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BGP

Key topics -BGP

RFC 4271 - A Border Gateway Protocol 4

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Key topics
Your go to book is "BGP design and Implementation by Zhang and ..."
Overview
The idea
Each AS has a number.
The route from one AS to another is described by an AS_PATH
The local AS number is not added to the AS_PATH until the route is advertised to the next AS
IGP= routing between node entities (Routers)
EGP=routing between AS entities (Autonomous Systems)
· IGPs announce networks and describe the cost to reach those networks.
· BGP announces AS pathways and the networks that are reachable at the end of the pathway.
BGP Characteristics
BGP is a path-vector protocol with the following enhancements:
· Reliable updates: BGP runs on top of TCP (port 179)
· Incremental, triggered updates only
· Periodic keepalive messages to verify TCP connectivity
· Rich metrics (called path vectors or attributes)
· Designed to scale to huge internetworks (e.g., the Internet)
· Uses variable TCP Window (uses bigger packets & more efficient update mechanism)
· carries a network mask for each advertised network
· supports both VLSM&CIDR ( classless interdomain routing)
BGP is categorized as an advanced distance vector protocol, but it is actually a path-vector protocol.
On Multihoming
BGP vs IGP
IGP
EGP
When to use BGP
Do not use BGP
BGP vs IGP Announcements
· IGPs announce networks and describe the cost to reach those networks.
· BGP announces AS pathways and the networks that are reachable at the end of the pathway. BGP describes the pathway by using attributes, which are similar to metrics.
BGP Internals
BGP processes and structures
BGP Communication
Neighbors
Two routers using BGP form a TCP connection with each other and exchange messages to open and confirm the connection parameters. These two BGP routers are called peer routers, or neighbors.
Peers can be either internal or external to the AS.
BGP Transmission
SYN (source Any, dest 179) ßà SYN/ACK (source 179, destin Any)
TCP uses a dynamic window,
TCP is designed to use a sliding window,
“BGP Connection Collision Detection”
Packet transmission-BGP vs IGPs
Manual adjacency
The finite machine diagram
BGP Message Types
Open message
· Version number:
· AS number:
· Holdtime:
· BGP router identifier (router ID):
· Optional parameters:
Keep alive message
Update message
· Withdrawn routes:
· Path attributes:
· Network-layer reachability information:
Notification message
Timers
Keep alive interval
Hold timer
Recommend ratio is Hd=KeepAlive/3
Keep alive negotiation when KA is not HT/3 and LKA!=RKA
Updates
Triggered updates
Batched Updates
Databases/Tables
Path Attributes
BGP metrics are called path attributes.
Well known
mandatory
they must be included in all BGP Update messages
ORIGIN
AS_PATH
NEXT_HOP
discretionary
they may or may not be sent in a specific Update message
LOCAL_PREF
ATOMIC_AGGREGATE
must be recognized
Optional
transitive
should be passed to its peers even if they are not supported
COMMUNITY
AGGREGATOR
nontransitive
-updates with these attributes are not passed farther
ORIGINATOR_ID
CLUSTER_LIST
MULTI_EXIT_DISC
not necessarily recognized
Administrative Weight
AS_SET
Peering
Peering conditions
AS Number
Update Source interface
Use of loop back
disable-connect-check
Multihop
TTL-security
Authentication
Peer group
Basic Routing
The BGP split horizon rule and Loop detection
Split horizon
Loop detection
BGP Path Selection Process
The BGP Decision Process
BGP path selections -criteria
Load balancing
Route Dampening
Adv. Routing
When a BGP-speaking router with no local policy configured receives NLRI from multiple IBGP sources for the same destination, the router chooses one IBGP path as the best path.
Recursive Route Lookup Process
Recursive lookup
CEF
Soft Switching: Fast switching, Optimum switching
Anouncing routes
BGP IGP Interaction
IGP BGP synchronization
Next Hop Behaviour
Default values for metrics when new routes are inserted
Summarizing
Redistributing
IGP to BGP
BGP to IGP
Conditional longer prefix injection
Route poisoning
BGP backdoor
Administrative Distance
Route filtering
Order of operations
AS_PATH filtering (neigh filter-list)
Prefix list filtering (neigh prefix-list)
Prefix Based Outbound Route Filtering (neigh capability orf prefix-list)
Route-map filtering (neigh route-map )
Applying filtering changes
Overview
Traditional Filtering Limitations (clear ip bgp * )
BGP Soft Reconfiguration ( clear ip bgp * soft in|out )
BGP Soft Reset Enhancement AKA Route Refresh
for inbound routing updates ( clear ip bgp * in|out )

Monitoring
Influencing Routing policies
Weight
Local preference
MED
Overview
Changing the Default MED
Advanced MED Configuration
bgp deterministic-med
always-compare-med
bgp bestpath-med confed
bgp bestpath-med missing-med-worst
Monitoring the MED
AS path prepending
BGP Hide Local-Autonomous System
Communities
Overview
In same cases using the MED to influence the preferred return path is not possible because the MED cannot be propagated across several autonomous systems.
• BGP communities are a means of tagging routes to ensure consistent filtering or route selection policy.
• Any BGP router can tag routes in incoming and outgoing routing updates or when doing redistribution
• Any BGP router can filter routes in incoming or outgoing updates or select preferred routes based on communities
• By default, communities are stripped in outgoing BGP updates.

Communities
Community-lists
Regular community
• The community attribute is a transitive optional attribute.
Its value is a 32-bit number (range 0 to 4,294,967,200).
Format: 32 bit decimal or 16bit:16bit decimals
Standard communities (well known communities)
Private communities
Extended community
Community attribute structure
Point of insertion (1B)
Community ID (1B)
Cost (4B)
Cost Community
Configuring communities
Route Tagging with BGP Communities
Configuring Community Propagation
Defining BGP Community-Lists
New Community format
Matching BGP Communities with Route-Maps
Monitoring Communities
• Communities are displayed in show ip bgp prefix printout.
• Communities are not displayed in debugging outputs.
• Routes in the BGP table tagged with a set of communities or routes matching a community-list can be displayed.
If you use the show ip bgp community-list command, all networks that are permitted by the community-list are listed.
DMZ Link Bandwidth
Scalability in transit AS
Overview
Scaling IGP and BGP in Service Provider Networks
Route reflectors and Clusters
Introducing Confederations
Configuring and Monitoring Confederations
Improving scalability and performance
Peer Groups
Peer groups and peer templates
Improving BGP convergence & Stability
Transiting AS
Customer to ISP connections
Scaling SP BGP networks
QA
References

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