Hands-On Workshop on Science DMZ and P4-DPDK Lab

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Hands-on Workshop on Science DMZ and P4-DPDK

Lab 1: Hot potato routing and BGP

local preferences

Ali AlSabeh, Jorge Crichigno

University of South Carolina (USC)

University of South Carolina (USC)

The Engagement and Performance Operations Center (EPOC)

Minority Serving - Cyberinfrastructure Consortium (MS-CC)

Tuesday, August 6, 2024.

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Overview of BGP Attributes

BGP – Best Path

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Is this the best path? Does

it meet the path selection

criteria? If so, add to

routing table.

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Rick Graziani, “Implementing Cisco IP Routing,” Cisco Press, 2015

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The main goal is to provide interdomain routing

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BGP selects one path as the best path

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It places the selected path in its routing table and propagates the path to its neighbors

BGP – Best Path

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1.

Prefer the path with 

highest weight 

(configured locally; set to 0 (default) for routes not originated by the router)

2.

If weights are the same, prefer the path with 

highest local preference 

(set to 100 by default)

3.

If the local preferences are the same, prefer the 

path that was originated by BGP running on the router or

redistributed from an Interior Gateway Protocol (IGP)

4.

If no route was originated, prefer the path with the 

shortest AS_PATH

5.

If the paths have the same AS_PATH length, prefer the path with the 

lowest origin type

 (IGP is lower than Exterior

Gateway Protocol (EGP), and EGP is lower than Incomplete)

6.

If the origin codes are the same, prefer the path with the 

lowest MED attribute 

(set to 0 by default)

7.

If the paths have the same MED, prefer the 

External path (EBGP) over the Internal path (IBGP)

8.

If the paths are still the same, prefer the path through the 

closest IGP neighbor (lowest IGP metric)

9.

If both paths are external, prefer the path that was 

received first (oldest one)

10.

If the paths are still the same, prefer the path from the 

BGP router with the lowest router ID

11.

If the router ID is the same for multiple paths, prefer the path with the 

lowest IP address

Rick Graziani, “Implementing Cisco IP Routing,” Cisco Press, 2015

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Without route manipulation, the most common reason for path selection is Step 4



Prefer the path with the 

shortest AS_PATH

BGP – Best Path

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Without route manipulation, the most common reason for path selection is Step 4



Prefer the path with the 

shortest AS_PATH

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If multiple paths have the same length, the second most common decision point is Step 7



If the paths have the same MED, prefer 

EBGP over (internal) IBGP



We sometimes refer to this as “hot potato” routing (i.e., asymmetric routing)

BGP – Best Path

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BGP – Best Path

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Hot potato routing is not always desirable for a number of reasons



Different paths might have different network throughput



Some network devices might not handle asymmetric traffic properly, causing network security and

filtering issues

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The network administrator can manipulate some attributes to modify the path



Weight and Local Preference modify the outbound path



MED modifies the inbound path

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A well-known discretionary attribute

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Indicates to routers in the AS which path is preferred to exit the AS (higher is better)

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Configured on a router

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Exchanged only among routers within the same AS (passed only via IBGP, not via EBGP)

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Default value on a Cisco/FRR router is 100

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Local Preference takes precedence over AS_PATH

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This is very frequently used in provider networks

Rick Graziani, “Implementing Cisco IP Routing,” Cisco Press, 2015

Local Preference Attribute – For “Outbound Route”

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A well-known discretionary attribute

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Indicates to routers in the AS which path is preferred to exit the AS (higher is better)

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Configured on a router

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Exchanged only among routers within the same AS (passed only via IBGP, not via EBGP)

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Default value on a Cisco/FRR router is 100

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Local Preference takes precedence over AS_PATH

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This is very frequently used in provider networks

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All traffic in AS 64520 to 172.16.0.0 is sent

   via router A

Rick Graziani, “Implementing Cisco IP Routing,” Cisco Press, 2015

Local Preference Attribute – For “Outbound Route”

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Lab 1: Hot potato routing and BGP local preferences

Lab activities are described in Lab 12, Introduction to BGP lab series

Lab Topology

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Nework-1 and Network-4 are in ISP-1 (AS

100) and ISP-2 (AS 200), respectively

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Traffic sent from Network-1 to Network-4

follows the route h1-r1-r3-r4-h4

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Traffic from Network-4 to Network-1

follows the route h4-r4-r2-r1

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Route h1-r1-r3-r4-h2 should not be used

as it suffers from 0.1% packet loss

Lab Configuration

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Verify hot-potato routing

Test throughout

Lab Configuration

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Configure LOCAL_PREF attribute on r1 and r2

Platform Information

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We will use NETLAB virtual platform for the hands-on activities

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URL: 

https://netlab.cec.sc.edu/

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Username: 

<Email address used for registration>

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Temporary Password: 

nsf2024 (note, you will set a new password after the first login)

For a step-by-step on how to schedule a lab, follow the guide:

https://research.cec.sc.edu/files/cyberinfra/files/Cyberinfrastructure_Training_User_Guide.pdf

For any issues in accessing your NETLAB account, feel free to contact: 

choueiri@email.sc.edu