems (ASes)
Decentralised routing
Allows an AS to advertise that it exists
Each AS advertises routes it has, leaves decision about whether to use them or not to peer
Originally supported IPv4; these days multi-protocol

3. Autonomous System set of routers under same administrative control
owned/operated by same entity
identified by a unique number
autonomous system number (ASN)
historically a 16-bit value
public range:
private range:

4. Autonomous Systems
AS3
AS4
AS1
AS2
AS5
AS6

5. Autonomous Systems
AS1
AS2
eBGP
iBGP

6. BGP Path-Vector protocol
Routing information includes the AS path a route has traversed
A router prepends its AS number to any route it receives before advertising it
A router discards any AS path that includes its ASN in it
Why?

7. Autonomous Systems AS3 AS4 AS1 AS2 192.168.0.0/24 AS5 AS6

8. BGP message types OPEN KEEPALIVE UPDATE NOTIFICATION
Establish BGP session with peer; negotiate hold time, advise ASN.
KEEPALIVE
Periodic message sent so a router knows a peer is still up in absence of updates
UPDATE
Routes added or withdrawn
NOTIFICATION
Error condition encountered

9. BGP Message Format marker length type
Marker: all ones in most cases; can be used for MD5 authentication.
Length: bytes
Type: one of four values (open, update, notification, keepalive)

10. UPDATE Message
An exterior gateway protocol calls for incremental changes to routing table.
reduce volume of traffic to synchronise routing through the network
An UPDATE message advises
when new routes are available
Network Layer Reachability Information (NLRI)
when a route is withdrawn

11. IP address prefix
| Length (1 octet) |
| Prefix (variable) |
IPv4 routes are described in CIDR format and encoded as <Length, Prefix>; e.g.
/15
(15) (130) (216)

12. UPDATE Message +-----------------------------------------------------+
| Withdrawn Routes Length (2 octets) |
| Withdrawn Routes (variable) |
| Total Path Attribute Length (2 octets) |
| Path Attributes (variable) |
| Network Layer Reachability Information (variable) |
Section 4.3, UPDATE message format, RFC 4271

13. Withdrawing Routes
The prefixes to withdraw (if any) are specified in the withdrawn routes section
The receiver finds the appropriate prefix previously announced by the peer and removes that one
When a BGP connection is closed, all routes advertised from that connection are withdrawn by the peers

14. Advertising Routes
New routes, and their attributes, are specified in the NLRI and path attributes sections
Mandatory attributes:
origin: IGP, EGP, ?
AS path: seq. of ASes in path to prefix
next hop: IP address of router to use

15. Other Attributes
MED (Multi Exit Discriminator)
Local Pref
Community …

16. Attributes
| Attr. Flags |Attr. Type Code|
Bit 0: optional (0) / well-known (1)
Bit 1: transitive (1) / non-transitive (2)
Bit 2: partial (1) / complete (0)
Bit 3: attr. length octet (0) or two octets (1)

17. Classes of Attributes Well-known mandatory Well-known discretionary
BGP speaker MUST understand it and MUST include it in NLRI
Well-known discretionary
BGP speaker MUST understand it and MAY include it in NLRI
Optional transitive
BGP speaker MAY understand it but MUST forward attribute if received in NLRI – sets partial bit to 1
Optional non-transitive
BGP speaker MAY understand it but MUST NOT forward attribute received in NLRI

18. Multiprotocol BGP (MBGP)
BGP was designed to organise global IPv4 routing
MBGP: a set of extensions to enable other types of routes to be chucked around using the attributes section
Optional, non-transitive
Has to be negotiated at OPEN.
MP-Reach-NLRI (type 14)
MP-Unreach-NLRI (type 15)
RFC 4760

19. MP-Reach-NLRI
| Address Family Identifier (2 octets) |
| Subsequent Address Family Identifier (1 octet) |
| Length of Next Hop Network Address (1 octet) |
| Network Address of Next Hop (variable) |
| Reserved (1 octet) |
| Network Layer Reachability Information (variable) |
Source: Page 3, RFC 4760

20. BGP decision process Step Attribute
Controlled by local or neighbour AS? 1
Highest LocalPref
local 2
Lowest AS path length
neighbour 3
Lowest origin type
neither 4
Lowest MED 5
eBGP-learned over iBGP-learned 6
Lowest IGP cost to border router 7
Lowest router ID (to break ties)
Source: BGP routing policies in ISP networks, Caesar and Rexford.

21. BGP Communities Administrative tag used to associate routes together
Assists in implementing administrative routing policy
Optional transitive attribute.
Well-known communities:
No-Export; do not advertise outside of this AS
No-Advertise; do not advertise further
No-Export-Subconfed; do not advertise outside selected portion of AS.

22. Processing Route Advertisements Applying Policy
Import policy
Filter routes from going further
Append or modify attributes
Decision process
Export policy
Which neighbours will receive the route
Don’t want to carry traffic you aren’t being paid to carry
Source: BGP routing policies in ISP networks, Caesar and Rexford.

23. Routing Policy Import policy Protect network from bad advertisements
Advertising private or unallocated address space
Customer advertising default route to provider
Promote aggregation through filtering NLRI with prefixes longer than particular value

24. Configuring Local Policies
Preference
add/delete/modify route attributes
Filtering
Eliminate certain routes from consideration; control who they will be exported to;
Tagging
Associate additional state with a route using community attribute

25. Further Reading
BGP routing policies in ISP networks; Matthew Caesar and Jennifer Rexford
RFC 4760: Multiprotocol Extensions for BGP-4
RFC 4271: A Border Gateway Protocol 4 (BGP-4)

26. Next lecture reading RFC 4364 Section 3 to 4.3.2 (pp. 8-19)