Route Aggregation in BGP ||

Methods for advertising aggregated prefixes with problems.

The AS_SET attribute.

Methods for advertising aggregated prefixes with problems

Why Aggregate?

-- A bgp update message has to carry all the routes , but if we aggregate them it reduces the size of BGP updates.This contributes to network resource conservation

-- This also reduces the size of the BGP tables

-- Contributes to overall internet scalability

--"HIDES" individual prefix instabilities

How to inject Aggregate Routes

Method 1 :: Static Route

-- Configure static route for aggregate address

-- Always use null interface for next hop.

-- inject aggregate into BGP with network statement.

When a packet that has been forwarded to the aggregate address arrives at the router, its destination address should match with one of the more specific routes and be forwarded to the correct destination.

If a more specific route does not exist, for example if the prefix has flapped, it will be forwarded to the null interface and dropped.

Method 2:: aggregate-address statement

-- A member of the aggregate must be in the BGP table.

--Provides more configuration options than the static route method. (For more information see IOS guide)

LAB

R1

ip route 172.16.0.0 255.255.0.0 null 0

router bgp 65520

network 172.16.0.0

R4

router bgp 64950

aggregate-address 192.168.0.0 255.255.0.0

Lets go back to R1

R1#sh ip bgp

BGP table version is 5, local router ID is 10.1.1.1

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

RPKI validation codes: V valid, I invalid, N Not found

Network Next Hop Metric LocPrf Weight Path

*> 10.5.0.0/16 10.1.1.2 0 65501 64950 ?

*> 172.16.0.0 0.0.0.0 0 32768 i

*> 192.168.0.0/16 10.1.1.2 0 65501 64950 i

*> 192.168.1.0 10.1.1.2 0 65501 64950 ?

The problem is that we still have route for 192.168.1.0 also. Even though we did aggregate it on R4.

router bgp 64950

aggregate-address 192.168.0.0 255.255.0.0 summary-only

So to Supress more SPECIFIC ROUTES

Now again on R1

R1#sh ip bgp

BGP table version is 6, local router ID is 10.1.1.1

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

RPKI validation codes: V valid, I invalid, N Not found

Network Next Hop Metric LocPrf Weight Path

*> 10.5.0.0/16 10.1.1.2 0 65501 64950 ?

*> 172.16.0.0 0.0.0.0 0 32768 i

*> 192.168.0.0/16 10.1.1.2 0 65501 64950

---

Aggregare Attributes

ATOMIC_AGGREGATE

--Indicates that the prefix is an aggregate and not to an actual destination

-- Formed when an aggregate includes at least one AS number of the aggregated prefixes

-- No practical use in modern networks.

AGGREGATOR

-- AS number and Router ID of the router originating the aggregate

-- Can be useful for troubleshooting aggregate routes

Notice the output of the two subnets that were configured.

The subnet which was aggregated on R1 doesn't contain the atomic-aggregate and aggragate attribute. Both are sub-part of path attributes

Subnet which was aggregated using aggregate-address summary only has atomic-aggregate and aggragate attribute in it.

atomic-aggregate - keyword is present

aggregate attribute - aggregated by 64590 is present (the entire line)

---

THE AS_SET Attribute

Potential Problems with Aggregation

Aggregation reduces route information (precise)

-- Best path to destination might not be chosen from multiple aggregate routes.

Potential misconfiguration

--Could result in misrouted packets

--Ensure that the configured aggregate correctly represents the aggregated prefixes

Potential for inter-AS loops

-- AS_PATH SET prevents this

THE AS_PATH SET Attribute

AS_PATH is normally an ordered sequence of AS numbers

--AS_PATH SEQUENCE

AS_SET is an unordered sequence of AS numbers

-- includes all AS numbers of all prefixes belonging to aggregate

--Same loop prevention rules as AS_PATH SEQUENCE

*Drop received routes that include local AS number

--Prevents looping to constituent AS behind aggregation point

LAB

Assuming neighborship is established between R5 and R2.

and R5 will advertise using aggregate command.

We will then aggregate the addresses from 65515 and 65520 to 172.16.0.0/12

R5#aggregate-address 172.17.0.0 255.255.0.0 summary-only

Now we will aggregate both the subnets

-------------------------------------------

R3(config-router)#router bgp 65501

R3(config-router)#aggregate-address 172.16.0.0 255.240.0.0 summary-only

We are not using AS_SET as we want to see the differences..

But lets see what is at R3 at this point

Lets now configure it with AS_SET command and compare the differnce.

R3(config-router)#router bgp 65501

aggregate-address 172.16.0.0 255.240.0.0 summary-only as-set

Now lets check the ouput at R4

Problems in AS_PATH SET

AS_SET is rarely used in practice

-- and when it is used , frequently used incorrectly

Loss of origin data can cause operational problems

--new security practices cannot authenticate route origin

--Loss of path information causes traffic engineering problems

Current recommendation is not to use AS_SET

--RFC 6472