Injecting Routes Prefixes into the BGP Table

Unsurprisingly, an individual BGP router adds entries to its local BGP table by using the same general methods used by IGPs: by using the network command, by hearing the topology information via an Update message from a neighbor, or by redistributing from another routing protocol. The next few sections show examples of how a local BGP router adds routes to the BGP table by methods other than learning them from a BGP neighbor.

The BGP network Command

This section, and the next section, assumes the BGP no auto-summary command has been configured. Note that as of the Cisco IOS Software Release 12.3 Mainline, no auto-summary is the default; earlier releases defaulted to use auto-summary. Following that, the section, "The Impact of

KEY POINT

Auto-Summary on Redistributed Routes and the network Command," discusses the impact of the auto-summary command on both the network command and the redistribute command.

The BGP network router subcommand differs significantly from the network command used by IGPs. The BGP network command instructs that router's BGP process to do the following:

Look for a route in the router's current IP routing table that exactly matches the parameters of the network command; if the IP route exists, put the equivalent NLRI into the local BGP table.

With this logic, connected routes, static routes, or IGP routes could be taken from the IP routing table and placed into the BGP table for later advertisement. When the router removes that route from its IP routing table, BGP then removes the NLRI from the BGP table, and notifies neighbors that the route has been withdrawn.

Note that the IP route must be matched exactly when the no auto-summary command is configured or used by default.

Table 12-5 lists a few of the key features of the BGP network command, whose generic syntax is:

network {network-number [mask network-mask]} [route-map map-tag] Table 12-5 Key Features of the BGP network Command network {network-number [mask network-mask]} [route-map map-tag] Table 12-5 Key Features of the BGP network Command

KEY POINT

Feature

Implication

No mask is configured

Assumes the default classful mask.

Matching logic with no auto-summary configured

An IP route must match both the prefix and prefix length (mask).

Matching logic with auto-summary configured

If the network command lists a classful network, it matches if any subnets of the classful network exist.

NEXT_HOP of BGP route added to the BGP table*

Uses next hop of IP route.

Maximum number injected by the network command into one BGP process

200

Purpose of the route-map option on the network command

Can be used to filter routes and manipulate PAs, including NEXT_HOP*.

NEXT_HOP is a BGP PA that denotes the next-hop IP address that should be used to reach the NLRI.

KEY POINT

NEXT_HOP is a BGP PA that denotes the next-hop IP address that should be used to reach the NLRI.

Example 12-4 shows an example network command as implemented on R5 of Figure 12-4 (R5's BGP neighbors have been shut down so that the BGP table shows only BGP table entries created by the network commands on R5). In Example 12-4, R5 uses two network commands to add 21.0.0.0/8 and 22.1.1.0/24 to its BGP table.

Figure 12-4 Sample BGP Network, with IP Addresses

Figure 12-4 Sample BGP Network, with IP Addresses

EIGRP 6

R9 1

Networks 30 Through 39

Via redistribute eigrp 6 Command

Example 12-4 Examples of Populating the BGP Table via the network Command

! On R5, the network commands specifically match prefixes 21.0.0.0/8 and 22.1.1.0/24. The ! omission of the mask on the first command implies the associated classful mask ! of 255.0.0.0, as the IP address listed (21.0.0.0) is a class A address, router bgp 45 no synchronization bgp log-neighbor-changes network 21.0.0.0

network 22.1.1.0 mask 255.255.255.0

! The neighbor commands are not shown, as they are not pertinent to the topics ! covered in this example.

! Next, the two routes matched by the network commands are indeed in the IP ! routing table. Note that the route to 21.0.0.0/8 is a connected route, and the ! route to 22.1.1.0/24 is a static route. R5# show ip route | incl 21 | 22

Example 12-4 Examples of Populating the BGP Table via the network Command (Continued)

C 21.0.0.0/8 is directly connected, Loopback20

22.0.0.0/24 is subnetted, 1 subnets S 22.1.1.0 [1/0] via 10.1.5.9

! Below, the prefixes have been added to the BGP table. Note that the NEXT_HOP ! PA has been set to 0.0.0.0 for the route (21.0.0.0/8) that was taken from a ! connected route, with the NEXT_HOP for 22.1.1.0/24 matching the IP route. R5# show ip bgp

BGP table version is 38, local router ID is 5.5.5.5

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

Redistributing from an IGP, Static, or Connected Route

The BGP redistribute subcommand can redistribute static, connected, and IGP-learned routes. The mechanics of the BGP redistribute command work very similarly with redistribution as covered in Chapter 11, "IGP Route Redistribution, Route Summarization, and Default Routing"; however, this section covers a few nuances that are unique to BGP.

BGP does not use the concept of calculating a metric for each alternate route to reach a particular prefix. Instead, BGP uses a step-wise decision process that examines various PAs to determine the best route. As a result, redistribution into BGP does not require any consideration of setting metrics. However, as covered in Chapter 13, a router might need to apply a route map to the redistribution function to manipulate PAs, which in turn affects the BGP decision process. If a metric is assigned to a route injected into BGP, BGP assigns that metric value to the BGP Multi-Exit Discriminator (MED) PA, which is commonly referred to as metric.

NOTE Although this point is not unique to BGP, keep in mind that redistribution from an IGP causes two types of routes to be taken from the routing table—those learned by the routing protocol, and those connected routes for which that routing protocol matches with a network command.

Example 12-5 shows R6 (from Figure 12-4) filling its BGP table through route redistribution from Enhanced IGRP (EIGRP) process 6 (as configured in Example 12-5 with the router eigrp 6 command) and redistributing a single static route. EIGRP on R6 learns routes only for networks 30 through 39. The goals of this example are as follows:

■ Redistribute EIGRP routes for networks 31 and 32

■ Redistribute the static route to network 34, and set the MED (metric) to 9

■ Do not accidentally redistribute the connected routes that are matched by EIGRP's network commands

■ Use the Cisco IOS 12.3 default setting of no auto-summary

Example 12-5 shows the mistake of accidentally redistributing additional routes—the connected subnets of network 10.0.0.0 matched by EIGRP network commands. Later in the example, a route map is added to prevent the problem.

Example 12-5 Example of Populating the BGP Table via Redistribution

R6 redistributes EIGRP 6 routes and static routes below, setting the metric on redistributed static routes to 9. Note that EIGRP 6 matches subnets 10.1.68.0/24 and 10.1.69.0/24 with its network command, router bgp 678 redistribute static metric 9 redistribute eigrp 6

router eigrp 6 network 10.0.0.0

Commands unrelated to populating the local BGP table are omitted.

R6 has met the goal of injecting 31 and 32 from EIGRP, and 34 from static.

It also accidentally picked up two subnets of 10.0.0.0/8 because EIGRP's network

_10.0.0.0 command matched these connected subnets.

R6# show ip bgp

BGP table version is 1, local router ID is 6.6.6.6

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

! Below, note the metrics for the two EIGRP routes. The show ip bgp command output ! above shows how BGP assigned the MED (metric) that same value. R6# show ip route eigrp

32.0.0.0/24 is subnetted, 1 subnets D 32.1.1.0 [90/156160] via 10.1.69.9, 00:12:17, FastEthernet0/0

D 31.0.0.0/8 [90/156160] via 10.1.69.9, 00:12:17, FastEthernet0/0 ! Below, the redistribute eigrp command has been changed to the following, using ! a route map to only allow routes in networks in the 30s. redist eigrp 6 route-map just-30-something

Example 12-5 Example of Populating the BGP Table via Redistribution (Continued)

! The route map and ACLs used for the filtering are shown next. As a result, the ! two subnets of 10.0.0.0/8 will not be redistributed into the BGP table. R6# show route-map route-map just-30-something, permit, sequence 10 Match clauses:

ip address (access-lists): permit-30-39 Set clauses:

Policy routing matches: 0 packets, 0 bytes R6# show access-list

Standard IP access list permit-30-39

10 permit 32.0.0.0, wildcard bits 7.255.255.255 (1538 matches) 20 permit 30.0.0.0, wildcard bits 1.255.255.255 (1130 matches)

Also note that the NEXT_HOP PA for each route either matches the next hop of the redistributed route or is 0.0.0.0 for connected routes and routes to null0.

The Impact of Auto-Summary on Redistributed Routes and the network Command

As it does with IGPs, the BGP auto-summary command causes a classful summary route to be created if any component subnet of that summary exists. However, unlike IGPs, the BGP auto-summary router subcommand causes BGP to summarize only those routes injected due to redistribution on that router. BGP auto-summary does not look for classful network boundaries in the topology, and it does not look at routes already in the BGP table. It simply looks for routes injected into the BGP due to the redistribute and network commands on that same router.

The logic differs slightly based on whether the route is injected with the redistribute command or the network command. The logic for the two commands is summarized as follows:

KEY ■ redistribute—When the redistribution process would normally inject subnets of a classful PO|NT network, do not inject the subnets into the routing table, but instead inject the classful network.

■ network—For network commands that list a classful network number and no mask parameter, inject the classful network if at least one subnet of that classful network exists in the IP routing table.

While the preceding definitions are concise for study purposes, a few points deserve further emphasis and explanation. First, for redistribution, the auto-summary command causes the redistribution process to inject only classful networks into the local BGP table, and no subnets. The network command, with auto-summary configured, still injects subnets based on the same logic already described in this chapter. In addition to that logic, if a network command matches the classful network number, BGP injects the classful network, as long as at least any one subnet of that classful network exists in the IP routing table.

Example 12-6 shows an example that points out the impact of the auto-summary command. The example follows these steps on router R5 from Figure 12-2:

1. 10.15.0.0/16 is injected into BGP due to the redistribute command.

2. Auto-summary is configured, BGP is cleared, and now only 10.0.0.0/8 is in the BGP table.

3. Auto-summary and redistribution are disabled.

4. The network 10.0.0.0 command, network 10.12.0.0 mask 255.254.0.0 command, and network 10.14.0.0 mask 255.255.0.0 command are configured. Only the last of these three commands exactly matches a current route, so only that route is injected into BGP.

5. Auto-summary is enabled, causing 10.0.0.0/8 to be injected, as well as the original 10.14.0.0/ 16 route.

Example 12-6 Auto-Summary Impact on Routing Tables

R5 has shut down all neighbor connections, so the output of show ip bgp only routes injected on R5.

shows

Step 1 is below. Only 10.15.0.0/16 is injected by the current configuration.

Note that

the unrelated lines of output have been removed, and route-map only15 only

matches 10.15.0.0/16.

R5# show run | be router bgp

router bgp 5

no synchronization

redistribute connected route-map only15

no auto-summary

Below, note the absence of 10.0.0.0/8 as a route, and the presence of 10.15.1

5.0/16,

as well as the rest of the routes used in the upcoming steps.

R5# show ip route 10.0.0.0

Routing entry for 10.0.0.0/8, 4 known subnets

Attached (4 connections)

Redistributing via eigrp 99, bgp 5

Advertised by bgp 5 route-map only15

C 10.14.0.0/16 is directly connected, Loopback10

C 10.15.0.0/16 is directly connected, Loopback10

C 10.12.0.0/16 is directly connected, Loopback10

C 10.13.0.0/16 is directly connected, Loopback10

Only 10.15.0.0/16 is injected into BGP.

R5# show ip bgp

BGP table version is 2, local router ID is 5.5.5.5

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal

r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

*> 10.15.0.0/16 0.0.0.0 0 32768 ?

Next, step 2, where auto-summary is enabled. Now, 10.15.0.0/16 is no longer

Example 12-6 Auto-Summary Impact on Routing Tables (Continued)

! injected into BGP, but classful 10.0.0.0/8 is. R5# conf t

Enter configuration commands, one per line. End with CNTL/Z.

R5(config)# router bgp 5

R5(config-router)# no auto-summary

R5(config-router)# *Z

R5# show ip bgp

BGP table version is 2, local router ID is 5.5.5.5

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

Now, at step 3, no auto-summary disables automatic summarization, redistribution is disabled, and at step 4, the network commands are added. Note that 10.12.0.0/15 is not injected, as there is no exact match, nor is 10.0.0.0/8, as there is no exact match. However, 10.14.0.0/16 is injected due to the exact match of the prefix and prefix length. R5# conf t

Enter configuration commands, one per line. End with CNTL/Z. R5(config)# router bgp 5 R5(config-router)# no auto-summary R5(config-router)# no redist conn route-map only15 R5(config-router)# no redist connected R5(config-router)# network 10.0.0.0

R5(config-router)# network 10.12.0.0 mask 255.254.0.0 R5(config-router)# network 10.14.0.0 mask 255.255.0.0

R5(config-router)# R5# clear ip bgp R5# sh ip bgp | begin network

Network Next Hop Metric LocPrf Weight Path

Finally, auto-summary is re-enabled (not shown in the example). 10.14.0.0/16 is still an exact match, so it is still injected. 10.0.0.0/8 is also injected because of the network 10.0.0.0 command. R5# sh ip bgp | begin network

Network Next Hop Metric LocPrf Weight Path

Manual Summaries and the AS_PATH Path Attribute

As covered in the last several pages, a router can add entries to its BGP table using the network command and route redistribution. Additionally, BGP can use manual route summarization to advertise summary routes to neighboring routers, causing the neighboring routers to learn additional BGP routes. BGP manual summarization with the aggregate-address command differs significantly from using the auto-summary command. It can summarize based on any routes in the BGP table, creating a summary of any prefix length. It does not always suppress the advertisement of the component subnets, although it can be configured to do so.

The aggregate route must include the AS_PATH PA, just like it is required for every other NLRI in the BGP table. However, to fully understand what this command does, you need to take a closer look at the AS_PATH PA.

The AS_PATH PA consists of up to four different components, called segments, as follows:

■ AS_CONFED_SEQ (short for AS Confederation Sequence)

The most commonly used segment is called AS_SEQ. AS_SEQ is the idea of AS_PATH as shown back in Figure 12-1, with the PA representing all ASNs, in order, through which the route has been advertised.

However, the aggregate-address command can create a summary route for which the AS_SEQ must be null. When the component subnets of the summary route have differing AS_SEQ values, the router simply can't create an accurate representation of AS_SEQ, so it uses a null AS_SEQ. However, this action introduces the possibility of creating routing loops, because the contents of AS_PATH, specifically AS_SEQ, are used to prevent a route from being re-advertised to an AS that has already heard about the route.

The AS_PATH AS_SET segment solves the problem when the summary route has a null AS_SEQ. The AS_SET segment holds an unordered list of all the ASNs in all the component subnets' AS_SEQ segments.

Example 12-7 shows an example in which the router does use a null AS_SEQ for a summary route, and then the same summary with the as-set option 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 section "Confederations."

The following list summarizes the actions taken by the aggregate-address command when it creates a summary route:

KEY POINT

It does not create the summary if the BGP table does not currently have any routes for NLRI inside the summary.

■ If all the component subnets are withdrawn from the aggregating router's BGP table, it also then withdraws the aggregate. (In other words, the router tells its neighbors that the aggregate route is no longer valid.)

■ It sets the NEXT_HOP address of the summary, as listed in the local BGP table, as 0.0.0.0.

■ It sets the NEXT_HOP address of the summary route, as advertised to neighbors, to the router's update source IP address for each neighbor, respectively.

■ If the component subnets inside the summary all have the same AS_SEQ, it sets the new summary route's AS_SEQ to be exactly like the AS_SEQ of the component subnets.

■ If the AS_SEQ of the component subnets differs in any way, it sets the AS_SEQ of the new summary route to null.

■ When the as-set option has been configured, the router creates an AS_SET segment for the aggregate route, but only if the summary route's AS_SEQ is null.

■ As usual, if the summary is advertised to an eBGP peer, the router prepends its own ASN to the AS_SEQ before sending the Update.

■ It suppresses the advertisement of all component 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 option is configured. (Refer to Chapter 13 for an example of using the suppress-map option.)

Example 12-7 shows R3 from Figure 12-4 summarizing 23.0.0.0/8. R3 advertises the summary with ASN 123 as the only AS in the AS_SEQ, because some component subnets have AS_PATHS

of 45, and others have 678 45. As a result, R3 uses a null AS_SEQ for the aggregate. The example goes on to show the impact of the as-set option.

Example 12-7 Route Aggregation and the as-set Option

! Note that R3's routes to network 23 all have the same AS_PATH except one new ! prefix, which has an AS_PATH that includes ASN 678. As a result, R3 will ! create a null AS_SEQ for the summary route. R3# show ip bgp | include 23

*> 23.3.0.0/20 *> 23.3.16.0/20 *> 23.3.32.0/19

Example 12-7 Route Aggregation and the as-set Option (Continued)

! The following 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 have a BGP table entry for 23.0.0.0/8; however, R3 will ! advertise this summary to its peers, because at least one component subnet ! exists.

R1 has learned the prefix, NEXT_HOP 3.3.3.3 (R3's update source IP address for R1), but the AS_PATH is now null because R1 is in the same AS as R3. (Had R3-R1 been an eBGP peering, R3 would have prepended its own ASN.)

_Note that the next command is on R1 R1 R1 R1.

R1# sh ip bgp | begin Network

Network Next Hop Metric LocPrf Weight Path

Next, R1 displays the AGGREGATOR PA, which identifies R3 (3.3.3.3) and its AS (123) as the aggregation point at which information is lost. Also, the phrase

! "atomic-aggregate" refers to the fact that the ATOMIC_AGGREGATE PA has also ! been set; this PA simply states that this NLRI is a summary. R1# show ip bgp 23.0.0.0

BGP routing table entry for 23.0.0.0/8, version 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 summary route from R1, but the lack of information in the current AS_PATH allows R6 to learn of the route, possibly causing a routing loop. (Remember, one of the component subnets, 23.4.0.0/16, came from

R6# sh ip bgp nei 172.16.16.1 received-routes | begin Network

Network Next Hop Metric LocPrf Weight Path

! The R3 configuration is changed as shown next 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 component of the AS_PATH PA, which includes an unordered ! list of all autonmous systems from all the component subnets' AS PATHs on R3.

R1# sh ip bgp | begin Network

Network Next Hop

Metric LocPrf Weight Path 0 100 0 45 i

Example 12-7 Route Aggregation and the as-set Option (Continued)

! Now R6 does not receive the 23.0.0.0 prefix due to R1s check of the AS_SET PA, ! noticing that ASN 678 is in the AS_SET and is also R6's ASN. R6# sh ip bgp nei 172.16.16.1 received-routes | begin Network

Network Next Hop Metric LocPrf Weight Path

NOTE Summary routes can also be added via another method. First, the router would create a static route, typically with destination of interface null0. Then, the prefix/length can be matched with the network command to inject the summary. This method does not filter any of the component subnets.

Table 12-6 summarizes the key points regarding summarization using the aggregate-address, auto-summary, and network commands.

Table 12-6 Summary: Injecting Summary Routes in BGP

Table 12-6 Summary: Injecting Summary Routes in BGP

KEY POINT

Command

Component Subnets Removed

Routes It Can Summarize

auto-summary (with redistribution)

All

Only 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

Only those injected into BGP on that router using the network command

KEY POINT

Adding Default Routes to BGP

The final method covered in this chapter for adding routes to a BGP table is to inject default routes into BGP. Default routes can be injected into BGP in one of three ways:

■ By injecting the default using the network command

■ By injecting the default using the redistribute command

■ By injecting a default route into BGP using the neighbor neighbor-id default-information [route-map route-map-name] BGP subcommand

When injecting a default route into BGP using the network command, a route to 0.0.0.0/0 must exist in the local routing table, and the network 0.0.0.0 command is required. The default IP route can be learned via any means, but if it is removed from the IP routing table, BGP removes the default route from the BGP table.

Injecting a default route through redistribution requires an additional configuration command— default-information originate. The default route must first exist in the IP routing table; for instance, a static default route to null0 could be created. Then, the redistribute static command could be used to redistribute that static default route. However, in the special case of the default route, Cisco IOS also requires the default-information originate BGP subcommand.

Injecting a default route into BGP by using the neighbor neighbor-id default-information [route-map route-map-name] BGP subcommand does not add a default route to the local BGP table; instead, it causes the advertisement of a default to the specified neighbor. In fact, this method does not even check for the existence of a default route in the IP routing table by default, but it can. With the route-map option, the referenced route map examines the entries in the IP routing table (not the BGP table); if a route map permit clause is matched, then the default route is advertised to the neighbor. Example 12-8 shows just such an example on R1, with route-map check-default checking for the existence of a default route before R1 would originate a default route to R3.

Example 12-8 Originating a Default Route to a Neighbor with the neighbor default-originate Command

! The pertinent parts of the R1 configuration are listed next, with the route map ! matching an IP route to 0.0.0.0/0 with a permit action, enabling the ! advertisement of a default route to neighbor 3.3.3.3 (R3). router bgp 123 neighbor 3.3.3.3 remote-as 123 neighbor 3.3.3.3 update-source Loopbackl neighbor 3.3.3.3 default-originate route-map check-default

ip prefix-list def-route seq 5 permit 0.0.0.0/0

route-map check-default permit 10 match ip address prefix-list def-route

! R1 indeed has a default route, as seen below.

S* 0.0.0.0/0 is directly connected, Null0

! R3 now learns a default route from R1, as seen below.

R3# show ip bgp | begin Network

Network Next Hop Metric LocPrf Weight Path

The ORIGIN Path Attribute

Depending on the method used to inject a route into a local BGP table, BGP assigns one of three BGP ORIGIN PA codes: IGP, EGP, or incomplete. The ORIGIN PA provides a general descriptor as to how a particular NLRI was first injected into a router's BGP table. The show ip bgp command includes the three possible values in the legend at the top of the command output, listing the actual ORIGIN code for each BGP route at the far right of each output line. Table 12-7 lists the three ORIGIN code names, the single-letter abbreviation used by Cisco IOS, and the reasons why a route is assigned a particular code.

The ORIGIN codes and meanings hide a few concepts that many people find counterintuitive. First, routes redistributed into BGP from an IGP actually have an ORIGIN code of incomplete. Also, do not confuse EGP with eBGP; an ORIGIN of EGP refers to Exterior Gateway Protocol, the very old and deprecated predecessor to BGP. In practice, the EGP ORIGIN code should not be seen today.

Table 12-7 BGP ORIGIN Codes

KEY POINT

Table 12-7 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 default-information originate command

The rules regarding the ORIGIN codes used for summary routes created with the aggregate-address command can also be a bit surprising. The rules are summarized as follows:

KEY POINT

■ If the as-set option is not used, the aggregate route uses ORIGIN code i.

■ If the as-set option is used, and all component subnets being summarized use ORIGIN code i, the aggregate has ORIGIN code i.

■ If the as-set option is used, and at least one of the component subnets has an ORIGIN code ?, the aggregate has ORIGIN code ?.

NOTE The BGP ORIGIN PA provides a minor descriptor for the origin of a BGP table entry, which is used as part of the BGP decision tree (covered in Chapter 13).

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