Internet Inter-AS Routing with BGP
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Internet inter-AS routing: BGP 🔗
BGP
(Border Gateway Protocol) is the de facto interdomain routing protocol.- It is the glue that holds the Internet together. BGP gives each AS the following capabilities:
- eBGP: Exchanges routing information between neighboring autonomous systems.
- iBGP: Exchanges routing information within an autonomous system. It allows a subnet to advertise its existence to the rest of the Internet.
eBGP and iBGP connections 🔗
- eBGP: A BGP connection between different autonomous systems.
- iBGP: A BGP connection within the same autonomous system.
Gateway routers run both eBGP and iBGP.
Basic BGP operation 🔗
BGP session
: Two BGP routers, or peers, exchange BGP messages over a semipermanent TCP connection.- They advertise paths to destination network prefixes; BGP is a path-vectorprotocol.
When AS3 gateway router 3a advertises the path 'AS3,X' to AS2 gateway router 2c, AS3 promises AS2 that it will forward datagrams toward X.
BGP route selection 🔗
BGP must select one route from several candidates.
- Local preference attribute: A policy decision.
- Shortest AS path.
- Closest next-hop router: Hot potato routing.
- Additional criteria.
Hot potato routing 🔗
Through iBGP, 2d learns that X is reachable through either 2a or 2c.
Hot potato routing selects the local gateway with the lowest intradomain cost.
- For example, 2d chooses 2a even though the route to X has more AS hops.
- It does not worry about interdomain costs!
Intra-AS vs. inter-AS routing 🔗
Policy:
- Intra-AS: There is a single administrator, so interadministrative policy decisions are unnecessary.
- Inter-AS: Administrators want control over how traffic is routed and which networks carry it.
Scale:
- Hierarchical routing reduces routing-table size and update traffic.
Performance:
- Intra-AS: Routing algorithms focus on performance.
- Inter-AS: Policy may take precedence over performance.
