Manual Summaries and the AS_PATH Path Attribute
As covered in the aftermost several pages, a router can add entries to its BGP table application the network
command and avenue redistribution. Additionally, BGP can use chiral avenue summarization to
advertise arbitrary routes to adjoining routers, causing the adjoining routers to learn
additional BGP routes. BGP chiral summarization with the aggregate-address command differs
significantly from application the auto-summary command. It can abridge based on any routes
in the BGP table, creating a arbitrary of any prefix length. It does not consistently abolish the
advertisement of the basic subnets, although it can be configured to do so.
The accumulated avenue charge accommodate the AS_PATH PA, aloof like it is appropriate for every added NLRI
in the BGP table. However, to absolutely accept what this command does, you charge to booty a closer
look at the AS_PATH PA.
The AS_PATH PA consists of up to four altered components, alleged segments, as follows:
■ AS_SEQ (short for AS Sequence)
■ AS_SET
■ AS_CONFED_SEQ (short for AS Confederation Sequence)
■ AS_CONFED_SET
The best frequently acclimated articulation is alleged AS_SEQ. AS_SEQ is the abstraction of AS_PATH as shown
back in Figure 11-1, with the PA apery all ASNs, in order, through which the avenue has been
advertised.
However, the aggregate-address command can actualize a arbitrary avenue for which the AS_SEQ
must be null. Back the basic subnets of the arbitrary avenue accept differing AS_SEQ values,
the router artlessly can’t actualize an authentic representation of AS_SEQ, so it uses a absent AS_SEQ.
However, this activity introduces the achievability of creating acquisition loops, because the capacity of
AS_PATH, accurately AS_SEQ, are acclimated to anticipate a avenue from actuality re-advertised to an AS
that has already heard about the route.
The AS_PATH AS_SET articulation solves the botheration back the arbitrary avenue has a absent AS_SEQ.
The AS_SET articulation holds an unordered account of all the ASNs in all the basic subnets’
AS_SEQ segments.
Example 11-7 shows an archetype in which the router does use a absent AS_SEQ for a arbitrary route,
and again the aforementioned arbitrary with the as-set advantage creating the AS_SET segment.
NOTE AS_PATH includes the AS_CONFED_SEQ and AS_CONFED_SET segments as
well, which are covered later, in the area “Confederations.”
Building the BGP Table 363
The afterward account summarizes the accomplishments taken by the aggregate-address command back it
creates a arbitrary route:
■ It does not actualize the arbitrary if the BGP table does not currently accept any routes for NLRI
inside the summary.
■ If all the basic subnets are aloof from the accumulation router’s BGP table, it also
then withdraws the aggregate. (In added words, the router tells its neighbors that the aggregate
route is no best valid.)
■ It sets the NEXT_HOP abode of the summary, as listed in the bounded BGP table, as 0.0.0.0.
■ It sets the NEXT_HOP abode of the arbitrary route, as advertised to neighbors, to the
router’s amend antecedent IP abode for anniversary neighbor, respectively.
■ If the basic subnets central the arbitrary all accept the aforementioned AS_SEQ, it sets the new
summary route’s AS_SEQ to be absolutely like the AS_SEQ of the basic subnets.
■ If the AS_SEQ of the basic subnets differs in any way, it sets the AS_SEQ of the new
summary avenue to null.
■ Back the as-set advantage has been configured, the router creates an AS_SET articulation for the
aggregate route, but alone if the arbitrary route’s AS_SEQ is null.
■ As usual, if the arbitrary is advertised to an eBGP peer, the router prepends its own ASN to
the AS_SEQ afore sending the Update.
■ It suppresses the advertisement of all basic subnets if the summary-only keyword is
used; advertises all of them if the summary-only keyword is omitted; or advertises a subset
if the suppress-map advantage is configured.
Example 11-7 shows R3 from Figure 11-4 summarizing 23.0.0.0/8. R3 advertises the summary
with ASN 123 as the alone AS in the AS_SEQ, because some basic subnets accept AS_PATHS
of 45, and others accept 678 45. As a result, R3 uses a absent AS_SEQ for the aggregate. The example
goes on to appearance the appulse of the as-set option.
Example 11-7 Avenue Accession and the as-set Option
! Note that R3’s routes to arrangement 23 all accept the aforementioned AS_PATH except one new
! prefix, which has an AS_PATH that includes ASN 678. As a result, R3 will
! actualize a absent AS_SEQ for the arbitrary route.
R3# appearance ip bgp | accommodate 23
*> 23.3.0.0/20 4.4.4.4 0 45 i
*> 23.3.16.0/20 4.4.4.4 0 45 i
*> 23.3.32.0/19 4.4.4.4 0 45 i
continues
364 Affiliate 11: BGP
*> 23.3.64.0/18 4.4.4.4 0 45 i
*> 23.3.128.0/17 4.4.4.4 0 45 i
*> 23.4.0.0/16 4.4.4.4 0 45 678 i
! The afterward command is now added to R3’s BGP configuration:
aggregate-address 23.0.0.0 255.0.0.0 summary-only
! Note: R3 will not accept a BGP table access for 23.0.0.0/8; however, R3 will
! acquaint this arbitrary to its peers, because at atomic one basic subnet
! exists.
! R1 has abstruse the prefix, NEXT_HOP 3.3.3.3 (R3’s amend antecedent IP abode for
! R1), but the AS_PATH is now absent because R1 is in the aforementioned AS as R3.
! (Had R3-R1 been an eBGP peering, R3 would accept prepended its own ASN.)
! Note that the abutting command is on R1 R1 R1 R1.
R1# sh ip bgp | activate Network
Network Abutting Hop Metric LocPrf Weight Path
*>i21.0.0.0 3.3.3.3 0 100 0 45 i
*>i23.0.0.0 3.3.3.3 0 100 0 i
! Next, R1 displays the AGGREGATOR PA, which identifies R3 (3.3.3.3) and its AS
! (123) as the accession point at which advice is lost. Also, the phrase
! “atomic-aggregate” refers to the actuality that the ATOMIC_AGGREGATE PA has also
! been set; this PA artlessly states that this NLRI is a summary.
R1# appearance ip bgp 23.0.0.0
BGP acquisition table access for 23.0.0.0/8, adaptation 45
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x800
Advertised to update-groups:
2
Local, (aggregated by 123 3.3.3.3), (received & used)
3.3.3.3 (metric 2302976) from 3.3.3.3 (3.3.3.3)
Origin IGP, metric 0, localpref 100, valid, internal, atomic-aggregate, best
! R6, in AS 678, receives the arbitrary avenue from R1, but the abridgement of information
! in the accepted AS_PATH allows R6 to apprentice of the route, possibly causing
! a acquisition loop. (Remember, one of the basic subnets, 23.4.0.0/16, came from
! ASN 678.)
R6# sh ip bgp nei 172.16.16.1 received-routes | activate Network
Network Abutting Hop Metric LocPrf Weight Path
*> 21.0.0.0 172.16.16.1 0 123 45 i
*> 23.0.0.0 172.16.16.1 0 123 i
! The R3 agreement is afflicted as apparent abutting to use the as-set option.
R3# aggregate-address 23.0.0.0 255.0.0.0 summary-only as-set
! R1 now has the AS_SET basic of the AS_PATH PA, which includes an unordered
! account of all autonmous systems from all the basic subnets’ AS_PATHs on R3.
R1# sh ip bgp | activate Network
Network Abutting Hop Metric LocPrf Weight Path
*>i21.0.0.0 3.3.3.3 0 100 0 45 i
*>i23.0.0.0 3.3.3.3 0 100 0 {45,678} i
Example 11-7 Avenue Accession and the as-set Advantage (Continued)
364 Affiliate 11: BGP
*> 23.3.64.0/18 4.4.4.4 0 45 i
*> 23.3.128.0/17 4.4.4.4 0 45 i
*> 23.4.0.0/16 4.4.4.4 0 45 678 i
! The afterward command is now added to R3’s BGP configuration:
aggregate-address 23.0.0.0 255.0.0.0 summary-only
! Note: R3 will not accept a BGP table access for 23.0.0.0/8; however, R3 will
! acquaint this arbitrary to its peers, because at atomic one basic subnet
! exists.
! R1 has abstruse the prefix, NEXT_HOP 3.3.3.3 (R3’s amend antecedent IP abode for
! R1), but the AS_PATH is now absent because R1 is in the aforementioned AS as R3.
! (Had R3-R1 been an eBGP peering, R3 would accept prepended its own ASN.)
! Note that the abutting command is on R1 R1 R1 R1.
R1# sh ip bgp | activate Network
Network Abutting Hop Metric LocPrf Weight Path
*>i21.0.0.0 3.3.3.3 0 100 0 45 i
*>i23.0.0.0 3.3.3.3 0 100 0 i
! Next, R1 displays the AGGREGATOR PA, which identifies R3 (3.3.3.3) and its AS
! (123) as the accession point at which advice is lost. Also, the phrase
! “atomic-aggregate” refers to the actuality that the ATOMIC_AGGREGATE PA has also
! been set; this PA artlessly states that this NLRI is a summary.
R1# appearance ip bgp 23.0.0.0
BGP acquisition table access for 23.0.0.0/8, adaptation 45
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x800
Advertised to update-groups:
2
Local, (aggregated by 123 3.3.3.3), (received & used)
3.3.3.3 (metric 2302976) from 3.3.3.3 (3.3.3.3)
Origin IGP, metric 0, localpref 100, valid, internal, atomic-aggregate, best
! R6, in AS 678, receives the arbitrary avenue from R1, but the abridgement of information
! in the accepted AS_PATH allows R6 to apprentice of the route, possibly causing
! a acquisition loop. (Remember, one of the basic subnets, 23.4.0.0/16, came from
! ASN 678.)
R6# sh ip bgp nei 172.16.16.1 received-routes | activate Network
Network Abutting Hop Metric LocPrf Weight Path
*> 21.0.0.0 172.16.16.1 0 123 45 i
*> 23.0.0.0 172.16.16.1 0 123 i
! The R3 agreement is afflicted as apparent abutting to use the as-set option.
R3# aggregate-address 23.0.0.0 255.0.0.0 summary-only as-set
! R1 now has the AS_SET basic of the AS_PATH PA, which includes an unordered
! account of all autonmous systems from all the basic subnets’ AS_PATHs on R3.
R1# sh ip bgp | activate Network
Network Abutting Hop Metric LocPrf Weight Path
*>i21.0.0.0 3.3.3.3 0 100 0 45 i
*>i23.0.0.0 3.3.3.3 0 100 0 {45,678} i
Example 11-7 Avenue Accession and the as-set Advantage (Continued)
Building the BGP Table 365
Table 11-6 summarizes the key credibility apropos summarization application the aggregate-address,
auto-summary, and arrangement commands.
Adding Absence Routes to BGP
The final adjustment covered in this affiliate for abacus routes to a BGP table is to inject absence routes
into BGP. Absence routes can be injected into BGP in one of three ways:
■ By injecting the absence application the arrangement command
■ By injecting the absence application the redistribute command
■ By injecting a absence avenue into BGP application the acquaintance neighbor-id default-information
[route-map route-map-name] BGP subcommand
When injecting a absence avenue into BGP application the arrangement command, a avenue to 0.0.0.0/0 must
exist in the bounded acquisition table, and the arrangement 0.0.0.0 command is required. The absence IP route
can be abstruse via any means, but if it is removed from the IP acquisition table, BGP removes the
default avenue from the BGP table.
Injecting a absence avenue through redistribution requires an added agreement command—
default-information originate. The absence avenue charge aboriginal abide in the IP acquisition table; for
instance, a changeless absence avenue to null0 could be created. Then, the redistribute changeless command
! Now R6 does not accept the 23.0.0.0 prefix due to R1’s analysis of the AS_SET PA,
! acquainted that ASN 678 is in the AS_SET and is additionally R6’s ASN.
R6# sh ip bgp nei 172.16.16.1 received-routes | activate Network
Network Abutting Hop Metric LocPrf Weight Path
*> 21.0.0.0 172.16.16.1 0 123 45 i
NOTE Arbitrary routes can additionally be added via addition method. First, the router would create
a changeless route, about with destination of interface null0. Then, the prefix/length can be
matched with the arrangement command to inject the summary. This adjustment does not clarify any
of the basic subnets.
Table 11-6 Summary: Injecting Arbitrary Routes in BGP
Command
Component
Subnets Removed Routes It Can Summarize
auto-summary (with
redistribution)
All Alone those injected into BGP on that router
using the redistribute command
aggregate-address All, none, or a subset Any prefixes already in the BGP table
auto-summary (with the
network command)
None Alone those injected into BGP on that router
using the arrangement command
Example 11-7 Avenue Accession and the as-set Advantage (Continued)
366 Affiliate 11: BGP
could be acclimated to redistribute that changeless absence route. However, in the appropriate case of the default
route, Cisco IOS additionally requires the default-information arise BGP subcommand.
Injecting a absence avenue into BGP by application the acquaintance neighbor-id default-information
[route-map route-map-name] BGP subcommand does not add a absence avenue to the bounded BGP
table; instead, it causes the advertisement of a absence to the defined neighbor. In fact, this method
does not alike analysis for the actuality of a absence avenue in the IP acquisition table by default, but it
can. With the route-map option, the referenced avenue map examines the entries in the IP routing
table (not the BGP table); if a avenue map admittance article is matched, again the absence avenue is advertised
to the neighbor. Archetype 11-8 shows aloof such an archetype on R1, with route-map check-default
checking for the actuality of a absence avenue afore R1 would arise a absence avenue to R3.
ORIGIN Path Attribute
Depending on the adjustment acclimated to inject a avenue into a bounded BGP table, BGP assigns one of three
BGP ORIGIN PA codes: IGP, EGP, or incomplete. The ORIGIN PA provides a accepted descriptor
as to how a accurate NLRI was aboriginal injected into a router’s BGP table. The appearance ip bgp
command includes the three accessible ethics in the fable at the top of the command output, listing
the absolute ORIGIN cipher for anniversary BGP avenue at the far appropriate of anniversary achievement line. Table 11-7 lists
the three ORIGIN cipher names, the single-letter abridgement acclimated by Cisco IOS, and the reasons
why a avenue is assigned a accurate code.
The ORIGIN codes and meanings adumbrate a few concepts that abounding bodies acquisition counterintuitive. First,
routes redistributed into BGP from an IGP absolutely accept an ORIGIN cipher of incomplete. Also, do
Example 11-8 Originating a Absence Avenue to a Acquaintance with the acquaintance default-originate Command
! The pertinent genitalia of the R1 agreement are listed next, with the avenue map
! analogous an IP avenue to 0.0.0.0/0 with a admittance action, enabling the
! advertisement of a absence avenue to acquaintance 3.3.3.3 (R3).
router bgp 123
neighbor 3.3.3.3 remote-as 123
neighbor 3.3.3.3 update-source Loopback1
neighbor 3.3.3.3 default-originate route-map check-default
!
ip avenue 0.0.0.0 0.0.0.0 Null0
!
ip prefix-list def-route seq 5 admittance 0.0.0.0/0
!
route-map check-default admittance 10
match ip abode prefix-list def-route
! R1 absolutely has a absence route, as apparent below.
R1# appearance ip avenue | accommodate 0.0.0.0/0
S* 0.0.0.0/0 is anon connected, Null0
! R3 now learns a absence avenue from R1, as apparent below.
R3# appearance ip bgp | activate Network
Network Abutting Hop Metric LocPrf Weight Path
*>i0.0.0.0 1.1.1.1 100 0 i
not abash EGP with eBGP; an ORIGIN of EGP refers to Exterior Gateway Protocol, the actual old
and deprecated antecedent to BGP. In practice, the EGP ORIGIN cipher should not be apparent today.
BGP ORIGIN Codes
ORIGIN Code
Cisco IOS
Notation
Used for Routes Injected Due to the Following
Commands
IGP i network, aggregate-address (in some cases), and neighbor
default-originate commands
EGP e Exterior Gateway Protocol (EGP). No specific commands apply.
Incomplete ? redistribute, aggregate-address (in some cases), and defaultinformation
originate command
The rules apropos the ORIGIN codes acclimated for arbitrary routes created with the aggregateaddress
command can additionally be a bit surprising. The rules are abbreviated as follows:
■ If the as-set advantage is not used, the accumulated avenue uses ORIGIN cipher i.
■ If the as-set advantage is used, and all basic subnets actuality abbreviated use ORIGIN cipher i,
the accumulated has ORIGIN cipher i.
■ If the as-set advantage is used, and at atomic one of the basic subnets has an ORIGIN cipher ?,
the accumulated has ORIGIN cipher ?.
NOTE The BGP ORIGIN PA provides a accessory descriptor for the agent of a BGP table entry,
which is acclimated as allotment of the BGP accommodation process.