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Constructing the BGP Table
Joe Astorinowww.astorinonetworks.com
@jastorino
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BGP Table Summary
The BGP table contains all the prefixes learned by BGP and the
path attributes (PA) associated with those prefixes In BGP,
prefixes are technically called NLRI
The BGP table is actually comprised of three databases
Raw NLRI / PA information received from peers
Adj-RIBs-In
The best NLRIs after route policies have been applied
Loc-RIB
Routes BGP will advertise to other BGP peers
Adj-RIBs-Out
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BGP Table Sources
The local BGP table is built from three different sources
Network command
Local Routes
BGP Updates
iBGP or eBGP
Redistribution
Redistributed
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BGP Network Command
The network command adds routes to the local BGP table that the
router wishes to advertise into BGP and ultimately send to BGP
peers
The network command specifies a prefix and a mask. For the
prefix to be added to the BGP table, the prefix must match exactly
a route already existing in the routing table!
The prefix can match existing routes from connected, static or
IGPs
R1#show ip interface brief | i LoopbackLoopback0 1.1.1.1 YES
NVRAM up up
R1#show ip route 1.1.1.1Routing entry for 1.1.1.1/32Known via
"connected", distance 0, metric 0 (connected, via interface)
R1#sh run | section bgprouter bgp 65123network 1.1.1.1 mask
255.255.255.255
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Redistribution into BGP
Connected, static and IGP routes may be redistributed into
BGP
Many times the redistribution is tied to a route-map in order to
manipulate the attributes of the redistributed prefixes
By default, when redistributing OSPF into BGP, only intra and
inter-area routes are allowed. This can be changed during the
redistribution
ip prefix-list AS65123 seq 5 permit 172.16.123.0/24 ge 26 le
26ip prefix-list AS65123 seq 10 permit 10.0.123.0/29!route-map
CONNECTED-BGP permit 10match ip address prefix-list AS65123set
metric 100set community 65123:110!router bgp 65123network 1.1.1.1
mask 255.255.255.255redistribute connected route-map
CONNECTED-BGPredistribute ospf 1 match internal external 1 external
2 nssa-external
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Auto Summarization
auto-summary summarizes prefixes added to BGP at
classfulboundaries
The behavior is different depending on if the prefixes were
added with the network command or through redistribution
The default is no auto-summary since IOS 12.3
Redistribution
Redistribute only the classful network
Network Command
Inject more specific and summary
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BGP Path Attributes
A BGP path attribute (PA) is a characteristic of a BGP
prefix
Ultimately, BGP uses path attributes and the BGP best path
selection algorithm to select a valid and best path for each
NLRI
Path attributes are categorized into one of four groups
Well-known
mandatory
discretionary
Optional
transitive
nontransitive
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Path Attributes
Attribute Class
ORIGIN well-known mandatory
AS_PATH well-known mandatory
NEXT_HOP well-known mandatory
LOCAL_PREF well-known discretionary
ATOMIC_AGGREGATE well-known discretionary
AGGREGATOR optional transitive
COMMUNITY optional transitive
MULTI_EXIT_DISC optional nontransitive
ORIGINATOR_ID optional nontransitive
CLUSTER_LIST optional nontransitive
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ORIGIN Path Attribute
How was this prefix entered into BGP? Where did it come
from?
IGP (i) Prefix was learned internal to the AS Prefixes added to
BGP with the network command
EGP (e) Prefix was learned via EGP
Incomplete (?) Prefix was learned through some other method
Redistributed routes have an origin of incomplete
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ORIGIN Path Attribute
R1#show ip bgp | i 1.1.1.1BGP table version is 188, local router
ID is 1.1.1.1*> 1.1.1.1/32 0.0.0.0 0 32768 i
R1#show ip bgp 1.1.1.1 | b OriginOrigin IGP, metric 0, localpref
100, weight 32768, valid, sourced,
local, best
R1#show ip bgp | i 10.0.45.0*>i10.0.45.0/30 2.2.2.2 0 100 0
65004 ?
R1#show ip bgp 10.0.45.0 | b OriginOrigin incomplete, metric 0,
localpref 100, valid, internal, best
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AS_PATH Path Attribute
Generally, AS_PATH lists all the AS a prefix has gone
through
A router drops any BGP prefix that contains its own AS in the
AS_PATH
AS_PATH influences inbound routing (shortest AS_PATH wins)
Technically, AS_PATH can contain four different
sub-components
AS_SEQ Ordered list of AS numbers
AS_SET Unordered set of AS numbers enclosed in { } Used when
prefixes have been aggregated
AS_CONFED_SEQUENCE Similar to AS_SEQ but used in
confederations
AS_CONFED_SET Similar to AS_SET but used in confederations
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AS_PATH Path Attribute
Router#sh ip bgp | b 10.64*> 10.64.0.0/16 192.168.100.161 500
0 13979 21326 4809 65103 i*> 10.65.0.0/16 192.168.100.161 500 0
13979 21326 4809 65106 i*> 10.66.0.0/16 192.168.100.161 500 0
13979 21326 4809 65107 i*> 10.67.0.0/16 192.168.100.161 500 0
13979 21326 65109 i
Router#sh ip bgp 10.64.0.0BGP routing table entry for
10.64.0.0/16, version 2698Paths: (1 available, best #1, table
default)
Advertised to update-groups:1 2
Refresh Epoch 113979 21326 4809 65103, (aggregated by 65103
192.168.101.3)
192.168.100.161 from 192.168.100.161 (12.123.71.116)Origin IGP,
localpref 500, valid, external, atomic-aggregate, bestCommunity:
13979:5000
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LOCAL_PREF Path Attribute
Local Preference
Influences outbound path. Default is 100 and highest wins
Well-known discretionary attribute only sent between iBGP
peers
Diagram courtesy cisco.com
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Configuring LOCAL_PREF
ip prefix-list PREFER_ME_PLEASE permit 1.1.1.1/32!route-map
SET_LOCAL_PREF permit 10match ip address prefix-list
PREFER_ME_PLEASEset local-preference 500!router bgp 65123neighbor
10.0.45.1 route-map SET_LOCAL_PREF in
!R5#sh ip bgp 1.1.1.1BGP routing table entry for 1.1.1.1/32,
version 183Paths: (2 available, best #2, table
Default-IP-Routing-Table)
Advertised to update-groups:2
65004 651236.6.6.6 (metric 156160) from 6.6.6.6 (6.6.6.6)
Origin IGP, metric 0, localpref 100, valid, internal65004
65123
10.0.45.1 from 10.0.45.1 (172.16.46.193)Origin IGP, localpref
500, valid, external, best
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MULTI_EXIT_DISC Path Attribute
Multi-Exit-Discriminator (MED), sometimes known as metric
Optional nontransitive attribute that influences inbound
routing
Default value is 0 and lowest wins
MED is only compared when prefixes are coming from the same
AS!!!
Diagram courtesy cisco.com
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Configuring MED
ip prefix-list PREFER_ME_PLEASE permit 1.1.1.1/32!route-map
SET_MED permit 10match ip address prefix-list PREFER_ME_PLEASEset
metric 50!route-map SET_MED permit 20set metric 100!router bgp
65123neighbor 10.0.45.1 route-map SET_MED out
!R5#sh ip bgp 1.1.1.1BGP routing table entry for 1.1.1.1/32,
version 183Paths: (2 available, best #2, table
Default-IP-Routing-Table)
Advertised to update-groups:2
65004 651236.6.6.6 (metric 156160) from 6.6.6.6 (6.6.6.6)
Origin IGP, metric 50, localpref 100, valid, internal65004
65123
10.0.45.1 from 10.0.45.1 (172.16.46.193)Origin IGP, localpref
500, valid, external, best
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ATOMIC_AGGREGATE Path Attribute
When aggregating prefixes that came from different AS_PATHs,
AS_PATH information is lost
When aggregating, BGP sets the ATOMIC_AGGREGATE path attribute
Signals to receiving routers that AS_PATH information has been
lost
A router that receives a prefix with ATOMIC_AGGREGATE set cannot
advertise a more specific prefix of the aggregate, and must keep
ATOMIC_AGGREGATE set on updates
Additionally, the AGGREGATOR path attribute is set AGGREGATOR
specifies the RID and AS that aggregated the prefix
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Configuring BGP Aggregation
At least one subnet of the aggregate must be present
By default, BGP will advertise the aggregate and more specific
prefix
R5#sh ip bgp | i 172.16.56*> 172.16.56.0/26 0.0.0.0 0 32768
?*> 172.16.56.64/26 0.0.0.0 0 32768 ?*>i172.16.56.128/26
6.6.6.6 0 100 0 ?*>i172.16.56.192/26 6.6.6.6 0 100 0 ?
router bgp 65056aggregate-address 172.16.56.0 255.255.255.0
R5#sh ip bgp | i 172.16.56*> 172.16.56.0/26 0.0.0.0 0 32768
?*> 172.16.56.0/24 0.0.0.0 32768 i*> 172.16.56.64/26 0.0.0.0
0 32768 ?*>i172.16.56.128/26 6.6.6.6 0 100 0
?*>i172.16.56.192/26 6.6.6.6 0 100 0 ?
router bgp 65056aggregate-address 172.16.56.0 255.255.255.0
summary-only
R5#sh ip bgp | i 172.16.56s> 172.16.56.0/26 0.0.0.0 0 32768
?*> 172.16.56.0/24 0.0.0.0 32768 is> 172.16.56.64/26 0.0.0.0
0 32768 ?s>i172.16.56.128/26 6.6.6.6 0 100 0
?s>i172.16.56.192/26 6.6.6.6 0 100 0 ?
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AS_SET
Due to AS_PATH information being lost during aggregation,
routing loops are a possibility
The AS_SET path attribute helps address this by placing an
unordered list of all the AS from the more specific prefixes into
AS_SET
Additionally, AS_SEQ is still set to the aggregating routers
AS
No more need to set ATOMIC_AGGREGATE
AS_SET is dynamic If more specific routes downstream fail,
AS_SETcan be updated on the fly, and those changes are
advertised
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Configuring AS_SET
RouterC(config)#router bgp 300aggregate-adddress 160.0.0.0
255.0.0.0 summary-only as-set
RouterD# show ip bgp | b Network
Network Next Hop Metric LocPrf Weight Path*> 160.0.0.0/8
4.4.4.1 0 300 {200,100} i
Diagram courtesy cisco.com
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Adding a Default Route to the BGP Table
There are multiple ways to get a default route into the BGP
table
network 0.0.0.0 0.0.0.0/0 must already exist in the routing
table
default-information originate 0.0.0.0/0 must already exist in
the routing table Requires redistribution of 0.0.0.0/0 to work
neighbor default-originate Unconditionally advertises 0.0.0.0/0
to a specific neighbor
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Conditional Default Routing
ip prefix-list MUST_EXIST permit 10.1.1.1/32!route-map CONDITION
permit 10match ip address prefix-list MUST_EXIST
!Router bgp 65123neighbor 10.0.123.2 default-originate route-map
CONDITION
ip prefix-list MUST_EXIST permit 10.1.1.1/32ip prefix-list
DEFAULT_ROUTE permit 0.0.0.0/0!route-map MUST_EXIST permit 10match
ip address prefix-list MUST_EXIST
!route-map DEFAULT_ROUTEmatch ip address prefix-list
DEFAULT_ROUTE
!router bgp 65123neighbor 10.0.123.2 advertise-map DEFAULT_ROUTE
exist-map MUST_EXIST
Option 1: route-map
Option 2: advertise-map
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BGP Lab Diagrams
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Lab L2/L3 Diagram
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Constructing the BGP TableBGP Table SummaryBGP Table SourcesBGP
Network CommandRedistribution into BGPAuto SummarizationBGP Path
AttributesPath AttributesORIGIN Path AttributeORIGIN Path
AttributeAS_PATH Path AttributeAS_PATH Path AttributeLOCAL_PREF
Path AttributeConfiguring LOCAL_PREFMULTI_EXIT_DISC Path
AttributeConfiguring MEDATOMIC_AGGREGATE Path AttributeConfiguring
BGP AggregationAS_SETConfiguring AS_SETAdding a Default Route to
the BGP TableConditional Default RoutingBGP Lab DiagramsLab L2/L3
DiagramSlide Number 25
