This document provides an overview of BGP path selection attributes and techniques for manipulating them, including BGP weight and AS path prepending. It begins by explaining that BGP selects the best path based on attributes rather than lowest metric like IGPs. It then details the priority of each attribute and provides a quick overview of each. The remainder focuses on BGP weight, explaining how it influences local path selection and can be set per neighbor or using route maps. It also covers AS path prepending and how it can be used to prepend an AS path inbound or outbound to influence path selection.
Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet.
BGP (Border Gateway Routing Protocol) is a standardized exterior gateway protocol designed to
exchange routing and reachability information between autonomous systems (AS) on the Internet. The
Border Gateway Protocol makes routing decisions based on paths, network policies or rule-sets
configured by a network administrator, and are involved in making core routing decisions.
BGP is a very robust and scalable routing protocol, as evidenced by the fact that BGP is the routing
protocol employed on the Internet.
Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet.
BGP (Border Gateway Routing Protocol) is a standardized exterior gateway protocol designed to
exchange routing and reachability information between autonomous systems (AS) on the Internet. The
Border Gateway Protocol makes routing decisions based on paths, network policies or rule-sets
configured by a network administrator, and are involved in making core routing decisions.
BGP is a very robust and scalable routing protocol, as evidenced by the fact that BGP is the routing
protocol employed on the Internet.
Open Shortest Path First (OSPF) || 2020 || Ser-2Nutan Singh
Open Shortest Path First (OSPF) is a routing protocol for Internet Protocol (IP) networks. It uses a link state routing (LSR) algorithm and falls into the group of interior gateway protocols (IGPs), operating within a single autonomous system (AS). It is defined as OSPF Version 2 in RFC 2328 (1998) for IPv4.
In this webinar, we cover how Border Gateway Protocol works. Starting from key concepts, you'll learn about Autonomous Systems, the BGP protocol, AS Path, learning and advertising routes, RIBs and route selection. See the webinar recording at https://www.thousandeyes.com/webinars/how-bgp-works
Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector routing protocol that is used on a computer network for automating routing decisions and configuration.
This Presentation was made by Ali Ibrahim and Aun Haider for the Class Activity purpose. We do hope that this Presentation may assist those students who are undergoing networking studies, presentation or projects in a fruitful and positive manner.
Open Shortest Path First (OSPF) || 2020 || Ser-2Nutan Singh
Open Shortest Path First (OSPF) is a routing protocol for Internet Protocol (IP) networks. It uses a link state routing (LSR) algorithm and falls into the group of interior gateway protocols (IGPs), operating within a single autonomous system (AS). It is defined as OSPF Version 2 in RFC 2328 (1998) for IPv4.
In this webinar, we cover how Border Gateway Protocol works. Starting from key concepts, you'll learn about Autonomous Systems, the BGP protocol, AS Path, learning and advertising routes, RIBs and route selection. See the webinar recording at https://www.thousandeyes.com/webinars/how-bgp-works
Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector routing protocol that is used on a computer network for automating routing decisions and configuration.
This Presentation was made by Ali Ibrahim and Aun Haider for the Class Activity purpose. We do hope that this Presentation may assist those students who are undergoing networking studies, presentation or projects in a fruitful and positive manner.
Describe the different metrics that BGP can use in building a routing.docxearleanp
Describe the different metrics that BGP can use in building a routing table
Solution
The metrics used by BGP in building a routing table are
1) Weight — weight is the first criterion used by the router and it is set locally on the user’s router. The Weight is not passed to the following router updates. In case there are multiple paths to a certain IP address, BGP always selects the path with the highest weight. The weight parameter can be set either through neighbor command, route maps or via the AS-path access list.
2) Local Preference — this criterion indicates which route has local preference and BGP selects the one with the highest preference. Local Preference default is 100.
3) Network or Aggregate — this criterion chooses the path that was originated locally via an aggregate or a network, as the aggregation of certain routes in one is quite effective and helps to save a lot of space on the network.
4) Shortest AS_PATH — this criterion is used by BGP only in case it detects two similar paths with nearly the same local preference, weight and locally originated or aggregate addresses.
5) Lowest origin type — this criterion assigns higher preference to Exterior Gateway Protocol (EGP) and lower preference to Interior Gateway Protocol (IGP).
6) Lowest multi-exit discriminator (MED) — this criterion, representing the external metric of a route, gives preference to the lower MED value.
7) eBGP over iBGP — just like the “Lowest origin type†criterion, this criterion prefers eBGP rather than iBGP.
8) Lowest IGP metric — this criterion selects the path with the lowest IGP metric to the BGP next hop.
9) Multiple paths — this criterion serves as indication whether multiple routes need to be installed in the routing table.
10) External paths — out of several external paths, this criterion selects the first received path.
11) Lowest router ID — this criterion selects the path which connects to the BGP router that has the lowest router ID.
12) Minimum cluster list — in case multiple paths have the same router ID or originator, this criterion selects the path with the minimum length of the cluster list.
13) Lowest neighbor address — this criterion selects the path, which originates from the lowest neighbor address.
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4. Difference Between BGP & IGP Path Selection
• BGP (Border Gateway Protocol)
routers usually receive multiple
paths to the same destination.
• BGP however, selects the best
path based on a set of
attributes.
• IGPs select the path with the
lowest metric. For example:
• RIP selects the path with the
lowest hop count.
• OSPF selects the path with the
lowest cost.
• EIGRP selects the path with the
highest bandwidth and lowest
delay (unless you change the K
values).
6. Path is selected based on the following attributes
Priority Attribute
1 Weight
2 Local Preference
3 Originate
4 AS path length
5 Origin code
6 MED (MULTI_EXIT_DISC)
7 eBGP path over iBGP path
8 Shortest IGP path to BGP next hop
9 Oldest path
10 Router ID
11 Neighbor IP address
7. Quick overview of Each Attribute (1 of 3)
• Weight: Prefer the path with the highest weight. This is a value that is local to the router
and it’s Cisco proprietary. The default value is 0 for all routes that are not originated by
the local router.
• Local Preference: The local preference is used within an autonomous system and
exchanged between iBGP routers. We prefer the path with the highest local preference.
The default value is 100.
• Originate: Prefer the path that the local router originated. In the BGP table, you will see
next hop 0.0.0.0. You can get a path in the BGP table through the BGP network
command, aggregation, or redistribution. A BGP router will prefer routes that it installed
into BGP itself over a route that another router installed in BGP.
• AS path length: Prefer the path with the shortest AS path length. For example, AS path 1
2 3 is preferred over AS path 1 2 3 4 5.
8. Quick overview of Each Attribute (2 of 3)
• Origin code: Prefer the lowest origin code. There are three origin
codes: IGP, EGP, INCOMPLETE. IGP is lower than EGP and EGP is lower
than INCOMPLETE.
• MED (MULTI_EXIT_DISC): Prefer the path with the lowest MED. The
MED is exchanged between autonomous systems.
• eBGP path over iBGP path: Prefer eBGP (external BGP) over iBGP
(internal BGP) paths.
• Shortest IGP path to BGP next hop: Prefer the path within the
autonomous system with the lowest IGP metric to the BGP next hop.
9. Quick overview of Each Attribute (3 of 3)
• Oldest Path: Prefer the path that we received first, in other words, the
oldest path.
• Router ID: Prefer the path with the lowest BGP neighbor router ID.
The router ID is based on the highest IP address. If you have a
loopback interface, then the IP address on the loopback will be used.
The router ID can also be manually configured.
• Neighbor IP address: Prefer the path with the lowest neighbor IP
address. If you have two eBGP routers and two links in between then
the router ID will be the same. In this case, the neighbor IP address is
the tiebreaker.
11. About BGP Weight Attribute (1 of 2)
• BGP weight attribute is a Cisco proprietary path attribute that is local
to the router and is used to influence local path selection on a certain
router.
• BGP weight is the first tie breaker for best path selection on a Cisco
router, the attribute is not propagated to other routers in the BGP
updates and only found on Cisco routers.
• The path with the highest weight is selected as the best toward a
destination.
12. About BGP Weight Attribute (2 of 2)
• The weight can be a number from 0 to 65,535. Paths that the router
originates have a weight of 32,768 by default, and other paths have a
weight of 0. As we mentioned earlier the path with the highest weight
value wins.
• BGP weight can be set using three ways on a Cisco router. Those are:
assigned per neighbor, using AS-Path access lists, using route-maps
(complex).
• BGP weight is the easiest way to manipulate the BGP path selection
on a single Cisco router for outbound traffic. The attribute is local and
will not be propagated to other routers in the network within the BGP
update messages.
13. BGP Weight Attribute Assigned per neighbor (1 of
4)
• R1 to get to R6 it has chosen the path through
R4-
When both paths are external prefer the oldest one –
This is the tiebreaker!
14. BGP Weight Attribute Assigned per neighbor (2 of
4)
• To prove that we can restart the bgp process on R4 to
make R5 the oldest route. On R4 I have done a clear ip
bgp * This now makes the route through R5 the oldest
one and is preferred.
15. BGP Weight Attribute Assigned per neighbor (3 of
4)
• Under BGP enter the following command – neighbor
10.0.14.4 weight 100
16. BGP Weight Attribute Assigned per neighbor (4 of
4)
• You will also notice that all prefixes received from R4 have
also had their weight changed to 100
17. BGP Weight Attribute Using AS-Path access lists (1
of 2)
• For this example we only want to set the weight for the
6.6.6.6/32 prefix. First we need to match this prefix in an
access list
This is now applied to the neighbor statement for R4 – we also need to remove the previous
weight attribute
18. BGP Weight Attribute Using AS-Path access lists (1
of 2)
• You can see now that we have changed the weight
attribute for the 6.6.6.6/32 prefix only. All other routes
have the default weight of 0.
20. About BGP AS-Path Prepending
• AS-Path prepending is a way to manipulate the AS-Path attribute of a
BGP route.
• It allows prepending multiple entries of AS to a BGP route.
• This can come as a workaround if a specific path is required to be
followed, and other means like Multi-Exit Discriminator (MED) is not
supported.
• AS-Path prepending can be applied to inbound and outbound
direction using route-maps.
21. Outbound AS-Path Prepending (1 of 2)
• AS-Path prepending can be
applied to outbound direction
on R3 router as below-
• Outbound AS-Path prepending on
R3-
22. Outbound AS-Path Prepending (2 of 2)
• When R3 router advertises its 10.1.1.0/24 network
to R2 router, it prepends its own AS 200 multiple
times. However, R1 router receives the original
BGP route with a single AS-path entry.
• BGP tables on R1 and R2-
R2 router has two paths to 10.1.1.0/24. However, it
chooses the path through R1 since that path has
shortest AS-Path (indicated by a > sign, meaning best
route). Hence, the direct path through R3 router can
act as a backup path.
23. Inbound AS-Path Prepending (1 of 2)
• Inbound AS-Path prepending is configured using
set as-path prepend last-as command under route-
map which is then applied in inbound direction.
The last-as keyword copies the AS number of the
neighbor advertising the BGP route that matches
the route-map.
• Inbound AS-Path prepending on R1-
24. Inbound AS-Path Prepending (2 of 2)
• This configuration causes R1 to prepend the AS
number of R3 twice before installing in the BGP
table. As seen below, R3 does not prepend any AS
number by itself.
• BGP tables on R1 and R3-