Well-Known BGP Attributes

Attribute Classification

BGP attributes are classified into four categories based on their properties:

Well-Known Mandatory Attributes

These attributes must be present in every BGP update message:

1. ORIGIN Attribute (Type Code 1)

# Network statement (IGP origin)
router bgp 65001
 network 192.168.1.0 mask 255.255.255.0

# Redistribution (INCOMPLETE origin)
router bgp 65001
 redistribute ospf 1

# Route map to set origin
route-map SET-ORIGIN permit 10
 set origin igp

2. AS_PATH Attribute (Type Code 2)

3. NEXT_HOP Attribute (Type Code 3)

# Set next hop to self for iBGP
router bgp 65001
 neighbor 192.168.1.2 remote-as 65001
 neighbor 192.168.1.2 next-hop-self

# Route map to set next hop
route-map SET-NEXTHOP permit 10
 set ip next-hop 192.168.1.1

Well-Known Discretionary Attributes

These attributes are recognized by all BGP speakers but are not required:

4. LOCAL_PREF Attribute (Type Code 5)

# Set default local preference
router bgp 65001
 bgp default local-preference 200

# Route map to set local preference
route-map SET-LOCAL-PREF permit 10
 set local-preference 150

# Apply to neighbor
router bgp 65001
 neighbor 192.168.1.2 route-map SET-LOCAL-PREF in

5. ATOMIC_AGGREGATE Attribute (Type Code 6)

Attribute Processing Rules

Attribute Flags

Each attribute has flags that define its properties:

Attribute Manipulation

Well-known attributes can be modified using route maps:

# Route map to modify multiple attributes
route-map MODIFY-ATTRIBUTES permit 10
 match ip address prefix-list CUSTOMER-ROUTES
 set origin igp
 set local-preference 150
 set as-path prepend 65001 65001

route-map MODIFY-ATTRIBUTES permit 20
 set local-preference 100

# Apply to neighbor
router bgp 65001
 neighbor 192.168.1.2 route-map MODIFY-ATTRIBUTES in

Common Attribute Scenarios

route-map PRIMARY-ISP permit 10
 set local-preference 200

route-map BACKUP-ISP permit 10
 set local-preference 100

route-map PREPEND-PATH permit 10
 set as-path prepend 65001 65001 65001

Attribute Validation

Troubleshooting Attribute Issues

# Show BGP table with attributes
show ip bgp 192.168.1.0/24

# Show detailed path information
show ip bgp 192.168.1.0/24 longer-prefixes

# Show attributes for specific neighbor
show ip bgp neighbor 192.168.1.2 received-routes

# Debug attribute processing
debug ip bgp updates

Attribute Display Example

BGP table version is 15, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 192.168.1.0/24   192.168.1.2              0    100      0 65002 i
*  192.168.1.0/24   192.168.1.3              0     80      0 65003 65002 i

Path details:
  Path 1: Origin IGP, Local Preference 100, AS Path: 65002
  Path 2: Origin IGP, Local Preference 80, AS Path: 65003 65002

Best Practices

Attribute Security

Optional BGP Attributes

Optional Attribute Types

Optional attributes are divided into two categories:

Multi-Exit Discriminator (MED) - Type Code 4

# Set MED for outbound routes
route-map SET-MED-PRIMARY permit 10
 set metric 100

route-map SET-MED-BACKUP permit 10
 set metric 200

# Apply to neighbors
router bgp 65001
 neighbor 192.168.1.2 route-map SET-MED-PRIMARY out
 neighbor 192.168.1.3 route-map SET-MED-BACKUP out

# Always compare MED
router bgp 65001
 bgp always-compare-med

COMMUNITY Attribute - Type Code 8

Well-Known Communities

# Define community list
ip community-list standard CUSTOMER permit 65001:100
ip community-list standard PEER permit 65001:200
ip community-list standard TRANSIT permit 65001:300

# Set communities with route map
route-map SET-COMMUNITY permit 10
 match ip address prefix-list CUSTOMER-ROUTES
 set community 65001:100
 set community no-export additive

# Apply communities
router bgp 65001
 neighbor 192.168.1.2 send-community
 neighbor 192.168.1.2 route-map SET-COMMUNITY out

BGP Path Selection Process

Path Selection Overview

When BGP receives multiple routes to the same destination, it uses a step-by-step process to select the best path. The process stops at the first step that produces a clear winner.

BGP Path Selection Algorithm

The standard BGP path selection process follows these steps:

Path Selection Example

Routes to 192.168.1.0/24:

Path A: Local Pref=100, AS Path=65002, MED=50, eBGP
Path B: Local Pref=200, AS Path=65003 65004, MED=100, eBGP
Path C: Local Pref=100, AS Path=65002, MED=75, iBGP

Selection Process:
1. All paths valid ✓
2. No weight differences
3. Path B wins (Local Pref=200 > 100)

Selected Path: Path B

Local Preference

Local Preference Overview

Local Preference (LOCAL_PREF) is a well-known discretionary attribute that indicates to other BGP speakers within the same AS which path is preferred for outbound traffic.

Setting Local Preference

# Create route map to set local preference
route-map SET-LOCAL-PREF permit 10
 match ip address prefix-list PRIMARY-ROUTES
 set local-preference 200

route-map SET-LOCAL-PREF permit 20
 match ip address prefix-list BACKUP-ROUTES
 set local-preference 50

route-map SET-LOCAL-PREF permit 30
 set local-preference 100

# Apply to neighbor
router bgp 65001
 neighbor 192.168.1.2 route-map SET-LOCAL-PREF in

Common Local Preference Scenarios

Primary/Backup Links

# Primary ISP (higher local preference)
route-map PRIMARY-ISP permit 10
 set local-preference 200

# Backup ISP (lower local preference)
route-map BACKUP-ISP permit 10
 set local-preference 100

# Apply to neighbors
router bgp 65001
 neighbor 192.168.1.2 remote-as 65002
 neighbor 192.168.1.2 route-map PRIMARY-ISP in
 neighbor 192.168.1.3 remote-as 65003
 neighbor 192.168.1.3 route-map BACKUP-ISP in

BGP Communities

Community Overview

BGP Communities are optional transitive attributes that can be attached to routes to implement routing policies. They act as "tags" that can be used to group routes and apply consistent policies.

Well-Known Communities

# Enable community sending
router bgp 65001
 neighbor 192.168.1.2 send-community

# Set community with route map
route-map SET-COMMUNITY permit 10
 match ip address prefix-list CUSTOMER-ROUTES
 set community 65001:100

# Set multiple communities
route-map SET-COMMUNITY permit 20
 match ip address prefix-list PEER-ROUTES
 set community 65001:200 65001:300

# Add communities (preserving existing)
route-map ADD-COMMUNITY permit 10
 set community 65001:400 additive

Business Relationship Classification