Bgp Routing Protocol

Core Workflow

Phase 1: Basic BGP Configuration

Establish BGP peering relationships:

! Basic eBGP configuration
router bgp 65001
 bgp router-id 1.1.1.1
 neighbor 10.0.0.2 remote-as 65002
 network 192.168.1.0 mask 255.255.255.0

! eBGP with loopback (requires ebgp-multihop)
router bgp 65001
 neighbor 2.2.2.2 remote-as 65002
 neighbor 2.2.2.2 update-source Loopback0
 neighbor 2.2.2.2 ebgp-multihop 3

! Static route for loopback reachability
ip route 2.2.2.2 255.255.255.255 Serial0/0

iBGP Configuration:

! iBGP peering (same AS)
router bgp 65001
 neighbor 3.3.3.3 remote-as 65001
 neighbor 3.3.3.3 update-source Loopback0
 neighbor 3.3.3.3 next-hop-self

Phase 2: BGP Path Selection Process

Understand the BGP best path selection algorithm:

Verification Commands:

show ip bgp
show ip bgp summary
show ip bgp neighbors
show ip bgp 192.168.1.0/24

Phase 3: Weight Attribute

Influence local path selection (Cisco-proprietary):

! Set weight for specific neighbor
router bgp 65001
 neighbor 10.0.0.2 weight 1000

! Set weight using route-map (preferred)
ip prefix-list NETWORK-A seq 5 permit 192.168.1.0/24

route-map SET-WEIGHT permit 10
 match ip address prefix-list NETWORK-A
 set weight 500
route-map SET-WEIGHT permit 20

router bgp 65001
 neighbor 10.0.0.2 route-map SET-WEIGHT in

Key Points:

• Default weight is 0 (32768 for locally originated)
• Higher weight preferred
• Only locally significant
• Not advertised to neighbors

Phase 4: Local Preference

Influence AS-wide path selection:

! Set local preference for incoming routes
ip prefix-list PREFER-PATH seq 5 permit 10.0.0.0/8

route-map SET-LOCPREF permit 10
 match ip address prefix-list PREFER-PATH
 set local-preference 200
route-map SET-LOCPREF permit 20

router bgp 65001
 neighbor 10.0.0.2 route-map SET-LOCPREF in

Key Points:

• Default local preference is 100
• Higher value preferred
• Exchanged between iBGP peers
• Used for outbound traffic engineering

Verification:

show ip bgp
! Look for LocPrf column

Phase 5: AS-Path Prepending

Influence inbound traffic by lengthening AS-path:

! Prepend own AS number to outgoing updates
route-map PREPEND permit 10
 set as-path prepend 65001 65001 65001

router bgp 65001
 neighbor 10.0.0.2 route-map PREPEND out

Key Points:

• Only prepend your own AS number
• Makes path less preferred to remote ASes
• Used for inbound traffic engineering
• Too much prepending can be ignored

Phase 6: MED (Multi-Exit Discriminator)

Influence inbound traffic between same AS connections:

! Set MED for outgoing routes
route-map SET-MED permit 10
 set metric 100

router bgp 65001
 neighbor 10.0.0.2 route-map SET-MED out

! Compare MED from different ASes (not default)
router bgp 65001
 bgp always-compare-med
 bgp bestpath as-path ignore

Key Points:

• Default MED is 0
• Lower MED preferred
• Only compared for paths from same AS by default
• Suggests preferred entry point to remote AS

Phase 7: Origin Attribute

Manipulate path preference via origin code:

! Set origin in route-map
route-map SET-ORIGIN permit 10
 set origin incomplete

route-map SET-ORIGIN-EGP permit 10
 set origin egp 1

router bgp 65001
 neighbor 10.0.0.2 route-map SET-ORIGIN out

Origin Codes:

• i - IGP (from network command) - Most preferred
• e - EGP (legacy) - Middle preference
• ? - Incomplete (redistributed) - Least preferred

Phase 8: BGP Confederations

Scale iBGP with sub-AS design:

! Sub-AS 65501 configuration
router bgp 65501
 bgp confederation identifier 65000
 bgp confederation peers 65502
 neighbor 10.0.0.2 remote-as 65502
 neighbor 10.0.0.2 next-hop-self
 neighbor 3.3.3.3 remote-as 65501

! Sub-AS 65502 configuration
router bgp 65502
 bgp confederation identifier 65000
 bgp confederation peers 65501
 neighbor 10.0.0.1 remote-as 65501
 neighbor 4.4.4.4 remote-as 65502

Key Points:

• Appears as single AS to external peers
• Sub-AS numbers typically private (65xxx)
• Confederation peers use special eBGP rules
• Reduces full-mesh iBGP requirement

Phase 9: Route Reflectors

Alternative iBGP scaling solution:

! Route Reflector configuration
router bgp 65001
 neighbor 2.2.2.2 remote-as 65001
 neighbor 2.2.2.2 route-reflector-client
 neighbor 3.3.3.3 remote-as 65001
 neighbor 3.3.3.3 route-reflector-client

! Client configuration (no special config needed)
router bgp 65001
 neighbor 1.1.1.1 remote-as 65001

Reflection Rules:

• Routes from eBGP peer → Reflect to all clients and non-clients
• Routes from client → Reflect to all clients and non-clients
• Routes from non-client → Reflect only to clients

Phase 10: BGP Communities

Tag routes for policy application:

! Enable community sending
router bgp 65001
 neighbor 10.0.0.2 send-community

! Set community on routes
route-map SET-COMMUNITY permit 10
 set community 65001:100

! Match community for filtering
ip community-list standard BLOCK permit 65001:999

route-map FILTER-COMMUNITY deny 10
 match community BLOCK
route-map FILTER-COMMUNITY permit 20

! Well-known communities
set community no-export       ! Don't advertise outside AS
set community no-advertise    ! Don't advertise to any peer
set community local-as        ! Don't advertise outside local AS

Peer Groups for Efficiency:

router bgp 65001
 neighbor INTERNAL peer-group
 neighbor INTERNAL remote-as 65001
 neighbor INTERNAL update-source Loopback0
 neighbor INTERNAL next-hop-self
 neighbor 2.2.2.2 peer-group INTERNAL
 neighbor 3.3.3.3 peer-group INTERNAL
 neighbor 4.4.4.4 peer-group INTERNAL

Quick Reference

BGP Attributes

Common Commands

Path Selection Shortcuts

Constraints and Limitations

Design Considerations

• iBGP requires full mesh or RR/Confederation
• eBGP peers must be directly connected (or ebgp-multihop)
• Synchronization rule with IGP
• Next-hop reachability critical

Security Implications

• BGP hijacking via unauthorized announcements
• Route leaks between providers
• Lack of built-in authentication (use MD5)
• Prefix filtering essential

Troubleshooting

Neighbor Not Establishing

Symptoms: State stuck in Active/Idle

Solutions:

1. Verify reachability between peers
2. Check AS number configuration
3. Verify update-source for loopback peering
4. Check ebgp-multihop for non-direct connections
5. Verify no ACL blocking TCP 179

Routes Not in Table

Symptoms: Neighbors up but missing routes

Solutions:

1. Verify network statement or redistribution
2. Check route-map filtering
3. Verify next-hop reachability
4. Check prefix-list/filter-list
5. Verify synchronization requirements

Path Selection Issues

Symptoms: Wrong path selected

Solutions:

1. Check weight settings (highest priority)
2. Verify local preference values
3. Compare AS-path lengths
4. Check MED values (if same AS)
5. Use show ip bgp <prefix> for detailed analysis