Introduction

Protocol Specifications

Protocol Type	Path vector
Peering mechanism	Manual peering between neighbors
eBGP AD	20
iBGP AD	200
Rights	Open standard
Supported protocols	IPv4, IPv6
Transport	TCP/179
Update mode	Only triggered
Timers	Hello (60 sec)
Authentication	None, MD5
Specifications	RFC 4271

Usage applications

BGP is needed for redundancy of servers. BGP is not used for providing redundancy to users for internet access.
Customer connected to multiple Internet service providers (ISPs).
Service provider networks (Transit autonomous system).
Network cores of very large enterprise networks (distribution or core layer)as a backup or redundant routing protocol due to its stability.

Customer's Network will become a Transit Autonomous system if ISP traffic passes through your AS.

Session Establishment facts

BGP neighbors are not discovered.
They must be configured manually on both sides of the connection.
TCP port 179 is used. 
Only one session remains if both connection attempts succeed.

Without tuning behaves like RIP, considers AS as a Hop.

BGP is an advanced Path Vector Protocol and has following advantages:

Reliable updates (using TCP)
Triggered updates only
Rich metric (Path attributes)
Scalable to massive networks
Updates are Incremental and Triggered

Only 1 instance of BGP can be run on a router. It will show an error if running it with another AS Number.

BGP States

Idle           Neighbor is not responding/Verifying Route to neighbor
Active         Attempting to connect
Connect        TCP session established
OpenSent       Open message sent
OpenConfirm    Neighbor replied with Open Message
Active(2nd)    Neighbor failed to reply or Mismatched Parameter
Established    Adjacency established

BGP Messages

Open            Starts the Session
Update          Network Reachability Exchanges
Keepalive       Sent every 60 seconds
Notification    Always indicate something is wrong

BGP Tables

Neighbor Table        Configured/Connected BGP Peers
BGP Table             List of All BGP Routes(Can be Huge)
Routing Table         List of Best Routes

BGP Route selection criteria^[1]

Mnemonics: N-WLLA-OMNI

Full Internet BGP routing table is more than 300K routes and a BGP router can receive multiple copies of that routing table from multiple providers, router has to compare those multiple entries and select only the best route for the routing table. It uses the BGP Best Path Selection Algorithm to do this. Routes installed by different BGP instances are compared by the general algorithm, i.e. route distances are compared and the route with lower distance is preferred.

Attribute	Which is better
Next Hop reachable	Route cannot be used if next hop is unreachable
Weight	Bigger
Local Preference	Bigger
Locally Injected	Locally injected is better than iBGP/eBGP learned
AS Path Length	Smaller
Origin	Prefer IGP over EGP & EGP over Unknown
MED	Smaller
Neighbor Type	Prefer eBGP over iBGP
IGP Metric to Next Hop	Smaller

Filter with Route Maps

Route maps are very powerful filtering tools, they can be used to accomplish the following tasks:

Filter on IP prefixes coming from a specific autonomous system
Filter on other BGP attributes
Modify BGP attributes

Match clauses in the BGP route map can be based on the following:

IP network numbers and subnet masks (prefix list or access list)
Route originator
Next hop
Origin code
Tag value attached to an Interior Gateway Protocol (IGP) route
Autonomous system path
Community
IGP route type

With a route map, the following can be set:

Origin
Next hop
Weight
Community
Local preference
MED

You can apply a route map on incoming or outgoing routing information for a neighbor.
The routing information must be permitted by the route map to be accepted.
If the route map has no statement explicitly permitting a route, the route is implicitly denied and dropped.

The syntax required is as follows:

Router(config-router)# neighbor ip-address route-map name in|out

What is a Route Reflector?^[2]

	This section is under construction.

Synchronization

Do not Use or Advertize a route learned by iBGP unless the same has been learned by IGP as well.
This is used to prevent the traffic form getting dropped by the intermediate routers.

Auto-Summarization

	This section is under construction.

Next Hop Processing

eBGP: Changes next hop address on advertized routes.
iBGP: Do not changes next hop address on advertized routes.
      iBGP was designed to be run in Frame Relay, Ethernet:

        [R1]    [R3]
          |       |
          ---------
              |
             [R2]

       Here if Peering is formed between R1-R2 & R2-R3.
       Traffic from R1 can reach R3 directly if the next hop IP is not changed.
       Else it needs to pass through R2 unnecessarily.
       Can be changed with: 
             # neighbor 1.1.1.1 next-hop-self

BGP Split Horizon

Do not send updates that you receive from iBGP to other iBGP peers

Peer Groups

neighbor IBGP_PEERS peer-group
neighbor IBGP_PEERS remote-as 5500 
neighbor IBGP_PEERS next-hop-self
neighbor IBGP_PEERS update-source lo1

neighbor 3.3.3.3 peer-group IBGP_PEERS
neighbor 2.2.2.2 peer-group IBGP_PEERS
neighbor 4.4.4.4 peer-group IBGP_PEERS

MED vs Local Preference

Multi-Exit Discriminator

The MED is an optional attribute that comes in handy when there are multiple entrance paths to an AS.
The remote AS sets MED values to tell the other AS which path to use.
The MED is passed between the two autonomous systems, but the value is not passed to any other ASs.
The path with the lowest MED is the preferred path.
This attribute is only used to influence entry INTO the AS.

Local Preference

LOCAL_PREF is a well-known attribute that is also used when multiple paths between autonomous systems exist.
The LOCAL_PREF attribute is just that… local and exclusive to the AS.
Routers within the local AS are told what path to use to exit that AS.
The local preference value is passed only among iBGP peers, and this value never leaves the local AS.

EBGP vs IBGP

EBGP	IBGP
Peering is between two different AS	Peering is between same AS
Routes learned from eBGP peer will be advertised to other peers(EBGP or IBGP)	Routes learned from IBGP peer will not be advertised to other IBGP peers, can be advertised to EBGP peer.
EBGP peers are set with TTL = 1, neighbors are assumed to be directly connected. Can change this behavior for EBGP by “neighbor x.x.x.x ebgp-multihop <TTL>”. Multihop is the term used in EBGP only.	For IBGP peers dont need to be directly connected.
Routes have AD of 20	Routes have AD of 200
Next hop is changed on advertised routers	Next-hop IP will not be changed when adv prefixes to another IBGP

Troubleshooting and Monitoring

Command	Description
show ip bgp neighbor ip-address	Displays detailed neighbor information
show ip bgp	Displays all the routes in the BGP table
show ip bgp ip-prefix [mask subnet-mask]	Displays detailed information about all paths for a single prefix
debug ip tcp transactions	Displays all TCP transactions
debug ip bgp events	Displays significant BGP events
debug ip bgp keepalives	Debugs BGP keepalive packets
debug ip bgp updates	Displays all incoming or outgoing BGP updates
debug ip bgp updates acl	Displays all incoming and sent updates matching an ACL
debug up bgp ip-address update [acl]	Displays all BGP updates received from or sent to a specific neighbor

BGP route not installing, route reasons:

Synchronisation is enabled & route unknown by IGP(run 'no sync' command)
Next Hop inaccessible (for iBGP run 'neighbor 1.1.1.1 next-hop-self' command)
AS path includes the local AS
Rejection by inbound policy

Blackhole formed in iGBP if all internal routers not running BGP, Solution:

Redistribute into IGP: Full Routing Table redistribution not possible, Redistribute partial routing table/specific routes.
Add a direct WAN Link between BGP Peers
Run iBGP between Peers
?? Configure Route Reflector

If any of the neighbors in below command output shows as in 'Active' state, it means some issue with the neighbor:

show ip bgp summary

Use Loopback interface for forming peers in router having multiple links.

When using eBGP, peers will not come up when using loopback as they need to be directly connected and should not have a Hop.
Use ebgp-multihop command to resovle this issue:
    # neighbor 1.1.1.1 ebgp-multihop 2

There are 2 ways to advertise networks into BGP:

Network Command
Redistribution

When using Network command:

Below command will advertize 50.0.0.0/8 into BGP

network 50.0.0.0

Therefore advertize exact subnet only:

network 50.1.1.0 mask 255.255.255.0

If the carot sign '>' is missing, the route is not the best one, so not installed in routing table:

 * valid, > best, i - internal, r RIB-failure

   Network          Next Hop            Metric LocPrf Weight Path
*> 10.1.1.1/32      0.0.0.0                  0         32768 i
*>i10.2.2.2/32      172.16.1.2               0    100      0 i

R&S Quick Notes

When using Communities, don’t forget “neighbor send-community”
Know your attributes and the direction which applied, when to used what.
“aggregate address” needs a more specific prefix in the BGP table for aggregate to be advertised.
Synchronization issue has 3 solutions, 1- Load BGP on all transit routers, 2- GRE tunnel, 3- Redistribution BGP>IGP.
“no bgp nexthop trigger” – Disables next-hop tracking between scanner intervals.
“no bgp fast-ext-fallover” – Force the router to wait for the dead-timer to expire, before generating notification messages , when a connected peer goes down.
“neighbor fall-over” – Will check neighbor connenctivity between scanner intervals, aka BGP Fast Peering.
Only the Holdtime is sent in update-msg. Two neighbors will use the lowest holdtime and then calculate the keepalive from that.
Know your Regular Expressions
Know the difference between Peer-Groups and Peer-Templates

LAB

BGP Basic Lab

GNS3 File: File:cbt nuggets bgp lab.zip

Objectives

Configure iBGP & eBGP
Establish Neighbors using Loopback interfaces
Using Update-Source command
Using eBGP-Multihop command
Advertising Networks into BGP
Turn off BGP Auto-Summary
BGP Synchronization
BGP Handling of Next Hop Address

Configurations

R1 Config

!
interface Loopback1
 ip address 1.1.1.1 255.255.255.255
!
!
interface Serial1/0
 ip address 10.1.13.1 255.255.255.0
 serial restart-delay 0
!
interface Serial1/1
 ip address 10.1.12.1 255.255.255.0
 serial restart-delay 0
!
!
router ospf 1
 log-adjacency-changes
 network 0.0.0.0 255.255.255.255 area 0
!
router bgp 5500
 no synchronization
 bgp log-neighbor-changes
 neighbor IBGP_PEERS peer-group
 neighbor IBGP_PEERS remote-as 5500
 neighbor IBGP_PEERS update-source Loopback1
 neighbor IBGP_PEERS next-hop-self
 neighbor 2.2.2.2 peer-group IBGP_PEERS
 neighbor 3.3.3.3 peer-group IBGP_PEERS
 neighbor 4.4.4.4 remote-as 5500
 neighbor 4.4.4.4 update-source Loopback1
 no auto-summary
!

R2 Config

!
interface Loopback1
 ip address 2.2.2.2 255.255.255.255
!
!
interface Serial1/0
 ip address 10.1.24.1 255.255.255.0
 serial restart-delay 0
!
interface Serial1/1
 ip address 10.1.12.2 255.255.255.0
 serial restart-delay 0
!
!
router ospf 1
 log-adjacency-changes
 network 0.0.0.0 255.255.255.255 area 0
!
router bgp 5500
 no synchronization
 bgp log-neighbor-changes
 neighbor IBGP_PEERS peer-group
 neighbor IBGP_PEERS remote-as 5500
 neighbor IBGP_PEERS update-source Loopback1
 neighbor IBGP_PEERS next-hop-self
 neighbor 1.1.1.1 peer-group IBGP_PEERS
 neighbor 3.3.3.3 peer-group IBGP_PEERS
 neighbor 4.4.4.4 peer-group IBGP_PEERS
 no auto-summary
!

R3 Config

!
interface Loopback1
 ip address 3.3.3.3 255.255.255.255
!
!
interface Serial1/0
 ip address 10.1.13.2 255.255.255.0
 serial restart-delay 0
!
interface Serial1/1
 ip address 10.1.34.1 255.255.255.0
 serial restart-delay 0
!
!
router ospf 1
 log-adjacency-changes
 network 0.0.0.0 255.255.255.255 area 0
!
router bgp 5500
 no synchronization
 bgp log-neighbor-changes
 neighbor IBGP_PEERS peer-group
 neighbor IBGP_PEERS remote-as 5500
 neighbor IBGP_PEERS update-source Loopback1
 neighbor IBGP_PEERS next-hop-self
 neighbor 1.1.1.1 peer-group IBGP_PEERS
 neighbor 2.2.2.2 peer-group IBGP_PEERS
 neighbor 4.4.4.4 peer-group IBGP_PEERS
 no auto-summary
!

R4 Config

!
interface Loopback1
 ip address 4.4.4.4 255.255.255.255
!
!
interface Serial1/0
 ip address 10.1.24.2 255.255.255.0
 serial restart-delay 0
!
interface Serial1/1
 ip address 10.1.34.2 255.255.255.0
 serial restart-delay 0
!
interface Serial1/2
 ip address 10.1.45.1 255.255.255.0
 serial restart-delay 0
!
!
router ospf 1
 log-adjacency-changes
 network 0.0.0.0 255.255.255.255 area 0
!
router bgp 5500
 no synchronization
 bgp log-neighbor-changes
 neighbor IBGP_PEERS peer-group
 neighbor IBGP_PEERS remote-as 5500
 neighbor IBGP_PEERS update-source Loopback1
 neighbor IBGP_PEERS next-hop-self
 neighbor 1.1.1.1 peer-group IBGP_PEERS
 neighbor 1.1.1.1 update-source Loopback1
 neighbor 2.2.2.2 peer-group IBGP_PEERS
 neighbor 3.3.3.3 peer-group IBGP_PEERS
 neighbor 5.5.5.5 remote-as 6500
 neighbor 5.5.5.5 ebgp-multihop 2
 neighbor 5.5.5.5 update-source Loopback1
 no auto-summary
!
ip route 5.5.5.5 255.255.255.255 10.1.45.2
!
!

R5 Config

!
interface Loopback0
 ip address 200.1.1.1 255.255.255.255
!
interface Loopback1
 ip address 200.1.2.1 255.255.255.255
!
interface Loopback2
 ip address 200.1.3.1 255.255.255.255
!
interface Loopback3
 ip address 200.1.4.1 255.255.255.255
!
interface Loopback4
 ip address 200.1.5.1 255.255.255.255
!
interface Loopback5
 ip address 200.1.6.1 255.255.255.255
!
interface Loopback6
 ip address 50.1.1.1 255.255.255.0
!
interface Loopback7
 ip address 5.5.5.5 255.255.255.255
!
interface Serial1/2
 ip address 10.1.45.2 255.255.255.0
 serial restart-delay 0
!
!
router bgp 6500
 no synchronization
 bgp log-neighbor-changes
 network 50.1.1.0 mask 255.255.255.0
 redistribute connected route-map FILTER
 neighbor 4.4.4.4 remote-as 5500
 neighbor 4.4.4.4 ebgp-multihop 2
 neighbor 4.4.4.4 update-source Loopback7
 no auto-summary
!
ip route 4.4.4.4 255.255.255.255 10.1.45.1
!
!
!
access-list 50 permit 200.1.1.1
access-list 50 permit 200.1.2.1
access-list 50 permit 200.1.3.1
access-list 50 permit 200.1.4.1
!
route-map FILTER permit 10
 match ip address 50
!

BGP Attributes Lab

Objectives

Configure below Attributes:

Weight
AS-Path
Next Hop Address
Origin
Local Preference
Metric

References

↑ www.accenture.com
↑ www.accenture.com

{{#widget:DISQUS |id=networkm |uniqid=BGP |url=https://aman.awiki.org/wiki/BGP }}

[1] www.accenture.com

[2] www.accenture.com

[1]

[2]

Introduction

BGP States

BGP Messages

BGP Tables

BGP Route selection criteria[1]

Filter with Route Maps

What is a Route Reflector?[2]

Synchronization

Auto-Summarization

Next Hop Processing

BGP Split Horizon

Peer Groups

MED vs Local Preference

EBGP vs IBGP

Troubleshooting and Monitoring

R&S Quick Notes

LAB

BGP Basic Lab

Objectives

Configurations

BGP Attributes Lab

Objectives

References

BGP Route selection criteria^[1]

What is a Route Reflector?^[2]