Preventing persistent oscillations and loops in IBGP configuration with route reflection

Authors:
Anuj Rawat;Mark A. Shayman
Affiliations:
Department of Electrical and Computer Engineering, University of Maryland, College Park, MD;Department of Electrical and Computer Engineering, University of Maryland, College Park, MD
Venue:
Computer Networks: The International Journal of Computer and Telecommunications Networking
Year:
2006

Citing 5
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Abstract

Internal Border Gateway Protocol (IBGP) is responsible for distributing external reachability information, obtained via External-BGP (EBGP) sessions, within an autonomous system (AS). To avoid a full mesh of IBGP sessions between all the BGP speakers of an AS, scaling schemes such as route reflection and AS confederations have been proposed. But it has been observed that employing these schemes may result in problems such as routing oscillations and forwarding loops due to Multi-Exit Discriminator (MED) attribute and path asymmetry in IBGP. In this paper we study the pathologies observed in IBGP when route reflection is used. We model the AS using the Interior Gateway Protocol (IGP) connectivity graph GI and IBGP peering graph GL. Then we state some simple conditions on GI and GL and prove that these conditions guarantee the absence of any persistent routing oscillations and forwarding loops due to MED attribute and IBGP path asymmetry. We consider the problem of constructing an IBGP configuration given the IGP connectivity such that there are no persistent oscillations and loops, and apply the conditions developed in the paper on this problem. We prove that solving the problem while minimizing some appropriate cost function is NP hard. We then give an Integer Linear Program (ILP) to construct a forwarding loop and persistent routing oscillation free IBGP configuration, for an AS with given IGP connectivity graph. which minimizes some appropriate cost while satisfying the resource constraints on all the BGP speaking nodes.