Webinar topic: BGP troubleshooting: route origin Presenter: Achmad Mardiansyah In this webinar, we discussed about troubleshooting BGP issue related to attribute: route origin. Please share your feedback or webinar ideas here: http://bit.ly/glcfeedback Check our schedule for future events: https://www.glcnetworks.com/en/schedule/ Follow our social media for updates: Facebook, Instagram, YouTube Channel, and telegram also discord Recording is available on youtube: https://youtu.be/mrN6FUjXAmE

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BGP troubleshooting:
route origin
GLC WEBINAR,
8 Dec 2022, 19:30 GMT+7
Achmad Mardiansyah
achmad@glcnetworks.com
GLC Networks, Indonesia

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Agenda
● Introduction
● Review prerequisite knowledge
● BGP
● BGP attribute, origin
● Live practice
● Q & A
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Introduction
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What is GLC?
● Garda Lintas Cakrawala (www.glcnetworks.com)
● Based in Bandung, Indonesia
● Areas: Training, IT Consulting
● Certified partner for: Mikrotik, Ubiquity, Linux foundation
● Product: GLC radius manager
● Regular event
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Trainer Introduction
● Name: Achmad Mardiansyah
● Base: bandung, Indonesia
● Linux user since 1999, mikrotik user since 2007, UBNT
2011
● Mikrotik Certified Trainer
(MTCNA/RE/WE/UME/INE/TCE/IPv6)
● Mikrotik/Linux Certified Consultant
● Website contributor: achmadjournal.com, mikrotik.tips,
asysadmin.tips
● More info: http://au.linkedin.com/in/achmadmardiansyah
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Past experience
● 2020-2022 (Congo DRC, PNG, Malaysia): network support,
radius/billing integration
● 2019, Congo (DRC): build a wireless ISP from ground-up
● 2018, Malaysia: network revamp, develop billing solution and
integration, setup dynamic routing
● 2017, Libya (north africa): remote wireless migration for a new
Wireless ISP
● 2016, United Kingdom: workshop for wireless ISP, migrating a
bridged to routed network
● 2015, Kalimantan, wireless support
● See our website for more details
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Please introduce yourself
● Your name
● Your company/university?
● Your networking experience?
● Your mikrotik experience?
● Your expectation from this course?
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Prerequisite
This presentation some prerequisite knowledge
● We assume you already know:
○ TCP/IP
○ Mikrotik
○ Routing protocol
○ Routed protocol
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Review prerequisite knowledge
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7 OSI layer & protocol
● OSI layer Is a conceptual model from ISO (International
Standard Organization) for project OSI (Open System
Interconnection)
● When you send a message with a courier, you need to
add more info to get your message arrived at the
destination (This process is called encapsulation)
● What is protocol
○ Is a set of rules for communication
○ Available on each layer
● Communication consist of series encapsulation
○ SDU: service data unit (before PDU)
○ PDU: protocol data unit (after header is added)
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Layered model (TCP/IP vs ISO) and encapsulation
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/ datagram

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Did you notice?
● There is a big overhead on encapsulation process
● More encapsulation means less payload?
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Router and Routing
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● Router is a network device that is used to forward packets, based on layer 3
information (layer 3 header)
● Routing is the process of selecting a path for traffic in a network, or between
or across multiple networks
Physical
router
Router
icon

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Typical connection (physical)
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R2
R1
R3
● Router connects layer 2 segments
● Router works on layer 3, connecting L2
segments
● Meaning, each layer 2 segment has network
ID

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Typical connection (logical) and routing table
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Routing table:
● A table at router that is used to forward packet
● Available on every devices (router and host)
● Entry is executed sequentially
192.168.0.0/26
R1
192.168.0.1/26
192.168.0.3/26
192.168.0.2/26
R3
R2
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.3.3/24
192.168.3.9/24
192.168.2.9/24
192.168.2.2/24
192.168.1.1/24
192.168.1.9/24
destination gateway
192.168.0.0/26 direct
192.168.1.0/24 direct
192.168.2.0/24 192.168.0.2
192.168.3.0/24 192.168.0.3
192.168.16.3/32 192.168.0.2
0.0.0.0/0 (default gw) 192.168.0.3

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Forwarding packets using routing table
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● It works like a firewall: match and action
● When a packet arrived, routing table is used to forward packets
● You should think in binary to understand how it works
destination gateway
192.168.16.3/32
11000000 10101000 00001000 00000011
192.168.0.2
192.168.0.0/26
11000000 10101000 00000000 00
direct
192.168.1.0/24
11000000 10101000 00000001
direct
192.168.2.0/24
11000000 10101000 00000010
192.168.0.2
192.168.3.0/24
11000000 10101000 00000011
192.168.0.3
0.0.0.0/0
(no match)
192.168.0.3

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A packet arrived at R1… (example)
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Destination IP address of the packet is 192.168.2.6, which gateway do we use?
A: 192.168.2.6 = (11000000 10101000 00000010 00000110)
destination gateway
192.168.16.3/32
11000000 10101000 00001000 00000011
192.168.0.2
192.168.0.0/26
11000000 10101000 00000000 00
direct
192.168.1.0/24
11000000 10101000 00000001
direct
192.168.2.0/24
11000000 10101000 00000010
192.168.0.2
192.168.3.0/24
11000000 10101000 00000011
192.168.0.3
0.0.0.0/0 192.168.0.3

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Where routing table lookup happens?
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How routing works & Administrative distance (analogy)
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CITY 1 100 km
CITY 2 120 km
CITY 2 90 km
CITY 3 500 km
CITY 4 250 km
10.10.10.0/24 192.168.0.1 10
10.10.20.0/24 192.168.0.2 12
10.10.20.0/24 192.168.0.3 9
10.10.30.0/24 192.168.0.3 50
10.10.40.0/24 192.168.0.4 25

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Administrative distance
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● Distance is considered when prefix length is
same
● Lowest distance wins
● Administrative distance policy is depends on
vendor
● Table on the right shows an example of
administrative distance on cisco router

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Static routing
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● Entries on routing table is created manually
● Admin must manage routing table in all
routers
● Admin have full control
192.168.0.0/26
R1
192.168.0.1/26
192.168.0.3/26
192.168.0.2/26
R3
R2
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.3.3/24
192.168.3.9/24
192.168.2.9/24
192.168.2.2/24
192.168.1.1/24
192.168.1.9/24
destination gateway
192.168.0.0/26 direct
192.168.1.0/24 direct
192.168.2.0/24 192.168.0.2
192.168.3.0/24 192.168.0.3
192.168.16.3/32 192.168.0.2
0.0.0.0/0 192.168.0.3

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Dynamic routing
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● Router will talk to each other with routing protocol (RIP,
OSPF, BGP)
● Entries on routing table is created automatically
● Admin must have a good knowledge about routing
protocol
192.168.0.0/26
R1
192.168.0.1/26
192.168.0.3/26
192.168.0.2/26
R3
R2
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.3.3/24
192.168.3.9/24
192.168.2.9/24
192.168.2.2/24
192.168.1.1/24
192.168.1.9/24
destination gateway
192.168.0.0/26 direct
192.168.1.0/24 direct
192.168.2.0/24 192.168.0.2
192.168.3.0/24 192.168.0.3
192.168.16.3/32 192.168.0.2
0.0.0.0/0 192.168.0.3

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Routing metric
● is value used by a router to make
routing decisions. It depends on
routing protocols
● OSPF: accumulated cost
● RIP: hop count
● IS-IS: cost
● EIGRP: bandwidth, load, delay,
reliability and MTU
● BGP: AS-Path, Next-hop, Origin,
Local preference, Atomic aggregate,
Multi Exit Discriminator (MED)
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Asymmetric routing
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● Currently, routing is done one-way only
● Forwarding process on router is based on destination IP address
● There is no guarantee incoming path is similar to outgoing path
● We can only control outgoing forwarding
R1
192.168.0.1/26
192.168.0.3/26 R3
R2
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.3.3/24
192.168.3.9/24
192.168.2.9/24
192.168.2.2/24
192.168.1.1/24
192.168.1.9/24

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BGP
Border Gateway Protocol
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● Dynamic routing protocol, Path-vector
● Connecting AS numbers
● Each router need to create BGP-peer
○ Only receive and send routing prefix to peer
● At Layer 5 (session): use for BGP peer session
● At Layer 4 (transport): TCP port 179
● Used by all routers in the internet
● Routes contents:
○ Full routes: all prefix in the internet
○ Partial routes: specific prefix
Need to consider:
● Requires knowledge to operate
● More complex (compared to RIP), lots of options
What is BGP?
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● A collection of Routers & Networks
● Under one administration (organisation, company, etc)
● Defined by AS number, given by IANA via RIR (APNIC, LACNIC, AFRINIC, ARIN, RIPE-NCC)
● We run BGP between AS
● Inside AS we run IGP (Interior gateway protocol): RIP, OSPF, IS-IS, EIGRP, etc
Autonomous number (AS), IGP, BGP
AS1
AS4
AS3
AS2
R1
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HOW BGP works?
BGP steps
● Each router must have router-id and AS number
● Each router must create BGP peer
● Setup BGP filter:
○ Inbound filter
○ Outbound filter
● Exchange routing tables between routers
● Send keep alive periodically
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BGP attributes
BGP metrics are called path attributes. Classifications:
● Well-known (all router must support)
○ Mandatory (must exist, in every update)
■ AS-Path
■ Next-hop
■ Origin
○ Discretionary (not always exist)
■ Local preference
■ Atomic aggregate
● Optional (some routers do not support)
○ Transitive (If not recognized, marked as partial and still propagated to neighbor)
■ Community
■ Aggregator
○ Non-Transitive (If not recognized, discard)
■ Multi Exit Discriminator (MED)
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BGP best path selection algorithm
1. Router is ignoring received path if the route is not valid. Route is valid if:
a. NEXT_HOP of the route is valid and reachable
b. AS_PATH received from external peers does not contain the local AS
c. route is not rejected by routing filters
2. The first path received is automatically considered 'best path'. Any further received paths are compared to first received to determine if the
new path is better.
3. Prefer the path with the highest WEIGHT.
a. WEIGHT parameter is local to the router on which it is configured. A route without assigned WEIGHT have a default value of 0.
4. Prefer the path with the highest LOCAL_PREF. It is used only within an AS.
a. A path without LOCAL_PREF attribute have a value of 100 by default.
5. Prefer the path with the shortest AS_PATH. (skipped if ignore-as-path-len set to yes)
a. Each AS_SET counts as 1, regardless of the set size. The AS_CONFED_SEQUENCE and AS_CONFED_SET are not included in the AS_PATH length.
6. Prefer the path that was locally originated via aggregate or BGP network
7. Prefer the path with the lowest ORIGIN type.
a. Interior Gateway Protocol (IGP) is lower than Exterior Gateway Protocol (EGP), and EGP is lower than INCOMPLETE, in other words IGP < EGP < INCOMPLETE
8. Prefer the path with the lowest multi-exit discriminator (MED).
a. The router compare MED attribute only for paths that have the same neighboring (leftmost) AS. Paths without explicit MED value are treated as with MED of 0
9. Prefer eBGP over iBGP paths
10. Prefer the route that comes from the BGP router with the lowest router ID. If a route carries the ORIGINATOR_ID attribute, then the
ORIGINATOR_ID is used instead of router ID.
11. Prefer the route with the shortest route reflection cluster list. Routes without a cluster list are considered to have a cluster list of length 0.
12. Prefer the path that comes from the lowest neighbor address
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BGP attribute: origin
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Topology 1
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E4
R1 R2
R21 R22
R41 R42
E2
E4
E4 E4
E4
E2
E2
E2
E3
E3
E6
R3 R4
R23 R24
R43 R44
E2
E4
E4 E4
E4
E2
E2
E2
E3
E3
E6
R5 R6
R25 R26
R45 R46
E2
E4
E4 E4
E4
E2
E2
E2
E3
E3
E6
R7 R8
R27 R28
R47 R48
E2
E4
E4 E4
E2
E2
E2
E3
E3
E6
R9 R10
R29 R30
R49 R50
E2
E4
E4
E4
E4
E2
E2
E2
E3
E3
E6
SW86
192.168.86.0/24
E5 E5 E5
E5 E5
R86
E6
R61 R62 R63 R64 R65 R66
R67 R68 R69 R70
E3 E3
E3 E3
E2
E2
E2
E2
E3 E3
E3 E3
E2
E2
E2
E2
E3 E3
E3 E3
E2
E2
E2
E2
E3 E3
E3 E3
E2
E2 E2
E2
E3 E3
E3 E3
E2
E2 E2
E2
E6
E5
E6 E5
SW51
E2
E2
SW52 SW53 SW54 SW55 SW55 SW59
SW58
SW55
SW60
E2
pc79 pc80
E2
E3 E4
E2
E3
E2
E4
pc78
E2
E2
E3 E4
pc71 pc72
E2
E2
E3 E4 E3 E4
pc73 pc74 pc75
E2
pc76
E2
pc77
E2
E2
E3 E4
E2
E2
E3 E4
E2
E2
E3 E4
E2
E3 E4
E2
E3 E4
E6
E5
E6
E5 E6 E5

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BGP attribute: Origin
Options
● Interior Gateway Protocol (IGP)
○ Assumes that the prefix came from an Interior Gateway Protocol
● Exterior Gateway Protocol (EGP)
○ Prefix coming from EGP protocol (before BGP exist). Egp is old
protocol
● INCOMPLETE
○ BGP is unsure how the prefix was injected into routing table. Can be
from: static route, redistribute
Priority:
● IGP < EGP < INCOMPLETE
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QnA
Any questions?
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Interested? Just come to our training...
● Topics are arranged in systematic and
logical way
● You will learn from experienced teacher
● Not only learn the materials, but also
sharing experiences, best-practices, and
networking
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End of slides
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